Expose PreviewImage & custom icons to py API.

This commit mainly:

* Exposes PreviewImage struct in RNA, including ways for user to set images data.
* Adds a new kind of PreviewImage, using a file path and IMB_thumb to get image.
* Adds a new kind of custom icon using PreviewImage, unrelated to ID previews system.
* Adds a python API (utils.previews) to allow python scripts to access those custom previews/icons.

Note that loading image from files' thumbnails is done when needed (deferred loading), not
when defining the custom preview/icon.

WARNING: for release addons who would want to use this, please keep it to a strict minimum, really needed level.
We do not want our UI to explode under hundreds of different flashy icons!

For more info, see also the release notes of Blender 2.75 (http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.75/Addons)
and the example/templates featured with Blender.

Patch by Campbell (ideasman42), Inês (brita) and Bastien (mont29).

Differential Revision: https://developer.blender.org/D1255

28 files changed, 1068 insertions, 239 deletions

diff --git a/source/blender/blenkernel/BKE_icons.h b/source/blender/blenkernel/BKE_icons.h
index 9af0d96884a..763a3874d4e 100644
--- a/source/blender/blenkernel/BKE_icons.h
+++ b/source/blender/blenkernel/BKE_icons.h
@@ -48,10 +48,14 @@ typedef struct Icon Icon;
 struct PreviewImage;
 struct ID;
 
+enum eIconSizes;
+
 void BKE_icons_init(int first_dyn_id);
 
 /* return icon id for library object or create new icon if not found */
-int BKE_icon_getid(struct ID *id);
+int BKE_icon_id_ensure(struct ID *id);
+
+int BKE_icon_preview_ensure(struct PreviewImage *preview);
 
 /* retrieve icon for id */
 struct Icon *BKE_icon_get(int icon_id);
@@ -60,8 +64,10 @@ struct Icon *BKE_icon_get(int icon_id);
 /* used for inserting the internal icons */
 void BKE_icon_set(int icon_id, struct Icon *icon);
 
-/* remove icon and free date if library object becomes invalid */
-void BKE_icon_delete(struct ID *id);
+/* remove icon and free data if library object becomes invalid */
+void BKE_icon_id_delete(struct ID *id);
+
+void BKE_icon_delete(int icon_id);
 
 /* report changes - icon needs to be recalculated */
 void BKE_icon_changed(int icon_id);
@@ -75,8 +81,17 @@ void BKE_previewimg_freefunc(void *link);
 /* free the preview image */
 void BKE_previewimg_free(struct PreviewImage **prv);
 
+/* clear the preview image or icon, but does not free it */
+void BKE_previewimg_clear(struct PreviewImage *prv);
+
+/* clear the preview image or icon at a specific size */
+void BKE_previewimg_clear_single(struct PreviewImage *prv, enum eIconSizes size);
+
+/* get the preview from any pointer */
+struct PreviewImage **BKE_previewimg_id_get_p(struct ID *id);
+
 /* free the preview image belonging to the id */
-void BKE_previewimg_free_id(struct ID *id);
+void BKE_previewimg_id_free(struct ID *id);
 
 /* create a new preview image */
 struct PreviewImage *BKE_previewimg_create(void);
@@ -85,6 +100,19 @@ struct PreviewImage *BKE_previewimg_create(void);
 struct PreviewImage *BKE_previewimg_copy(struct PreviewImage *prv);
 
 /* retrieve existing or create new preview image */
-struct PreviewImage *BKE_previewimg_get(struct ID *id);
+struct PreviewImage *BKE_previewimg_id_ensure(struct ID *id);
+
+void BKE_previewimg_ensure(struct PreviewImage *prv, const int size);
+
+struct PreviewImage *BKE_previewimg_cached_get(const char *name);
+
+struct PreviewImage *BKE_previewimg_cached_ensure(const char *name);
+
+struct PreviewImage *BKE_previewimg_cached_thumbnail_read(
+        const char *name, const char *path, const int source, bool force_update);
+
+void BKE_previewimg_cached_release(const char *name);
+
+#define ICON_RENDER_DEFAULT_HEIGHT 32
 
 #endif /*  __BKE_ICONS_H__ */
diff --git a/source/blender/blenkernel/intern/icons.c b/source/blender/blenkernel/intern/icons.c
index d3225f3fa35..450c1388f37 100644
--- a/source/blender/blenkernel/intern/icons.c
+++ b/source/blender/blenkernel/intern/icons.c
@@ -45,6 +45,7 @@
 
 #include "BLI_utildefines.h"
 #include "BLI_ghash.h"
+#include "BLI_string.h"
 
 #include "BKE_icons.h"
 #include "BKE_global.h" /* only for G.background test */
@@ -53,6 +54,10 @@
 
 #include "GPU_extensions.h"
 
+#include "IMB_imbuf.h"
+#include "IMB_imbuf_types.h"
+#include "IMB_thumbs.h"
+
 /* GLOBALS */
 
 static GHash *gIcons = NULL;
@@ -61,6 +66,7 @@ static int gNextIconId = 1;
 
 static int gFirstIconId = 1;
 
+static GHash *gCachedPreviews = NULL;
 
 static void icon_free(void *val)
 {
@@ -105,30 +111,50 @@ void BKE_icons_init(int first_dyn_id)
 	gFirstIconId = first_dyn_id;
 
 	if (!gIcons)
-		gIcons = BLI_ghash_int_new("icons_init gh");
+		gIcons = BLI_ghash_int_new(__func__);
+
+	if (!gCachedPreviews) {
+		gCachedPreviews = BLI_ghash_str_new(__func__);
+	}
 }
 
 void BKE_icons_free(void)
 {
-	if (gIcons)
+	if (gIcons) {
 		BLI_ghash_free(gIcons, NULL, icon_free);
-	gIcons = NULL;
+		gIcons = NULL;
+	}
+
+	if (gCachedPreviews) {
+		BLI_ghash_free(gCachedPreviews, MEM_freeN, BKE_previewimg_freefunc);
+		gCachedPreviews = NULL;
+	}
 }
 
-PreviewImage *BKE_previewimg_create(void)
+static PreviewImage *previewimg_create_ex(size_t deferred_data_size)
 {
 	PreviewImage *prv_img = NULL;
 	int i;
 
-	prv_img = MEM_callocN(sizeof(PreviewImage), "img_prv");
+	prv_img = MEM_mallocN(sizeof(PreviewImage) + deferred_data_size, "img_prv");
+	memset(prv_img, 0, sizeof(*prv_img));  /* leave deferred data dirty */
+
+	if (deferred_data_size) {
+		prv_img->use_deferred = true;
+	}
 
 	for (i = 0; i < NUM_ICON_SIZES; ++i) {
-		prv_img->changed[i] = 1;
+		prv_img->flag[i] |= PRV_CHANGED;
 		prv_img->changed_timestamp[i] = 0;
 	}
 	return prv_img;
 }
 
+PreviewImage *BKE_previewimg_create(void)
+{
+	return previewimg_create_ex(0);
+}
+
 void BKE_previewimg_freefunc(void *link)
 {
 	PreviewImage *prv = (PreviewImage *)link;
@@ -138,7 +164,6 @@ void BKE_previewimg_freefunc(void *link)
 		for (i = 0; i < NUM_ICON_SIZES; ++i) {
 			if (prv->rect[i]) {
 				MEM_freeN(prv->rect[i]);
-				prv->rect[i] = NULL;
 			}
 			if (prv->gputexture[i])
 				GPU_texture_free(prv->gputexture[i]);
@@ -156,6 +181,25 @@ void BKE_previewimg_free(PreviewImage **prv)
 	}
 }
 
+void BKE_previewimg_clear_single(struct PreviewImage *prv, enum eIconSizes size)
+{
+	MEM_SAFE_FREE(prv->rect[size]);
+	if (prv->gputexture[size]) {
+		GPU_texture_free(prv->gputexture[size]);
+	}
+	prv->h[size] = prv->w[size] = 0;
+	prv->flag[size] |= PRV_CHANGED;
+	prv->changed_timestamp[size] = 0;
+}
+
+void BKE_previewimg_clear(struct PreviewImage *prv)
+{
+	int i;
+	for (i = 0; i < NUM_ICON_SIZES; ++i) {
+		BKE_previewimg_clear_single(prv, i);
+	}
+}
+
 PreviewImage *BKE_previewimg_copy(PreviewImage *prv)
 {
 	PreviewImage *prv_img = NULL;
@@ -167,79 +211,186 @@ PreviewImage *BKE_previewimg_copy(PreviewImage *prv)
 			if (prv->rect[i]) {
 				prv_img->rect[i] = MEM_dupallocN(prv->rect[i]);
 			}
-			else {
-				prv_img->rect[i] = NULL;
-			}
 			prv_img->gputexture[i] = NULL;
 		}
 	}
 	return prv_img;
 }
 
-void BKE_previewimg_free_id(ID *id) 
+PreviewImage **BKE_previewimg_id_get_p(ID *id)
 {
-	if (GS(id->name) == ID_MA) {
-		Material *mat = (Material *)id;
-		BKE_previewimg_free(&mat->preview);
-	}
-	else if (GS(id->name) == ID_TE) {
-		Tex *tex = (Tex *)id;
-		BKE_previewimg_free(&tex->preview);
-	}
-	else if (GS(id->name) == ID_WO) {
-		World *wo = (World *)id;
-		BKE_previewimg_free(&wo->preview);
+	switch (GS(id->name)) {
+#define ID_PRV_CASE(id_code, id_struct) case id_code: { return &((id_struct *)id)->preview; }
+		ID_PRV_CASE(ID_MA, Material);
+		ID_PRV_CASE(ID_TE, Tex);
+		ID_PRV_CASE(ID_WO, World);
+		ID_PRV_CASE(ID_LA, Lamp);
+		ID_PRV_CASE(ID_IM, Image);
+		ID_PRV_CASE(ID_BR, Brush);
+#undef ID_PRV_CASE
 	}
-	else if (GS(id->name) == ID_LA) {
-		Lamp *la  = (Lamp *)id;
-		BKE_previewimg_free(&la->preview);
-	}
-	else if (GS(id->name) == ID_IM) {
-		Image *img  = (Image *)id;
-		BKE_previewimg_free(&img->preview);
+
+	return NULL;
+}
+
+void BKE_previewimg_id_free(ID *id)
+{
+	PreviewImage **prv_p = BKE_previewimg_id_get_p(id);
+	if (prv_p) {
+		BKE_previewimg_free(prv_p);
 	}
-	else if (GS(id->name) == ID_BR) {
-		Brush *br  = (Brush *)id;
-		BKE_previewimg_free(&br->preview);
+}
+
+PreviewImage *BKE_previewimg_id_ensure(ID *id)
+{
+	PreviewImage **prv_p = BKE_previewimg_id_get_p(id);
+
+	if (prv_p) {
+		if (*prv_p == NULL) {
+			*prv_p = BKE_previewimg_create();
+		}
+		return *prv_p;
 	}
+
+	return NULL;
 }
 
-PreviewImage *BKE_previewimg_get(ID *id)
+PreviewImage *BKE_previewimg_cached_get(const char *name)
 {
-	PreviewImage *prv_img = NULL;
+	return BLI_ghash_lookup(gCachedPreviews, name);
+}
 
-	if (GS(id->name) == ID_MA) {
-		Material *mat = (Material *)id;
-		if (!mat->preview) mat->preview = BKE_previewimg_create();
-		prv_img = mat->preview;
+/**
+ * Generate an empty PreviewImage, if not yet existing.
+ */
+PreviewImage *BKE_previewimg_cached_ensure(const char *name)
+{
+	PreviewImage *prv = NULL;
+	void **prv_p;
+
+	if (!BLI_ghash_ensure_p_ex(gCachedPreviews, name, &prv_p, (GHashKeyCopyFP)BLI_strdup)) {
+		*prv_p = BKE_previewimg_create();
 	}
-	else if (GS(id->name) == ID_TE) {
-		Tex *tex = (Tex *)id;
-		if (!tex->preview) tex->preview = BKE_previewimg_create();
-		prv_img = tex->preview;
+	prv = *prv_p;
+	BLI_assert(prv);
+
+	return prv;
+}
+
+/**
+ * Generate a PreviewImage from given file path, using thumbnails management, if not yet existing.
+ */
+PreviewImage *BKE_previewimg_cached_thumbnail_read(
+        const char *name, const char *path, const int source, bool force_update)
+{
+	PreviewImage *prv = NULL;
+	void **prv_p;
+
+	prv_p = BLI_ghash_lookup_p(gCachedPreviews, name);
+
+	if (prv_p) {
+		prv = *prv_p;
+		BLI_assert(prv);
 	}
-	else if (GS(id->name) == ID_WO) {
-		World *wo = (World *)id;
-		if (!wo->preview) wo->preview = BKE_previewimg_create();
-		prv_img = wo->preview;
+
+	if (prv && force_update) {
+		const char *prv_deferred_data = PRV_DEFERRED_DATA(prv);
+		if (((int)prv_deferred_data[0] == source) && STREQ(&prv_deferred_data[1], path)) {
+			/* If same path, no need to re-allocate preview, just clear it up. */
+			BKE_previewimg_clear(prv);
+		}
+		else {
+			BKE_previewimg_free(&prv);
+		}
 	}
-	else if (GS(id->name) == ID_LA) {
-		Lamp *la  = (Lamp *)id;
-		if (!la->preview) la->preview = BKE_previewimg_create();
-		prv_img = la->preview;
+
+	if (!prv) {
+		/* We pack needed data for lazy loading (source type, in a single char, and path). */
+		const size_t deferred_data_size = strlen(path) + 2;
+		char *deferred_data;
+
+		prv = previewimg_create_ex(deferred_data_size);
+		deferred_data = PRV_DEFERRED_DATA(prv);
+		deferred_data[0] = source;
+		memcpy(&deferred_data[1], path, deferred_data_size - 1);
+
+		force_update = true;
 	}
-	else if (GS(id->name) == ID_IM) {
-		Image *img  = (Image *)id;
-		if (!img->preview) img->preview = BKE_previewimg_create();
-		prv_img = img->preview;
+
+	if (force_update) {
+		if (prv_p) {
+			*prv_p = prv;
+		}
+		else {
+			BLI_ghash_insert(gCachedPreviews, BLI_strdup(name), prv);
+		}
 	}
-	else if (GS(id->name) == ID_BR) {
-		Brush *br  = (Brush *)id;
-		if (!br->preview) br->preview = BKE_previewimg_create();
-		prv_img = br->preview;
+
+	return prv;
+}
+
+void BKE_previewimg_cached_release(const char *name)
+{
+	PreviewImage *prv = BLI_ghash_popkey(gCachedPreviews, name, MEM_freeN);
+
+	if (prv) {
+		if (prv->icon_id) {
+			BKE_icon_delete(prv->icon_id);
+		}
+		BKE_previewimg_freefunc(prv);
 	}
+}
 
-	return prv_img;
+/** Handle deferred (lazy) loading/generation of preview image, if needed.
+ * For now, only used with file thumbnails. */
+void BKE_previewimg_ensure(PreviewImage *prv, const int size)
+{
+	if (prv->use_deferred) {
+		const bool do_icon = ((size == ICON_SIZE_ICON) && !prv->rect[ICON_SIZE_ICON]);
+		const bool do_preview = ((size == ICON_SIZE_PREVIEW) && !prv->rect[ICON_SIZE_PREVIEW]);
+
+		if (do_icon || do_preview) {
+			ImBuf *thumb;
+			char *prv_deferred_data = PRV_DEFERRED_DATA(prv);
+			int source =  prv_deferred_data[0];
+			char *path = &prv_deferred_data[1];
+			int icon_w, icon_h;
+
+			thumb = IMB_thumb_manage(path, THB_LARGE, source);
+
+			if (thumb) {
+				/* PreviewImage assumes premultiplied alhpa... */
+				IMB_premultiply_alpha(thumb);
+
+				if (do_preview) {
+					prv->w[ICON_SIZE_PREVIEW] = thumb->x;
+					prv->h[ICON_SIZE_PREVIEW] = thumb->y;
+					prv->rect[ICON_SIZE_PREVIEW] = MEM_dupallocN(thumb->rect);
+					prv->flag[ICON_SIZE_PREVIEW] &= ~(PRV_CHANGED | PRV_USER_EDITED);
+				}
+				if (do_icon) {
+					if (thumb->x > thumb->y) {
+						icon_w = ICON_RENDER_DEFAULT_HEIGHT;
+						icon_h = (thumb->y * icon_w) / thumb->x + 1;
+					}
+					else if (thumb->x < thumb->y) {
+						icon_h = ICON_RENDER_DEFAULT_HEIGHT;
+						icon_w = (thumb->x * icon_h) / thumb->y + 1;
+					}
+					else {
+						icon_w = icon_h = ICON_RENDER_DEFAULT_HEIGHT;
+					}
+
+					IMB_scaleImBuf(thumb, icon_w, icon_h);
+					prv->w[ICON_SIZE_ICON] = icon_w;
+					prv->h[ICON_SIZE_ICON] = icon_h;
+					prv->rect[ICON_SIZE_ICON] = MEM_dupallocN(thumb->rect);
+					prv->flag[ICON_SIZE_ICON] &= ~(PRV_CHANGED | PRV_USER_EDITED);
+				}
+				IMB_freeImBuf(thumb);
+			}
+		}
+	}
 }
 
 void BKE_icon_changed(int id)
@@ -251,20 +402,20 @@ void BKE_icon_changed(int id)
 	icon = BLI_ghash_lookup(gIcons, SET_INT_IN_POINTER(id));
 	
 	if (icon) {
-		PreviewImage *prv = BKE_previewimg_get((ID *)icon->obj);
+		PreviewImage *prv = BKE_previewimg_id_ensure((ID *)icon->obj);
 
 		/* all previews changed */
 		if (prv) {
 			int i;
 			for (i = 0; i < NUM_ICON_SIZES; ++i) {
-				prv->changed[i] = 1;
+				prv->flag[i] |= PRV_CHANGED;
 				prv->changed_timestamp[i]++;
 			}
 		}
 	}
 }
 
-int BKE_icon_getid(struct ID *id)
+int BKE_icon_id_ensure(struct ID *id)
 {
 	Icon *new_icon = NULL;
 
@@ -277,11 +428,11 @@ int BKE_icon_getid(struct ID *id)
 	id->icon_id = get_next_free_id();
 
 	if (!id->icon_id) {
-		printf("BKE_icon_getid: Internal error - not enough IDs\n");
+		printf("%s: Internal error - not enough IDs\n", __func__);
 		return 0;
 	}
 
-	new_icon = MEM_callocN(sizeof(Icon), "texicon");
+	new_icon = MEM_mallocN(sizeof(Icon), __func__);
 
 	new_icon->obj = id;
 	new_icon->type = GS(id->name);
@@ -295,6 +446,40 @@ int BKE_icon_getid(struct ID *id)
 	return id->icon_id;
 }
 
+/**
+ * Return icon id of given preview, or create new icon if not found.
+ */
+int BKE_icon_preview_ensure(PreviewImage *preview)
+{
+	Icon *new_icon = NULL;
+
+	if (!preview || G.background)
+		return 0;
+
+	if (preview->icon_id)
+		return preview->icon_id;
+
+	preview->icon_id = get_next_free_id();
+
+	if (!preview->icon_id) {
+		printf("%s: Internal error - not enough IDs\n", __func__);
+		return 0;
+	}
+
+	new_icon = MEM_mallocN(sizeof(Icon), __func__);
+
+	new_icon->obj = preview;
+	new_icon->type = 0;  /* Special, tags as non-ID icon/preview. */
+
+	/* next two lines make sure image gets created */
+	new_icon->drawinfo = NULL;
+	new_icon->drawinfo_free = NULL;
+
+	BLI_ghash_insert(gIcons, SET_INT_IN_POINTER(preview->icon_id), new_icon);
+
+	return preview->icon_id;
+}
+
 Icon *BKE_icon_get(int icon_id)
 {
 	Icon *icon = NULL;
@@ -302,7 +487,7 @@ Icon *BKE_icon_get(int icon_id)
 	icon = BLI_ghash_lookup(gIcons, SET_INT_IN_POINTER(icon_id));
 	
 	if (!icon) {
-		printf("BKE_icon_get: Internal error, no icon for icon ID: %d\n", icon_id);
+		printf("%s: Internal error, no icon for icon ID: %d\n", __func__, icon_id);
 		return NULL;
 	}
 
@@ -314,18 +499,39 @@ void BKE_icon_set(int icon_id, struct Icon *icon)
 	void **val_p;
 
 	if (BLI_ghash_ensure_p(gIcons, SET_INT_IN_POINTER(icon_id), &val_p)) {
-		printf("BKE_icon_set: Internal error, icon already set: %d\n", icon_id);
+		printf("%s: Internal error, icon already set: %d\n", __func__, icon_id);
 		return;
 	}
 
 	*val_p = icon;
 }
 
-void BKE_icon_delete(struct ID *id)
+void BKE_icon_id_delete(struct ID *id)
 {
-
 	if (!id->icon_id) return;  /* no icon defined for library object */
 
 	BLI_ghash_remove(gIcons, SET_INT_IN_POINTER(id->icon_id), NULL, icon_free);
 	id->icon_id = 0;
 }
+
+/**
+ * Remove icon and free data.
+ */
+void BKE_icon_delete(int icon_id)
+{
+	Icon *icon;
+
+	if (!icon_id) return;  /* no icon defined for library object */
+
+	icon = BLI_ghash_popkey(gIcons, SET_INT_IN_POINTER(icon_id), NULL);
+
+	if (icon) {
+		if (icon->type) {
+			((ID *)(icon->obj))->icon_id = 0;
+		}
+		else {
+			((PreviewImage *)(icon->obj))->icon_id = 0;
+		}
+		icon_free(icon);
+	}
+}
diff --git a/source/blender/blenkernel/intern/image.c b/source/blender/blenkernel/intern/image.c
index f3f4775dbf7..29576e4f1f6 100644
--- a/source/blender/blenkernel/intern/image.c
+++ b/source/blender/blenkernel/intern/image.c
@@ -338,7 +338,7 @@ void BKE_image_free(Image *ima)
 
 	image_free_packedfiles(ima);
 
-	BKE_icon_delete(&ima->id);
+	BKE_icon_id_delete(&ima->id);
 	ima->id.icon_id = 0;
 
 	BKE_previewimg_free(&ima->preview);
@@ -3001,7 +3001,7 @@ static void image_initialize_after_load(Image *ima, ImBuf *ibuf)
 {
 	/* preview is NULL when it has never been used as an icon before */
 	if (G.background == 0 && ima->preview == NULL)
-		BKE_icon_changed(BKE_icon_getid(&ima->id));
+		BKE_icon_changed(BKE_icon_id_ensure(&ima->id));
 
 	/* fields */
 	if (ima->flag & IMA_FIELDS) {
diff --git a/source/blender/blenkernel/intern/lamp.c b/source/blender/blenkernel/intern/lamp.c
index 96b0b95adf3..44e35c645de 100644
--- a/source/blender/blenkernel/intern/lamp.c
+++ b/source/blender/blenkernel/intern/lamp.c
@@ -232,7 +232,7 @@ void BKE_lamp_free(Lamp *la)
 	}
 	
 	BKE_previewimg_free(&la->preview);
-	BKE_icon_delete(&la->id);
+	BKE_icon_id_delete(&la->id);
 	la->id.icon_id = 0;
 }
 
diff --git a/source/blender/blenkernel/intern/material.c b/source/blender/blenkernel/intern/material.c
index eaaf1319167..283c7a6fc88 100644
--- a/source/blender/blenkernel/intern/material.c
+++ b/source/blender/blenkernel/intern/material.c
@@ -106,7 +106,7 @@ void BKE_material_free_ex(Material *ma, bool do_id_user)
 	
 	if (ma->preview)
 		BKE_previewimg_free(&ma->preview);
-	BKE_icon_delete((struct ID *)ma);
+	BKE_icon_id_delete((struct ID *)ma);
 	ma->id.icon_id = 0;
 	
 	/* is no lib link block, but material extension */
diff --git a/source/blender/blenkernel/intern/texture.c b/source/blender/blenkernel/intern/texture.c
index 8e4fcc5d1cc..246e3f2a316 100644
--- a/source/blender/blenkernel/intern/texture.c
+++ b/source/blender/blenkernel/intern/texture.c
@@ -567,7 +567,7 @@ void BKE_texture_free(Tex *tex)
 	BKE_animdata_free((struct ID *)tex);
 	
 	BKE_previewimg_free(&tex->preview);
-	BKE_icon_delete((struct ID *)tex);
+	BKE_icon_id_delete((struct ID *)tex);
 	tex->id.icon_id = 0;
 	
 	if (tex->nodetree) {
diff --git a/source/blender/blenkernel/intern/world.c b/source/blender/blenkernel/intern/world.c
index f26cbf7880a..e4736b1f54c 100644
--- a/source/blender/blenkernel/intern/world.c
+++ b/source/blender/blenkernel/intern/world.c
@@ -74,7 +74,7 @@ void BKE_world_free_ex(World *wrld, bool do_id_user)
 	if (wrld->gpumaterial.first)
 		GPU_material_free(&wrld->gpumaterial);
 	
-	BKE_icon_delete((struct ID *)wrld);
+	BKE_icon_id_delete((struct ID *)wrld);
 	wrld->id.icon_id = 0;
 }
 
diff --git a/source/blender/editors/include/ED_render.h b/source/blender/editors/include/ED_render.h
index 2bc0566f4a6..414126cac13 100644
--- a/source/blender/editors/include/ED_render.h
+++ b/source/blender/editors/include/ED_render.h
@@ -60,11 +60,15 @@ struct Scene *ED_render_job_get_current_scene(const struct bContext *C);
  * - PR_BUTS_RENDER: preview is rendered for buttons window
  * - PR_ICON_RENDER: preview is rendered for icons. hopefully fast enough for at least 32x32
  * - PR_NODE_RENDER: preview is rendered for node editor
+ * - PR_ICON_DEFERRED: No render, we just ensure deferred icon data gets generated.
  */
 
-#define PR_BUTS_RENDER  0
-#define PR_ICON_RENDER  1
-#define PR_NODE_RENDER  2
+enum {
+	PR_BUTS_RENDER   = 0,
+	PR_ICON_RENDER   = 1,
+	PR_NODE_RENDER   = 2,
+	PR_ICON_DEFERRED = 3,
+};
 
 void ED_preview_init_dbase(void);
 void ED_preview_free_dbase(void);
diff --git a/source/blender/editors/include/UI_interface_icons.h b/source/blender/editors/include/UI_interface_icons.h
index 9d190fa81c7..92f3b0180c6 100644
--- a/source/blender/editors/include/UI_interface_icons.h
+++ b/source/blender/editors/include/UI_interface_icons.h
@@ -49,7 +49,7 @@ typedef struct IconFile {
 #define ICON_DEFAULT_HEIGHT_SCALE ((int)(UI_UNIT_Y * 0.8f))
 #define ICON_DEFAULT_WIDTH_SCALE  ((int)(UI_UNIT_X * 0.8f))
 
-#define PREVIEW_DEFAULT_HEIGHT 96
+#define PREVIEW_DEFAULT_HEIGHT 128
 
 /*
  * Resizable Icons for Blender
diff --git a/source/blender/editors/interface/interface.c b/source/blender/editors/interface/interface.c
index b63871f5c82..a9cc926b275 100644
--- a/source/blender/editors/interface/interface.c
+++ b/source/blender/editors/interface/interface.c
@@ -3237,6 +3237,18 @@ static uiBut *ui_def_but(
 	return but;
 }
 
+void ui_def_but_icon(uiBut *but, const int icon, const int flag) {
+	if (icon) {
+		ui_icon_ensure_deferred(but->block->evil_C, icon, (flag & UI_BUT_ICON_PREVIEW) != 0);
+	}
+	but->icon = (BIFIconID)icon;
+	but->flag |= flag;
+
+	if (but->str && but->str[0]) {
+		but->drawflag |= UI_BUT_ICON_LEFT;
+	}
+}
+
 static void ui_def_but_rna__disable(uiBut *but)
 {
 	but->flag |= UI_BUT_DISABLED;
@@ -3500,11 +3512,7 @@ static uiBut *ui_def_but_rna(
 		but->rnaindex = 0;
 
 	if (icon) {
-		but->icon = (BIFIconID)icon;
-		but->flag |= UI_HAS_ICON;
-		if (str[0]) {
-			but->drawflag |= UI_BUT_ICON_LEFT;
-		}
+		ui_def_but_icon(but, icon, UI_HAS_ICON);
 	}
 	
 	if ((type == UI_BTYPE_MENU) && (but->dt == UI_EMBOSS_PULLDOWN)) {
@@ -3691,8 +3699,7 @@ int UI_autocomplete_end(AutoComplete *autocpl, char *autoname)
 static void ui_but_update_and_icon_set(uiBut *but, int icon)
 {
 	if (icon) {
-		but->icon = (BIFIconID) icon;
-		but->flag |= UI_HAS_ICON;
+		ui_def_but_icon(but, icon, UI_HAS_ICON);
 	}
 
 	ui_but_update(but);
@@ -4066,7 +4073,7 @@ void UI_but_drag_set_value(uiBut *but)
 void UI_but_drag_set_image(uiBut *but, const char *path, int icon, struct ImBuf *imb, float scale)
 {
 	but->dragtype = WM_DRAG_PATH;
-	but->icon = icon; /* no flag UI_HAS_ICON, so icon doesnt draw in button */
+	ui_def_but_icon(but, icon, 0);  /* no flag UI_HAS_ICON, so icon doesnt draw in button */
 	but->dragpoin = (void *)path;
 	but->imb = imb;
 	but->imb_scale = scale;
@@ -4220,8 +4227,7 @@ uiBut *uiDefIconTextMenuBut(uiBlock *block, uiMenuCreateFunc func, void *arg, in
 {
 	uiBut *but = ui_def_but(block, UI_BTYPE_PULLDOWN, 0, str, x, y, width, height, arg, 0.0, 0.0, 0.0, 0.0, tip);
 
-	but->icon = (BIFIconID) icon;
-	but->flag |= UI_HAS_ICON;
+	ui_def_but_icon(but, icon, UI_HAS_ICON);
 
 	but->drawflag |= UI_BUT_ICON_LEFT;
 	but->flag |= UI_BUT_ICON_SUBMENU;
@@ -4236,8 +4242,7 @@ uiBut *uiDefIconMenuBut(uiBlock *block, uiMenuCreateFunc func, void *arg, int ic
 {
 	uiBut *but = ui_def_but(block, UI_BTYPE_PULLDOWN, 0, "", x, y, width, height, arg, 0.0, 0.0, 0.0, 0.0, tip);
 
-	but->icon = (BIFIconID) icon;
-	but->flag |= UI_HAS_ICON;
+	ui_def_but_icon(but, icon, UI_HAS_ICON);
 	but->drawflag &= ~UI_BUT_ICON_LEFT;
 
 	but->menu_create_func = func;
@@ -4253,7 +4258,7 @@ uiBut *uiDefIconTextBlockBut(uiBlock *block, uiBlockCreateFunc func, void *arg,
 	
 	/* XXX temp, old menu calls pass on icon arrow, which is now UI_BUT_ICON_SUBMENU flag */
 	if (icon != ICON_RIGHTARROW_THIN) {
-		but->icon = (BIFIconID) icon;
+		ui_def_but_icon(but, icon, 0);
 		but->drawflag |= UI_BUT_ICON_LEFT;
 	}
 	but->flag |= UI_HAS_ICON;
@@ -4270,9 +4275,8 @@ uiBut *uiDefIconBlockBut(uiBlock *block, uiBlockCreateFunc func, void *arg, int
 {
 	uiBut *but = ui_def_but(block, UI_BTYPE_BLOCK, retval, "", x, y, width, height, arg, 0.0, 0.0, 0.0, 0.0, tip);
 	
-	but->icon = (BIFIconID) icon;
-	but->flag |= UI_HAS_ICON;
-	
+	ui_def_but_icon(but, icon, UI_HAS_ICON);
+
 	but->drawflag |= UI_BUT_ICON_LEFT;
 	
 	but->block_create_func = func;
@@ -4305,9 +4309,8 @@ uiBut *uiDefSearchBut(uiBlock *block, void *arg, int retval, int icon, int maxle
 {
 	uiBut *but = ui_def_but(block, UI_BTYPE_SEARCH_MENU, retval, "", x, y, width, height, arg, 0.0, maxlen, a1, a2, tip);
 	
-	but->icon = (BIFIconID) icon;
-	but->flag |= UI_HAS_ICON;
-	
+	ui_def_but_icon(but, icon, UI_HAS_ICON);
+
 	but->drawflag |= UI_BUT_ICON_LEFT | UI_BUT_TEXT_LEFT;
 	
 	ui_but_update(but);
diff --git a/source/blender/editors/interface/interface_icons.c b/source/blender/editors/interface/interface_icons.c
index 6536c0a869e..7c05d5f4378 100644
--- a/source/blender/editors/interface/interface_icons.c
+++ b/source/blender/editors/interface/interface_icons.c
@@ -55,6 +55,7 @@
 
 #include "IMB_imbuf.h"
 #include "IMB_imbuf_types.h"
+#include "IMB_thumbs.h"
 
 #include "BIF_gl.h"
 #include "BIF_glutil.h"
@@ -904,10 +905,13 @@ void UI_icons_init(int first_dyn_id)
 static int preview_render_size(enum eIconSizes size)
 {
 	switch (size) {
-		case ICON_SIZE_ICON:    return 32;
-		case ICON_SIZE_PREVIEW: return PREVIEW_DEFAULT_HEIGHT;
+		case ICON_SIZE_ICON:
+			return ICON_RENDER_DEFAULT_HEIGHT;
+		case ICON_SIZE_PREVIEW:
+			return PREVIEW_RENDER_DEFAULT_HEIGHT;
+		default:
+			return 0;
 	}
-	return 0;
 }
 
 /* Create rect for the icon
@@ -923,12 +927,49 @@ static void icon_create_rect(struct PreviewImage *prv_img, enum eIconSizes size)
 	else if (!prv_img->rect[size]) {
 		prv_img->w[size] = render_size;
 		prv_img->h[size] = render_size;
-		prv_img->changed[size] = 1;
+		prv_img->flag[size] |= PRV_CHANGED;
 		prv_img->changed_timestamp[size] = 0;
 		prv_img->rect[size] = MEM_callocN(render_size * render_size * sizeof(unsigned int), "prv_rect");
 	}
 }
 
+void ui_icon_ensure_deferred(const bContext *C, const int icon_id, const bool big)
+{
+	Icon *icon = BKE_icon_get(icon_id);
+
+	if (icon) {
+		DrawInfo *di = (DrawInfo *)icon->drawinfo;
+
+		if (!di) {
+			di = icon_create_drawinfo();
+
+			icon->drawinfo = di;
+			icon->drawinfo_free = UI_icons_free_drawinfo;
+		}
+
+		if (di) {
+			if (di->type == ICON_TYPE_PREVIEW) {
+				PreviewImage *prv = (icon->type != 0) ? BKE_previewimg_id_ensure((ID *)icon->obj) : icon->obj;
+
+				if (prv) {
+					const int size = big ? ICON_SIZE_PREVIEW : ICON_SIZE_ICON;
+
+					if (!prv->use_deferred || prv->rect[size] || (prv->flag[size] & PRV_USER_EDITED)) {
+						return;
+					}
+
+					icon_create_rect(prv, size);
+
+					/* Always using job (background) version. */
+					ED_preview_icon_job(C, prv, NULL, prv->rect[size], prv->w[size], prv->h[size]);
+
+					prv->flag[size] &= ~PRV_CHANGED;
+				}
+			}
+		}
+	}
+}
+
 /* only called when icon has changed */
 /* only call with valid pointer from UI_icon_draw */
 static void icon_set_image(
@@ -940,6 +981,11 @@ static void icon_set_image(
 		return;
 	}
 
+	if (prv_img->flag[size] & PRV_USER_EDITED) {
+		/* user-edited preview, do not auto-update! */
+		return;
+	}
+
 	icon_create_rect(prv_img, size);
 
 	if (use_job) {
@@ -961,22 +1007,32 @@ PreviewImage *UI_icon_to_preview(int icon_id)
 	
 	if (icon) {
 		DrawInfo *di = (DrawInfo *)icon->drawinfo;
-		if (di && di->data.buffer.image) {
-			ImBuf *bbuf;
-			
-			bbuf = IMB_ibImageFromMemory(di->data.buffer.image->datatoc_rect, di->data.buffer.image->datatoc_size, IB_rect, NULL, "<matcap buffer>");
-			if (bbuf) {
-				PreviewImage *prv = BKE_previewimg_create();
-				
-				prv->rect[0] = bbuf->rect;
+		if (di) {
+			if (di->type == ICON_TYPE_PREVIEW) {
+				PreviewImage *prv = (icon->type != 0) ? BKE_previewimg_id_ensure((ID *)icon->obj) : icon->obj;
 
-				prv->w[0] = bbuf->x;
-				prv->h[0] = bbuf->y;
-				
-				bbuf->rect = NULL;
-				IMB_freeImBuf(bbuf);
-				
-				return prv;
+				if (prv) {
+					return BKE_previewimg_copy(prv);
+				}
+			}
+			else if (di->data.buffer.image) {
+				ImBuf *bbuf;
+
+				bbuf = IMB_ibImageFromMemory(di->data.buffer.image->datatoc_rect, di->data.buffer.image->datatoc_size,
+				                             IB_rect, NULL, __func__);
+				if (bbuf) {
+					PreviewImage *prv = BKE_previewimg_create();
+
+					prv->rect[0] = bbuf->rect;
+
+					prv->w[0] = bbuf->x;
+					prv->h[0] = bbuf->y;
+
+					bbuf->rect = NULL;
+					IMB_freeImBuf(bbuf);
+
+					return prv;
+				}
 			}
 		}
 	}
@@ -987,6 +1043,10 @@ static void icon_draw_rect(float x, float y, int w, int h, float UNUSED(aspect),
                            unsigned int *rect, float alpha, const float rgb[3], const bool is_preview)
 {
 	ImBuf *ima = NULL;
+	int draw_w = w;
+	int draw_h = h;
+	int draw_x = x;
+	int draw_y = y;
 
 	/* sanity check */
 	if (w <= 0 || h <= 0 || w > 2000 || h > 2000) {
@@ -1006,21 +1066,34 @@ static void icon_draw_rect(float x, float y, int w, int h, float UNUSED(aspect),
 	}
 
 	/* rect contains image in 'rendersize', we only scale if needed */
-	if (rw != w && rh != h) {
+	if (rw != w || rh != h) {
+		/* preserve aspect ratio and center */
+		if (rw > rh) {
+			draw_w = w;
+			draw_h = (int)(((float)rh / (float)rw) * (float)w);
+			draw_y += (h - draw_h) / 2;
+		}
+		else if (rw < rh) {
+			draw_w = (int)(((float)rw / (float)rh) * (float)h);
+			draw_h = h;
+			draw_x += (w - draw_w) / 2;
+		}
+		/* if the image is squared, the draw_ initialization values are good */
+
 		/* first allocate imbuf for scaling and copy preview into it */
 		ima = IMB_allocImBuf(rw, rh, 32, IB_rect);
 		memcpy(ima->rect, rect, rw * rh * sizeof(unsigned int));
-		IMB_scaleImBuf(ima, w, h); /* scale it */
+		IMB_scaleImBuf(ima, draw_w, draw_h); /* scale it */
 		rect = ima->rect;
 	}
 
 	/* draw */
 	if (is_preview) {
-		glaDrawPixelsSafe(x, y, w, h, w, GL_RGBA, GL_UNSIGNED_BYTE, rect);
+		glaDrawPixelsSafe(draw_x, draw_y, draw_w, draw_h, draw_w, GL_RGBA, GL_UNSIGNED_BYTE, rect);
 	}
 	else {
-		glRasterPos2f(x, y);
-		glDrawPixels(w, h, GL_RGBA, GL_UNSIGNED_BYTE, rect);
+		glRasterPos2f(draw_x, draw_y);
+		glDrawPixels(draw_w, draw_h, GL_RGBA, GL_UNSIGNED_BYTE, rect);
 	}
 
 	if (ima)
@@ -1081,10 +1154,13 @@ static void icon_draw_texture(
 static int get_draw_size(enum eIconSizes size)
 {
 	switch (size) {
-		case ICON_SIZE_ICON: return ICON_DEFAULT_HEIGHT;
-		case ICON_SIZE_PREVIEW: return PREVIEW_DEFAULT_HEIGHT;
+		case ICON_SIZE_ICON:
+			return ICON_DEFAULT_HEIGHT;
+		case ICON_SIZE_PREVIEW:
+			return PREVIEW_DEFAULT_HEIGHT;
+		default:
+			return 0;
 	}
-	return 0;
 }
 
 
@@ -1147,9 +1223,12 @@ static void icon_draw_size(
 		glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
 	}
 	else if (di->type == ICON_TYPE_PREVIEW) {
-		PreviewImage *pi = BKE_previewimg_get((ID *)icon->obj);
+		PreviewImage *pi = (icon->type != 0) ? BKE_previewimg_id_ensure((ID *)icon->obj) : icon->obj;
 
 		if (pi) {
+			/* Do deferred loading/generation if needed. */
+//			BKE_previewimg_ensure(pi, size);
+
 			/* no create icon on this level in code */
 			if (!pi->rect[size]) return;  /* something has gone wrong! */
 			
@@ -1164,17 +1243,17 @@ static void icon_draw_size(
 static void ui_id_preview_image_render_size(
         const bContext *C, Scene *scene, ID *id, PreviewImage *pi, int size, const bool use_job)
 {
-	if ((pi->changed[size] || !pi->rect[size])) { /* changed only ever set by dynamic icons */
+	if (((pi->flag[size] & PRV_CHANGED) || !pi->rect[size])) { /* changed only ever set by dynamic icons */
 		/* create the rect if necessary */
 		icon_set_image(C, scene, id, pi, size, use_job);
 
-		pi->changed[size] = 0;
+		pi->flag[size] &= ~PRV_CHANGED;
 	}
 }
 
 void UI_id_icon_render(const bContext *C, Scene *scene, ID *id, const bool big, const bool use_job)
 {
-	PreviewImage *pi = BKE_previewimg_get(id);
+	PreviewImage *pi = BKE_previewimg_id_ensure(id);
 
 	if (pi) {
 		if (big)
@@ -1186,7 +1265,7 @@ void UI_id_icon_render(const bContext *C, Scene *scene, ID *id, const bool big,
 
 static void ui_id_brush_render(const bContext *C, ID *id)
 {
-	PreviewImage *pi = BKE_previewimg_get(id); 
+	PreviewImage *pi = BKE_previewimg_id_ensure(id);
 	enum eIconSizes i;
 	
 	if (!pi)
@@ -1195,9 +1274,9 @@ static void ui_id_brush_render(const bContext *C, ID *id)
 	for (i = 0; i < NUM_ICON_SIZES; i++) {
 		/* check if rect needs to be created; changed
 		 * only set by dynamic icons */
-		if ((pi->changed[i] || !pi->rect[i])) {
+		if (((pi->flag[i] & PRV_CHANGED) || !pi->rect[i])) {
 			icon_set_image(C, NULL, id, pi, i, true);
-			pi->changed[i] = 0;
+			pi->flag[i] &= ~PRV_CHANGED;
 		}
 	}
 }
@@ -1208,7 +1287,7 @@ static int ui_id_brush_get_icon(const bContext *C, ID *id)
 	Brush *br = (Brush *)id;
 
 	if (br->flag & BRUSH_CUSTOM_ICON) {
-		BKE_icon_getid(id);
+		BKE_icon_id_ensure(id);
 		ui_id_brush_render(C, id);
 	}
 	else {
@@ -1270,7 +1349,7 @@ int ui_id_icon_get(const bContext *C, ID *id, const bool big)
 		case ID_IM: /* fall through */
 		case ID_WO: /* fall through */
 		case ID_LA: /* fall through */
-			iconid = BKE_icon_getid(id);
+			iconid = BKE_icon_id_ensure(id);
 			/* checks if not exists, or changed */
 			UI_id_icon_render(C, NULL, id, big, true);
 			break;
diff --git a/source/blender/editors/interface/interface_intern.h b/source/blender/editors/interface/interface_intern.h
index a3192d3b8d0..9461547a164 100644
--- a/source/blender/editors/interface/interface_intern.h
+++ b/source/blender/editors/interface/interface_intern.h
@@ -465,6 +465,8 @@ extern bool ui_but_string_set(struct bContext *C, uiBut *but, const char *str) A
 extern bool ui_but_string_set_eval_num(struct bContext *C, uiBut *but, const char *str, double *value) ATTR_NONNULL();
 extern int  ui_but_string_get_max_length(uiBut *but);
 extern uiBut *ui_but_drag_multi_edit_get(uiBut *but);
+
+void ui_def_but_icon(uiBut *but, const int icon, const int flag);
 extern uiButExtraIconType ui_but_icon_extra_get(uiBut *but);
 
 extern void ui_but_default_set(struct bContext *C, const bool all, const bool use_afterfunc);
@@ -673,6 +675,7 @@ void ui_draw_preview_item(struct uiFontStyle *fstyle, rcti *rect, const char *na
 void uiStyleInit(void);
 
 /* interface_icons.c */
+void ui_icon_ensure_deferred(const struct bContext *C, const int icon_id, const bool big);
 int ui_id_icon_get(const struct bContext *C, struct ID *id, const bool big);
 
 /* resources.c */
diff --git a/source/blender/editors/interface/interface_templates.c b/source/blender/editors/interface/interface_templates.c
index fe1a1b9096d..9a34e4be980 100644
--- a/source/blender/editors/interface/interface_templates.c
+++ b/source/blender/editors/interface/interface_templates.c
@@ -430,8 +430,8 @@ static void template_ID(
 
 		but = uiDefBlockButN(block, id_search_menu, MEM_dupallocN(template), "", 0, 0, UI_UNIT_X * 6, UI_UNIT_Y * 6,
 		                     TIP_(template_id_browse_tip(type)));
-		but->icon = id ? ui_id_icon_get(C, id, true) : RNA_struct_ui_icon(type);
-		UI_but_flag_enable(but, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
+		ui_def_but_icon(but, id ? ui_id_icon_get(C, id, true) : RNA_struct_ui_icon(type),
+		                UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
 
 		if ((idfrom && idfrom->lib) || !editable)
 			UI_but_flag_enable(but, UI_BUT_DISABLED);
@@ -441,10 +441,9 @@ static void template_ID(
 	else if (flag & UI_ID_BROWSE) {
 		but = uiDefBlockButN(block, id_search_menu, MEM_dupallocN(template), "", 0, 0, UI_UNIT_X * 1.6, UI_UNIT_Y,
 		                     TIP_(template_id_browse_tip(type)));
-		but->icon = RNA_struct_ui_icon(type);
+		ui_def_but_icon(but, RNA_struct_ui_icon(type), UI_HAS_ICON);
 		/* default dragging of icon for id browse buttons */
 		UI_but_drag_set_id(but, id);
-		UI_but_flag_enable(but, UI_HAS_ICON);
 		UI_but_drawflag_enable(but, UI_BUT_ICON_LEFT);
 
 		if ((idfrom && idfrom->lib) || !editable)
@@ -1666,9 +1665,9 @@ static uiBlock *ui_icon_view_menu_cb(bContext *C, ARegion *ar, void *arg_litem)
 		
 		icon = item[a].icon;
 		value = item[a].value;
-		but = uiDefIconButR_prop(block, UI_BTYPE_ROW, 0, icon, x, y, UI_UNIT_X * 5, UI_UNIT_Y * 5,
+		but = uiDefIconButR_prop(block, UI_BTYPE_ROW, 0, ICON_NONE, x, y, UI_UNIT_X * 5, UI_UNIT_Y * 5,
 		                         &cb.ptr, cb.prop, -1, 0, value, -1, -1, NULL);
-		UI_but_flag_enable(but, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
+		ui_def_but_icon(but, icon, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
 	}
 
 	UI_block_bounds_set_normal(block, 0.3f * U.widget_unit);
@@ -1708,8 +1707,7 @@ void uiTemplateIconView(uiLayout *layout, PointerRNA *ptr, const char *propname)
 
 	but = uiDefBlockButN(block, ui_icon_view_menu_cb, cb, "", 0, 0, UI_UNIT_X * 6, UI_UNIT_Y * 6, "");
 
-	but->icon = icon;
-	UI_but_flag_enable(but, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
+	ui_def_but_icon(but, icon, UI_HAS_ICON | UI_BUT_ICON_PREVIEW);
 
 	if (free_items) {
 		MEM_freeN(items);
diff --git a/source/blender/editors/render/render_preview.c b/source/blender/editors/render/render_preview.c
index 65d4c2301a9..5112cd84d09 100644
--- a/source/blender/editors/render/render_preview.c
+++ b/source/blender/editors/render/render_preview.c
@@ -79,7 +79,7 @@
 
 #include "IMB_imbuf.h"
 #include "IMB_imbuf_types.h"
-
+#include "IMB_thumbs.h"
 
 #include "BIF_gl.h"
 #include "BIF_glutil.h"
@@ -128,7 +128,7 @@ ImBuf *get_brush_icon(Brush *brush)
 				}
 
 				if (brush->icon_imbuf)
-					BKE_icon_changed(BKE_icon_getid(&brush->id));
+					BKE_icon_changed(BKE_icon_id_ensure(&brush->id));
 			}
 		}
 	}
@@ -935,65 +935,87 @@ static void set_alpha(char *cp, int sizex, int sizey, char alpha)
 static void icon_preview_startjob(void *customdata, short *stop, short *do_update)
 {
 	ShaderPreview *sp = customdata;
-	ID *id = sp->id;
-	short idtype = GS(id->name);
-	
-	if (idtype == ID_IM) {
-		Image *ima = (Image *)id;
-		ImBuf *ibuf = NULL;
-		ImageUser iuser = {NULL};
 
-		/* ima->ok is zero when Image cannot load */
-		if (ima == NULL || ima->ok == 0)
-			return;
+	if (sp->pr_method == PR_ICON_DEFERRED) {
+		PreviewImage *prv = sp->owner;
+		ImBuf *thumb;
+		char *deferred_data = PRV_DEFERRED_DATA(prv);
+		int source =  deferred_data[0];
+		char *path = &deferred_data[1];
 
-		/* setup dummy image user */
-		iuser.ok = iuser.framenr = 1;
-		iuser.scene = sp->scene;
-		
-		/* elubie: this needs to be changed: here image is always loaded if not
-		 * already there. Very expensive for large images. Need to find a way to 
-		 * only get existing ibuf */
-		ibuf = BKE_image_acquire_ibuf(ima, &iuser, NULL);
-		if (ibuf == NULL || ibuf->rect == NULL) {
-			BKE_image_release_ibuf(ima, ibuf, NULL);
-			return;
-		}
-		
-		icon_copy_rect(ibuf, sp->sizex, sp->sizey, sp->pr_rect);
+//		printf("generating deferred %d×%d preview for %s\n", sp->sizex, sp->sizey, path);
+
+		thumb = IMB_thumb_manage(path, THB_LARGE, source);
 
-		*do_update = true;
+		if (thumb) {
+			/* PreviewImage assumes premultiplied alhpa... */
+			IMB_premultiply_alpha(thumb);
 
-		BKE_image_release_ibuf(ima, ibuf, NULL);
+			icon_copy_rect(thumb, sp->sizex, sp->sizey, sp->pr_rect);
+			IMB_freeImBuf(thumb);
+		}
 	}
-	else if (idtype == ID_BR) {
-		Brush *br = (Brush *)id;
+	else {
+		ID *id = sp->id;
+		short idtype = GS(id->name);
+
+		if (idtype == ID_IM) {
+			Image *ima = (Image *)id;
+			ImBuf *ibuf = NULL;
+			ImageUser iuser = {NULL};
+
+			/* ima->ok is zero when Image cannot load */
+			if (ima == NULL || ima->ok == 0)
+				return;
+
+			/* setup dummy image user */
+			iuser.ok = iuser.framenr = 1;
+			iuser.scene = sp->scene;
+
+			/* elubie: this needs to be changed: here image is always loaded if not
+			 * already there. Very expensive for large images. Need to find a way to
+			 * only get existing ibuf */
+			ibuf = BKE_image_acquire_ibuf(ima, &iuser, NULL);
+			if (ibuf == NULL || ibuf->rect == NULL) {
+				BKE_image_release_ibuf(ima, ibuf, NULL);
+				return;
+			}
 
-		br->icon_imbuf = get_brush_icon(br);
+			icon_copy_rect(ibuf, sp->sizex, sp->sizey, sp->pr_rect);
 
-		memset(sp->pr_rect, 0x88, sp->sizex * sp->sizey * sizeof(unsigned int));
+			*do_update = true;
 
-		if (!(br->icon_imbuf) || !(br->icon_imbuf->rect))
-			return;
+			BKE_image_release_ibuf(ima, ibuf, NULL);
+		}
+		else if (idtype == ID_BR) {
+			Brush *br = (Brush *)id;
 
-		icon_copy_rect(br->icon_imbuf, sp->sizex, sp->sizey, sp->pr_rect);
+			br->icon_imbuf = get_brush_icon(br);
 
-		*do_update = true;
-	}
-	else {
-		/* re-use shader job */
-		shader_preview_startjob(customdata, stop, do_update);
+			memset(sp->pr_rect, 0x88, sp->sizex * sp->sizey * sizeof(unsigned int));
+
+			if (!(br->icon_imbuf) || !(br->icon_imbuf->rect))
+				return;
 
-		/* world is rendered with alpha=0, so it wasn't displayed 
-		 * this could be render option for sky to, for later */
-		if (idtype == ID_WO) {
-			set_alpha((char *)sp->pr_rect, sp->sizex, sp->sizey, 255);
+			icon_copy_rect(br->icon_imbuf, sp->sizex, sp->sizey, sp->pr_rect);
+
+			*do_update = true;
 		}
-		else if (idtype == ID_MA) {
-			Material *ma = (Material *)id;
+		else {
+			/* re-use shader job */
+			shader_preview_startjob(customdata, stop, do_update);
 
-			if (ma->material_type == MA_TYPE_HALO)
+			/* world is rendered with alpha=0, so it wasn't displayed
+			 * this could be render option for sky to, for later */
+			if (idtype == ID_WO) {
 				set_alpha((char *)sp->pr_rect, sp->sizex, sp->sizey, 255);
+			}
+			else if (idtype == ID_MA) {
+				Material *ma = (Material *)id;
+
+				if (ma->material_type == MA_TYPE_HALO)
+					set_alpha((char *)sp->pr_rect, sp->sizex, sp->sizey, 255);
+			}
 		}
 	}
 }
@@ -1005,7 +1027,7 @@ static void common_preview_startjob(void *customdata, short *stop, short *do_upd
 {
 	ShaderPreview *sp = customdata;
 
-	if (sp->pr_method == PR_ICON_RENDER)
+	if (ELEM(sp->pr_method, PR_ICON_RENDER, PR_ICON_DEFERRED))
 		icon_preview_startjob(customdata, stop, do_update);
 	else
 		shader_preview_startjob(customdata, stop, do_update);
@@ -1041,29 +1063,34 @@ static void icon_preview_startjob_all_sizes(void *customdata, short *stop, short
 	const bool use_new_shading = BKE_scene_use_new_shading_nodes(ip->scene);
 
 	while (cur_size) {
+		PreviewImage *prv = ip->owner;
 		ShaderPreview *sp = MEM_callocN(sizeof(ShaderPreview), "Icon ShaderPreview");
+		const bool is_render = !prv->use_deferred;
 
 		/* construct shader preview from image size and previewcustomdata */
 		sp->scene = ip->scene;
 		sp->owner = ip->owner;
 		sp->sizex = cur_size->sizex;
 		sp->sizey = cur_size->sizey;
-		sp->pr_method = PR_ICON_RENDER;
+		sp->pr_method = is_render ? PR_ICON_RENDER : PR_ICON_DEFERRED;
 		sp->pr_rect = cur_size->rect;
 		sp->id = ip->id;
 
-		if (use_new_shading) {
-			/* texture icon rendering is hardcoded to use BI,
-			 * so don't even think of using cycle's bmain for
-			 * texture icons
-			 */
-			if (GS(ip->id->name) != ID_TE)
-				sp->pr_main = G_pr_main_cycles;
-			else
+		if (is_render) {
+			BLI_assert(ip->id);
+			if (use_new_shading) {
+				/* texture icon rendering is hardcoded to use BI,
+				 * so don't even think of using cycle's bmain for
+				 * texture icons
+				 */
+				if (GS(ip->id->name) != ID_TE)
+					sp->pr_main = G_pr_main_cycles;
+				else
+					sp->pr_main = G_pr_main;
+			}
+			else {
 				sp->pr_main = G_pr_main;
-		}
-		else {
-			sp->pr_main = G_pr_main;
+			}
 		}
 
 		common_preview_startjob(sp, stop, do_update, progress);
@@ -1145,7 +1172,7 @@ void ED_preview_icon_job(const bContext *C, void *owner, ID *id, unsigned int *r
 
 	/* customdata for preview thread */
 	ip->scene = CTX_data_scene(C);
-	ip->owner = id;
+	ip->owner = owner;
 	ip->id = id;
 
 	icon_preview_add_size(ip, rect, sizex, sizey);
diff --git a/source/blender/editors/render/render_update.c b/source/blender/editors/render/render_update.c
index d3d5cb12ac8..dedcbb144aa 100644
--- a/source/blender/editors/render/render_update.c
+++ b/source/blender/editors/render/render_update.c
@@ -270,7 +270,7 @@ static void material_changed(Main *bmain, Material *ma)
 	int texture_draw = false;
 
 	/* icons */
-	BKE_icon_changed(BKE_icon_getid(&ma->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&ma->id));
 
 	/* glsl */
 	if (ma->gpumaterial.first)
@@ -285,7 +285,7 @@ static void material_changed(Main *bmain, Material *ma)
 			continue;
 		}
 
-		BKE_icon_changed(BKE_icon_getid(&parent->id));
+		BKE_icon_changed(BKE_icon_id_ensure(&parent->id));
 
 		if (parent->gpumaterial.first)
 			GPU_material_free(&parent->gpumaterial);
@@ -325,7 +325,7 @@ static void lamp_changed(Main *bmain, Lamp *la)
 	Material *ma;
 
 	/* icons */
-	BKE_icon_changed(BKE_icon_getid(&la->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&la->id));
 
 	/* glsl */
 	for (ob = bmain->object.first; ob; ob = ob->id.next)
@@ -361,7 +361,7 @@ static void texture_changed(Main *bmain, Tex *tex)
 	bool texture_draw = false;
 
 	/* icons */
-	BKE_icon_changed(BKE_icon_getid(&tex->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&tex->id));
 
 	/* paint overlays */
 	for (scene = bmain->scene.first; scene; scene = scene->id.next)
@@ -372,7 +372,7 @@ static void texture_changed(Main *bmain, Tex *tex)
 		if (!material_uses_texture(ma, tex))
 			continue;
 
-		BKE_icon_changed(BKE_icon_getid(&ma->id));
+		BKE_icon_changed(BKE_icon_id_ensure(&ma->id));
 
 		if (ma->gpumaterial.first)
 			GPU_material_free(&ma->gpumaterial);
@@ -403,7 +403,7 @@ static void texture_changed(Main *bmain, Tex *tex)
 			continue;
 		}
 
-		BKE_icon_changed(BKE_icon_getid(&wo->id));
+		BKE_icon_changed(BKE_icon_id_ensure(&wo->id));
 		
 		if (wo->gpumaterial.first)
 			GPU_material_free(&wo->gpumaterial);		
@@ -451,7 +451,7 @@ static void world_changed(Main *bmain, World *wo)
 	Material *ma;
 
 	/* icons */
-	BKE_icon_changed(BKE_icon_getid(&wo->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&wo->id));
 	
 	/* glsl */
 	for (ma = bmain->mat.first; ma; ma = ma->id.next)
@@ -470,7 +470,7 @@ static void image_changed(Main *bmain, Image *ima)
 	Tex *tex;
 
 	/* icons */
-	BKE_icon_changed(BKE_icon_getid(&ima->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&ima->id));
 
 	/* textures */
 	for (tex = bmain->tex.first; tex; tex = tex->id.next)
diff --git a/source/blender/editors/space_image/image_ops.c b/source/blender/editors/space_image/image_ops.c
index 75e6f77ba49..ddb1cec9180 100644
--- a/source/blender/editors/space_image/image_ops.c
+++ b/source/blender/editors/space_image/image_ops.c
@@ -1346,7 +1346,7 @@ static int image_replace_exec(bContext *C, wmOperator *op)
 	/* XXX unpackImage frees image buffers */
 	ED_preview_kill_jobs(CTX_wm_manager(C), CTX_data_main(C));
 	
-	BKE_icon_changed(BKE_icon_getid(&sima->image->id));
+	BKE_icon_changed(BKE_icon_id_ensure(&sima->image->id));
 	BKE_image_signal(sima->image, &sima->iuser, IMA_SIGNAL_RELOAD);
 	WM_event_add_notifier(C, NC_IMAGE | NA_EDITED, sima->image);
 
diff --git a/source/blender/imbuf/IMB_thumbs.h b/source/blender/imbuf/IMB_thumbs.h
index f977a633c57..2cb56832a00 100644
--- a/source/blender/imbuf/IMB_thumbs.h
+++ b/source/blender/imbuf/IMB_thumbs.h
@@ -61,6 +61,8 @@ typedef enum ThumbSource {
 /* don't generate thumbs for images bigger then this (100mb) */
 #define THUMB_SIZE_MAX (100 * 1024 * 1024)
 
+#define PREVIEW_RENDER_DEFAULT_HEIGHT 128
+
 /* create thumbnail for file and returns new imbuf for thumbnail */
 ImBuf *IMB_thumb_create(const char *path, ThumbSize size, ThumbSource source, ImBuf *ibuf);
 
diff --git a/source/blender/imbuf/intern/thumbs.c b/source/blender/imbuf/intern/thumbs.c
index a608aa0db34..626f02e8ade 100644
--- a/source/blender/imbuf/intern/thumbs.c
+++ b/source/blender/imbuf/intern/thumbs.c
@@ -314,10 +314,10 @@ static ImBuf *thumb_create_ex(
 
 	switch (size) {
 		case THB_NORMAL:
-			tsize = 128;
+			tsize = PREVIEW_RENDER_DEFAULT_HEIGHT;
 			break;
 		case THB_LARGE:
-			tsize = 256;
+			tsize = PREVIEW_RENDER_DEFAULT_HEIGHT * 2;
 			break;
 		case THB_FAIL:
 			tsize = 1;
diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index b51b53cc801..f68de03614b 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -155,20 +155,36 @@ typedef struct Library {
 
 enum eIconSizes {
 	ICON_SIZE_ICON = 0,
-	ICON_SIZE_PREVIEW = 1
+	ICON_SIZE_PREVIEW = 1,
+
+	NUM_ICON_SIZES
+};
+
+/* for PreviewImage->flag */
+enum ePreviewImage_Flag {
+	PRV_CHANGED          = (1 << 0),
+	PRV_USER_EDITED      = (1 << 1),  /* if user-edited, do not auto-update this anymore! */
 };
-#define NUM_ICON_SIZES (ICON_SIZE_PREVIEW + 1)
 
 typedef struct PreviewImage {
 	/* All values of 2 are really NUM_ICON_SIZES */
 	unsigned int w[2];
 	unsigned int h[2];
-	short changed[2];
+	short flag[2];
 	short changed_timestamp[2];
 	unsigned int *rect[2];
+
+	/* Runtime-only data. */
 	struct GPUTexture *gputexture[2];
+	int icon_id;  /* Used by previews outside of ID context. */
+
+	char pad[3];
+	char use_deferred;  /* for now a mere bool, if we add more deferred loading methods we can switch to bitflag. */
 } PreviewImage;
 
+#define PRV_DEFERRED_DATA(prv) \
+	(CHECK_TYPE_INLINE(prv, PreviewImage *), BLI_assert((prv)->use_deferred), (void *)((prv) + 1))
+
 /**
  * Defines for working with IDs.
  *
diff --git a/source/blender/makesrna/RNA_access.h b/source/blender/makesrna/RNA_access.h
index cff9e424f31..965d1d91614 100644
--- a/source/blender/makesrna/RNA_access.h
+++ b/source/blender/makesrna/RNA_access.h
@@ -280,6 +280,7 @@ extern StructRNA RNA_IKParam;
 extern StructRNA RNA_Image;
 extern StructRNA RNA_ImageFormatSettings;
 extern StructRNA RNA_ImagePaint;
+extern StructRNA RNA_ImagePreview;
 extern StructRNA RNA_ImageSequence;
 extern StructRNA RNA_ImageTexture;
 extern StructRNA RNA_ImageUser;
diff --git a/source/blender/makesrna/intern/rna_ID.c b/source/blender/makesrna/intern/rna_ID.c
index db25786c599..b87b455b36f 100644
--- a/source/blender/makesrna/intern/rna_ID.c
+++ b/source/blender/makesrna/intern/rna_ID.c
@@ -34,6 +34,8 @@
 
 #include "BLI_utildefines.h"
 
+#include "BKE_icons.h"
+
 #include "RNA_access.h"
 #include "RNA_define.h"
 #include "RNA_enum_types.h"
@@ -406,6 +408,183 @@ static void rna_Library_filepath_set(PointerRNA *ptr, const char *value)
 	BKE_library_filepath_set(lib, value);
 }
 
+/* ***** ImagePreview ***** */
+
+static void rna_ImagePreview_is_custom_set(PointerRNA *ptr, int value, enum eIconSizes size)
+{
+	ID *id = ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	if ((value && (prv_img->flag[size] & PRV_USER_EDITED)) || (!value && !(prv_img->flag[size] & PRV_USER_EDITED))) {
+		return;
+	}
+
+	if (value)
+		prv_img->flag[size] |= PRV_USER_EDITED;
+	else
+		prv_img->flag[size] &= ~PRV_USER_EDITED;
+
+	prv_img->flag[size] |= PRV_CHANGED;
+
+	BKE_previewimg_clear_single(prv_img, size);
+}
+
+static void rna_ImagePreview_size_get(PointerRNA *ptr, int *values, enum eIconSizes size)
+{
+	ID *id = (ID *)ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	BKE_previewimg_ensure(prv_img, size);
+
+	values[0] = prv_img->w[size];
+	values[1] = prv_img->h[size];
+}
+
+static void rna_ImagePreview_size_set(PointerRNA *ptr, const int *values, enum eIconSizes size)
+{
+	ID *id = (ID *)ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	BKE_previewimg_ensure(prv_img, size);
+
+	if (values[0] && values[1]) {
+		prv_img->rect[size] = MEM_callocN(values[0] * values[1] * sizeof(unsigned int), "prv_rect");
+	}
+
+	prv_img->w[size] = values[0];
+	prv_img->h[size] = values[1];
+
+	prv_img->flag[size] |= (PRV_CHANGED | PRV_USER_EDITED);
+}
+
+static int rna_ImagePreview_pixels_get_length(PointerRNA *ptr, int length[RNA_MAX_ARRAY_DIMENSION], enum eIconSizes size)
+{
+	ID *id = ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	BKE_previewimg_ensure(prv_img, size);
+
+	length[0] = prv_img->w[size] * prv_img->h[size];
+
+	return length[0];
+}
+
+static void rna_ImagePreview_pixels_get(PointerRNA *ptr, int *values, enum eIconSizes size)
+{
+	ID *id = ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	BKE_previewimg_ensure(prv_img, size);
+
+	memcpy(values, prv_img->rect[size], prv_img->w[size] * prv_img->h[size] * sizeof(unsigned int));
+}
+
+static void rna_ImagePreview_pixels_set(PointerRNA *ptr, const int *values, enum eIconSizes size)
+{
+	ID *id = ptr->id.data;
+	PreviewImage *prv_img = (PreviewImage *)ptr->data;
+
+	if (id != NULL) {
+		BLI_assert(prv_img == BKE_previewimg_id_ensure(id));
+	}
+
+	memcpy(prv_img->rect[size], values, prv_img->w[size] * prv_img->h[size] * sizeof(unsigned int));
+	prv_img->flag[size] |= PRV_USER_EDITED;
+}
+
+static void rna_ImagePreview_is_image_custom_set(PointerRNA *ptr, int value)
+{
+	rna_ImagePreview_is_custom_set(ptr, value, ICON_SIZE_PREVIEW);
+}
+
+static void rna_ImagePreview_image_size_get(PointerRNA *ptr, int *values)
+{
+	rna_ImagePreview_size_get(ptr, values, ICON_SIZE_PREVIEW);
+}
+
+static void rna_ImagePreview_image_size_set(PointerRNA *ptr, const int *values)
+{
+	rna_ImagePreview_size_set(ptr, values, ICON_SIZE_PREVIEW);
+}
+
+static int rna_ImagePreview_image_pixels_get_length(PointerRNA *ptr, int length[RNA_MAX_ARRAY_DIMENSION])
+{
+	return rna_ImagePreview_pixels_get_length(ptr, length, ICON_SIZE_PREVIEW);
+}
+
+static void rna_ImagePreview_image_pixels_get(PointerRNA *ptr, int *values)
+{
+	rna_ImagePreview_pixels_get(ptr, values, ICON_SIZE_PREVIEW);
+}
+
+static void rna_ImagePreview_image_pixels_set(PointerRNA *ptr, const int *values)
+{
+	rna_ImagePreview_pixels_set(ptr, values, ICON_SIZE_PREVIEW);
+}
+
+static void rna_ImagePreview_is_icon_custom_set(PointerRNA *ptr, int value)
+{
+	rna_ImagePreview_is_custom_set(ptr, value, ICON_SIZE_ICON);
+}
+
+static void rna_ImagePreview_icon_size_get(PointerRNA *ptr, int *values)
+{
+	rna_ImagePreview_size_get(ptr, values, ICON_SIZE_ICON);
+}
+
+static void rna_ImagePreview_icon_size_set(PointerRNA *ptr, const int *values)
+{
+	rna_ImagePreview_size_set(ptr, values, ICON_SIZE_ICON);
+}
+
+static int rna_ImagePreview_icon_pixels_get_length(PointerRNA *ptr, int length[RNA_MAX_ARRAY_DIMENSION])
+{
+	return rna_ImagePreview_pixels_get_length(ptr, length, ICON_SIZE_ICON);
+}
+
+static void rna_ImagePreview_icon_pixels_get(PointerRNA *ptr, int *values)
+{
+	rna_ImagePreview_pixels_get(ptr, values, ICON_SIZE_ICON);
+}
+
+static void rna_ImagePreview_icon_pixels_set(PointerRNA *ptr, const int *values)
+{
+	rna_ImagePreview_pixels_set(ptr, values, ICON_SIZE_ICON);
+}
+
+static int rna_ImagePreview_icon_id_get(PointerRNA *ptr)
+{
+	/* Using a callback here allows us to only generate icon matching that preview when icon_id is requested. */
+	return BKE_icon_preview_ensure((PreviewImage *)(ptr->data));
+}
+static void rna_ImagePreview_icon_reload(PreviewImage *prv)
+{
+	/* will lazy load on next use, but only in case icon is not user-modified! */
+	if (!(prv->flag[ICON_SIZE_ICON] & PRV_USER_EDITED) && !(prv->flag[ICON_SIZE_PREVIEW] & PRV_USER_EDITED)) {
+		BKE_previewimg_clear(prv);
+	}
+}
+
 #else
 
 static void rna_def_ID_properties(BlenderRNA *brna)
@@ -524,6 +703,62 @@ static void rna_def_ID_materials(BlenderRNA *brna)
 	RNA_def_boolean(func, "update_data", 0, "", "Update data by re-adjusting the material slots assigned");
 }
 
+static void rna_def_image_preview(BlenderRNA *brna)
+{
+	StructRNA *srna;
+	FunctionRNA *func;
+	PropertyRNA *prop;
+
+	srna = RNA_def_struct(brna, "ImagePreview", NULL);
+	RNA_def_struct_sdna(srna, "PreviewImage");
+	RNA_def_struct_ui_text(srna, "Image Preview", "Preview image and icon");
+
+	prop = RNA_def_property(srna, "is_image_custom", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag[ICON_SIZE_PREVIEW]", PRV_USER_EDITED);
+	RNA_def_property_boolean_funcs(prop, NULL, "rna_ImagePreview_is_image_custom_set");
+	RNA_def_property_ui_text(prop, "Custom Image", "True if this preview image has been modified by py script,"
+	                         "and is no more auto-generated by Blender");
+
+	prop = RNA_def_int_vector(srna, "image_size", 2, NULL, 0, 0, "Image Size",
+	                          "Width and height in pixels", 0, 0);
+	RNA_def_property_subtype(prop, PROP_PIXEL);
+	RNA_def_property_int_funcs(prop, "rna_ImagePreview_image_size_get", "rna_ImagePreview_image_size_set", NULL);
+
+	prop = RNA_def_property(srna, "image_pixels", PROP_INT, PROP_NONE);
+	RNA_def_property_flag(prop, PROP_DYNAMIC);
+	RNA_def_property_multi_array(prop, 1, NULL);
+	RNA_def_property_ui_text(prop, "Image Pixels", "Image pixels, as bytes (always RGBA 32bits)");
+	RNA_def_property_dynamic_array_funcs(prop, "rna_ImagePreview_image_pixels_get_length");
+	RNA_def_property_int_funcs(prop, "rna_ImagePreview_image_pixels_get", "rna_ImagePreview_image_pixels_set", NULL);
+
+
+	prop = RNA_def_property(srna, "is_icon_custom", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag[ICON_SIZE_ICON]", PRV_USER_EDITED);
+	RNA_def_property_boolean_funcs(prop, NULL, "rna_ImagePreview_is_icon_custom_set");
+	RNA_def_property_ui_text(prop, "Custom Icon", "True if this preview icon has been modified by py script,"
+	                         "and is no more auto-generated by Blender");
+
+	prop = RNA_def_int_vector(srna, "icon_size", 2, NULL, 0, 0, "Icon Size",
+	                          "Width and height in pixels", 0, 0);
+	RNA_def_property_subtype(prop, PROP_PIXEL);
+	RNA_def_property_int_funcs(prop, "rna_ImagePreview_icon_size_get", "rna_ImagePreview_icon_size_set", NULL);
+
+	prop = RNA_def_property(srna, "icon_pixels", PROP_INT, PROP_NONE);
+	RNA_def_property_flag(prop, PROP_DYNAMIC);
+	RNA_def_property_multi_array(prop, 1, NULL);
+	RNA_def_property_ui_text(prop, "Icon Pixels", "Icon pixels, as bytes (always RGBA 32bits)");
+	RNA_def_property_dynamic_array_funcs(prop, "rna_ImagePreview_icon_pixels_get_length");
+	RNA_def_property_int_funcs(prop, "rna_ImagePreview_icon_pixels_get", "rna_ImagePreview_icon_pixels_set", NULL);
+
+	prop = RNA_def_int(srna, "icon_id", 0, INT_MIN, INT_MAX, "Icon ID",
+	                   "Unique integer identifying this preview as an icon (zero means invalid)", INT_MIN, INT_MAX);
+	RNA_def_property_clear_flag(prop, PROP_EDITABLE);
+	RNA_def_property_int_funcs(prop, "rna_ImagePreview_icon_id_get", NULL, NULL);
+
+	func = RNA_def_function(srna, "reload", "rna_ImagePreview_icon_reload");
+	RNA_def_function_ui_description(func, "Reload the preview from its source path");
+}
+
 static void rna_def_ID(BlenderRNA *brna)
 {
 	StructRNA *srna;
@@ -651,6 +886,7 @@ void RNA_def_ID(BlenderRNA *brna)
 	RNA_def_struct_ui_text(srna, "Any Type", "RNA type used for pointers to any possible data");
 
 	rna_def_ID(brna);
+	rna_def_image_preview(brna);
 	rna_def_ID_properties(brna);
 	rna_def_ID_materials(brna);
 	rna_def_library(brna);
diff --git a/source/blender/makesrna/intern/rna_brush.c b/source/blender/makesrna/intern/rna_brush.c
index 362cf77a332..e9e4282772d 100644
--- a/source/blender/makesrna/intern/rna_brush.c
+++ b/source/blender/makesrna/intern/rna_brush.c
@@ -342,8 +342,8 @@ static void rna_Brush_reset_icon(Brush *br, const char *UNUSED(type))
 		return;
 
 	if (id->icon_id >= BIFICONID_LAST) {
-		BKE_icon_delete(id);
-		BKE_previewimg_free_id(id);
+		BKE_icon_id_delete(id);
+		BKE_previewimg_id_free(id);
 	}
 
 	id->icon_id = 0;
@@ -415,8 +415,8 @@ static void rna_Brush_icon_update(Main *UNUSED(bmain), Scene *UNUSED(scene), Poi
 	br->id.icon_id = 0;
 
 	if (br->flag & BRUSH_CUSTOM_ICON) {
-		BKE_previewimg_get(&br->id);
-		BKE_icon_changed(BKE_icon_getid(&br->id));
+		BKE_previewimg_id_ensure(&br->id);
+		BKE_icon_changed(BKE_icon_id_ensure(&br->id));
 	}
 
 	WM_main_add_notifier(NC_BRUSH | NA_EDITED, br);
diff --git a/source/blender/makesrna/intern/rna_main_api.c b/source/blender/makesrna/intern/rna_main_api.c
index 168f23ceaa1..8b5ed66e217 100644
--- a/source/blender/makesrna/intern/rna_main_api.c
+++ b/source/blender/makesrna/intern/rna_main_api.c
@@ -35,6 +35,7 @@
 
 #include "DNA_ID.h"
 #include "DNA_modifier_types.h"
+#include "DNA_space_types.h"
 
 #include "BLI_utildefines.h"
 #include "BLI_path_util.h"
@@ -58,6 +59,7 @@
 #include "BKE_library.h"
 #include "BKE_object.h"
 #include "BKE_material.h"
+#include "BKE_icons.h"
 #include "BKE_image.h"
 #include "BKE_texture.h"
 #include "BKE_scene.h"
@@ -807,11 +809,12 @@ static int rna_Main_linestyle_is_updated_get(PointerRNA *ptr) { return DAG_id_ty
 
 #else
 
-void RNA_api_main(StructRNA *srna)
+void RNA_api_main(StructRNA *UNUSED(srna))
 {
 #if 0
 	FunctionRNA *func;
 	PropertyRNA *parm;
+
 	/* maybe we want to add functions in 'bpy.data' still?
 	 * for now they are all in collections bpy.data.images.new(...) */
 	func = RNA_def_function(srna, "add_image", "rna_Main_add_image");
@@ -820,8 +823,6 @@ void RNA_api_main(StructRNA *srna)
 	RNA_def_property_flag(parm, PROP_REQUIRED);
 	parm = RNA_def_pointer(func, "image", "Image", "", "New image");
 	RNA_def_function_return(func, parm);
-#else
-	(void)srna;
 #endif
 }
 
diff --git a/source/blender/python/intern/CMakeLists.txt b/source/blender/python/intern/CMakeLists.txt
index 8296027f044..ed04152182e 100644
--- a/source/blender/python/intern/CMakeLists.txt
+++ b/source/blender/python/intern/CMakeLists.txt
@@ -31,6 +31,7 @@ set(INC
 	../../blenloader
 	../../editors/include
 	../../gpu
+	../../imbuf
 	../../makesdna
 	../../makesrna
 	../../windowmanager
@@ -69,6 +70,7 @@ set(SRC
 	bpy_rna_callback.c
 	bpy_traceback.c
 	bpy_util.c
+	bpy_utils_previews.c
 	bpy_utils_units.c
 	stubs.c
 
@@ -94,6 +96,7 @@ set(SRC
 	bpy_rna_callback.h
 	bpy_traceback.h
 	bpy_util.h
+	bpy_utils_previews.h
 	bpy_utils_units.h
 	../BPY_extern.h
 )
diff --git a/source/blender/python/intern/bpy.c b/source/blender/python/intern/bpy.c
index ec3c017a7ed..9a5e488850e 100644
--- a/source/blender/python/intern/bpy.c
+++ b/source/blender/python/intern/bpy.c
@@ -48,6 +48,7 @@
 #include "bpy_props.h"
 #include "bpy_library.h"
 #include "bpy_operator.h"
+#include "bpy_utils_previews.h"
 #include "bpy_utils_units.h"
 
 #include "../generic/py_capi_utils.h"
@@ -330,6 +331,7 @@ void BPy_init_modules(void)
 	PyModule_AddObject(mod, "ops", BPY_operator_module());
 	PyModule_AddObject(mod, "app", BPY_app_struct());
 	PyModule_AddObject(mod, "_utils_units", BPY_utils_units());
+	PyModule_AddObject(mod, "_utils_previews", BPY_utils_previews_module());
 
 	/* bpy context */
 	RNA_pointer_create(NULL, &RNA_Context, (void *)BPy_GetContext(), &ctx_ptr);
diff --git a/source/blender/python/intern/bpy_utils_previews.c b/source/blender/python/intern/bpy_utils_previews.c
new file mode 100644
index 00000000000..5509ff50f5f
--- /dev/null
+++ b/source/blender/python/intern/bpy_utils_previews.c
@@ -0,0 +1,188 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * Contributor(s): Bastien Montagne
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/python/intern/bpy_utils_previews.c
+ *  \ingroup pythonintern
+ *
+ * This file defines a singleton py object accessed via 'bpy.utils.previews',
+ * which exposes low-level API for custom previews/icons.
+ * It is replaced in final API by an higher-level python wrapper, that handles previews by addon,
+ * and automatically release them on deletion.
+ */
+
+#include <Python.h>
+#include <structmember.h>
+
+#include "BLI_utildefines.h"
+
+#include "RNA_types.h"
+#include "RNA_access.h"
+
+#include "BPY_extern.h"
+#include "bpy_utils_previews.h"
+#include "bpy_rna.h"
+
+#include "MEM_guardedalloc.h"
+
+#include "IMB_imbuf.h"
+#include "IMB_imbuf_types.h"
+#include "IMB_thumbs.h"
+
+#include "BKE_icons.h"
+
+#include "DNA_ID.h"
+
+#include "../generic/python_utildefines.h"
+
+#define STR_SOURCE_TYPES "'IMAGE', 'MOVIE', 'BLEND', 'FONT'"
+
+PyDoc_STRVAR(bpy_utils_previews_new_doc,
+".. method:: new(name)\n"
+"\n"
+"   Generate a new empty preview, or return existing one matching ``name``.\n"
+"\n"
+"   :arg name: The name (unique id) identifying the preview.\n"
+"   :type name: string\n"
+"   :return: The Preview matching given name, or a new empty one.\n"
+"   :rtype: :class:`bpy.types.ImagePreview`\n"
+);
+static PyObject *bpy_utils_previews_new(PyObject *UNUSED(self), PyObject *args)
+{
+	char *name;
+	PreviewImage *prv;
+	PointerRNA ptr;
+
+	if (!PyArg_ParseTuple(args, "s:new", &name)) {
+		return NULL;
+	}
+
+	prv = BKE_previewimg_cached_ensure(name);
+	RNA_pointer_create(NULL, &RNA_ImagePreview, prv, &ptr);
+
+	return pyrna_struct_CreatePyObject(&ptr);
+}
+
+PyDoc_STRVAR(bpy_utils_previews_load_doc,
+".. method:: load(name, path, path_type, force_reload)\n"
+"\n"
+"   Generate a new preview from given file path, or return existing one matching ``name``.\n"
+"\n"
+"   :arg name: The name (unique id) identifying the preview.\n"
+"   :type name: string\n"
+"   :arg path: The file path to generate the preview from.\n"
+"   :type path: string\n"
+"   :arg path_type: The type of file, needed to generate the preview in [" STR_SOURCE_TYPES "].\n"
+"   :type path_type: string\n"
+"   :arg force_reload: If True, force running thumbnail manager even if preview already exists in cache.\n"
+"   :type force_reload: bool\n"
+"   :return: The Preview matching given name, or a new empty one.\n"
+"   :rtype: :class:`bpy.types.ImagePreview`\n"
+);
+static PyObject *bpy_utils_previews_load(PyObject *UNUSED(self), PyObject *args)
+{
+	char *name, *path, *path_type_s;
+	int path_type, force_reload = false;
+
+	PreviewImage *prv;
+	PointerRNA ptr;
+
+	if (!PyArg_ParseTuple( args, "sss|p:load", &name, &path, &path_type_s, &force_reload)) {
+		return NULL;
+	}
+
+	if (STREQ(path_type_s, "IMAGE")) {
+		path_type = THB_SOURCE_IMAGE;
+	}
+	else if (STREQ(path_type_s, "MOVIE")) {
+		path_type = THB_SOURCE_MOVIE;
+	}
+	else if (STREQ(path_type_s, "BLEND")) {
+		path_type = THB_SOURCE_BLEND;
+	}
+	else if (STREQ(path_type_s, "FONT")) {
+		path_type = THB_SOURCE_FONT;
+	}
+	else {
+		PyErr_Format(PyExc_ValueError,
+		             "load: invalid '%' path type, only [" STR_SOURCE_TYPES "] "
+		             "are supported", path_type_s);
+		return NULL;
+	}
+
+	prv = BKE_previewimg_cached_thumbnail_read(name, path, path_type, force_reload);
+	RNA_pointer_create(NULL, &RNA_ImagePreview, prv, &ptr);
+
+	return pyrna_struct_CreatePyObject(&ptr);
+}
+
+PyDoc_STRVAR(bpy_utils_previews_release_doc,
+".. method:: release(name)\n"
+"\n"
+"   Release (free) a previously created preview.\n"
+"\n"
+"\n"
+"   :arg name: The name (unique id) identifying the preview.\n"
+"   :type name: string\n"
+);
+static PyObject *bpy_utils_previews_release(PyObject *UNUSED(self), PyObject *args)
+{
+	char *name;
+
+	if (!PyArg_ParseTuple(args, "s:release", &name)) {
+		return NULL;
+	}
+
+	BKE_previewimg_cached_release(name);
+
+	Py_RETURN_NONE;
+}
+
+static struct PyMethodDef bpy_utils_previews_methods[] = {
+	/* Can't use METH_KEYWORDS alone, see http://bugs.python.org/issue11587 */
+	{"new", (PyCFunction)bpy_utils_previews_new, METH_VARARGS, bpy_utils_previews_new_doc},
+	{"load", (PyCFunction)bpy_utils_previews_load, METH_VARARGS, bpy_utils_previews_load_doc},
+	{"release", (PyCFunction)bpy_utils_previews_release, METH_VARARGS, bpy_utils_previews_release_doc},
+	{NULL, NULL, 0, NULL}
+};
+
+PyDoc_STRVAR(bpy_utils_previews_doc,
+"This object contains basic static methods to handle cached (non-ID) previews in Blender\n"
+"(low-level API, not exposed to final users)."
+);
+static struct PyModuleDef bpy_utils_previews_module = {
+	PyModuleDef_HEAD_INIT,
+	"bpy._utils_previews",
+	bpy_utils_previews_doc,
+	0,
+	bpy_utils_previews_methods,
+	NULL, NULL, NULL, NULL
+};
+
+
+PyObject *BPY_utils_previews_module(void)
+{
+	PyObject *submodule;
+
+	submodule = PyModule_Create(&bpy_utils_previews_module);
+
+	return submodule;
+}
diff --git a/source/blender/python/intern/bpy_utils_previews.h b/source/blender/python/intern/bpy_utils_previews.h
new file mode 100644
index 00000000000..3d7ade04b9f
--- /dev/null
+++ b/source/blender/python/intern/bpy_utils_previews.h
@@ -0,0 +1,32 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * Contributor(s): Bastien Montagne
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/python/intern/bpy_utils_previews.h
+ *  \ingroup pythonintern
+ */
+
+#ifndef __BPY_UTILS_PREVIEWS_H__
+#define __BPY_UTILS_PREVIEWS_H__
+
+PyObject *BPY_utils_previews_module(void);
+
+#endif /* __BPY_UTILS_PREVIEWS_H__ */
diff --git a/source/blender/windowmanager/intern/wm_init_exit.c b/source/blender/windowmanager/intern/wm_init_exit.c
index b5c267e8a39..77d3e3762a8 100644
--- a/source/blender/windowmanager/intern/wm_init_exit.c
+++ b/source/blender/windowmanager/intern/wm_init_exit.c
@@ -172,6 +172,17 @@ void WM_init(bContext *C, int argc, const char **argv)
 	
 	BLF_lang_set(NULL);
 
+	if (!G.background) {
+		GPU_init();
+
+		GPU_set_mipmap(!(U.gameflags & USER_DISABLE_MIPMAP));
+		GPU_set_linear_mipmap(true);
+		GPU_set_anisotropic(U.anisotropic_filter);
+		GPU_set_gpu_mipmapping(U.use_gpu_mipmap);
+
+		UI_init();
+	}
+
 	ED_spacemacros_init();
 
 	/* note: there is a bug where python needs initializing before loading the
@@ -197,17 +208,6 @@ void WM_init(bContext *C, int argc, const char **argv)
 
 	wm_init_reports(C); /* reports cant be initialized before the wm */
 
-	if (!G.background) {
-		GPU_init();
-
-		GPU_set_mipmap(!(U.gameflags & USER_DISABLE_MIPMAP));
-		GPU_set_linear_mipmap(true);
-		GPU_set_anisotropic(U.anisotropic_filter);
-		GPU_set_gpu_mipmapping(U.use_gpu_mipmap);
-
-		UI_init();
-	}
-	
 	clear_matcopybuf();
 	ED_render_clear_mtex_copybuf();