Make .blend file thumbnail reading simpler and more coherent, read/store them when reading in background mode.

Primary goal of this commit is to fix an annoying issue - when processing and saving .blend
files in background mode you lose their thumbnails, since it can only be generated with
an OpenGL context.

Solution to that is to read .blend thumbnail while reading .blend file (only done in background
mode currently), and store it in Main struct.

Also, this lead to removing .blend file reading code from thumb_blend (no need to have doublons).
We now have a small interface in regular reading code area, which keeps it reasonbaly light
by only reading/parsing header info, and first few BHead blocks.

This makes code reading .blend thumbnail about 3 to 4 times slower than previous highly specialized
one in blend_thumb.c, but overall thumbnail generation of a big .blend files folder only grows
of about 1%, think we can bare with it.

Finally, since thumbnail is now optionally stored in Main struct, it makes it easy to allow user
to define their own custom one (instead of auto-generated one). RNA API for this was not added though,
accessing that kind of .blend meta-data has to be rethought a bit on a bigger level first.

Reviewers: sergey, campbellbarton

Subscribers: Severin, psy-fi

Differential Revision: https://developer.blender.org/D1469

10 files changed, 270 insertions, 132 deletions

diff --git a/source/blender/blenkernel/BKE_library.h b/source/blender/blenkernel/BKE_library.h
index 6ecc955e26c..5b12858fc7d 100644
--- a/source/blender/blenkernel/BKE_library.h
+++ b/source/blender/blenkernel/BKE_library.h
@@ -38,8 +38,10 @@ extern "C" {
 
 #include "BLI_compiler_attrs.h"
 
+struct BlendThumbnail;
 struct ListBase;
 struct ID;
+struct ImBuf;
 struct Main;
 struct Library;
 struct wmWindowManager;
@@ -87,6 +89,10 @@ void BKE_main_free(struct Main *mainvar);
 void BKE_main_lock(struct Main *bmain);
 void BKE_main_unlock(struct Main *bmain);
 
+struct BlendThumbnail *BKE_main_thumbnail_from_imbuf(struct Main *bmain, struct ImBuf *img);
+struct ImBuf *BKE_main_thumbnail_to_imbuf(struct Main *bmain, struct BlendThumbnail *data);
+void BKE_main_thumbnail_create(struct Main *bmain);
+
 void BKE_main_id_tag_idcode(struct Main *mainvar, const short type, const bool tag);
 void BKE_main_id_tag_listbase(struct ListBase *lb, const bool tag);
 void BKE_main_id_tag_all(struct Main *mainvar, const bool tag);
diff --git a/source/blender/blenkernel/BKE_main.h b/source/blender/blenkernel/BKE_main.h
index ec654ea4b71..6a00961fbb3 100644
--- a/source/blender/blenkernel/BKE_main.h
+++ b/source/blender/blenkernel/BKE_main.h
@@ -50,6 +50,13 @@ struct EvaluationContext;
 struct Library;
 struct MainLock;
 
+/* Blender thumbnail, as written on file (width, height, and data as char RGBA). */
+/* We pack pixel data after that struct. */
+typedef struct BlendThumbnail {
+	int width, height;
+	char rect[0];
+} BlendThumbnail;
+
 typedef struct Main {
 	struct Main *next, *prev;
 	char name[1024]; /* 1024 = FILE_MAX */
@@ -58,6 +65,8 @@ typedef struct Main {
 	uint64_t build_commit_timestamp; /* commit's timestamp from buildinfo */
 	char build_hash[16];  /* hash from buildinfo */
 	short recovered;	/* indicate the main->name (file) is the recovered one */
+
+	BlendThumbnail *blen_thumb;
 	
 	struct Library *curlib;
 	ListBase scene;
@@ -109,7 +118,10 @@ typedef struct Main {
 #define MAIN_VERSION_OLDER(main, ver, subver) \
 	((main)->versionfile < (ver) || (main->versionfile == (ver) && (main)->subversionfile < (subver)))
 
-	
+#define BLEN_THUMB_SIZE 128
+
+#define BLEN_THUMB_MEMSIZE(_x, _y) (sizeof(BlendThumbnail) + (size_t)((_x) * (_y)) * sizeof(int))
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/source/blender/blenkernel/intern/library.c b/source/blender/blenkernel/intern/library.c
index 8bb2864a604..1466cdfa921 100644
--- a/source/blender/blenkernel/intern/library.c
+++ b/source/blender/blenkernel/intern/library.c
@@ -119,6 +119,9 @@
 
 #include "RNA_access.h"
 
+#include "IMB_imbuf.h"
+#include "IMB_imbuf_types.h"
+
 #ifdef WITH_PYTHON
 #include "BPY_extern.h"
 #endif
@@ -1097,6 +1100,8 @@ void BKE_main_free(Main *mainvar)
 	ListBase *lbarray[MAX_LIBARRAY];
 	int a;
 
+	MEM_SAFE_FREE(mainvar->blen_thumb);
+
 	a = set_listbasepointers(mainvar, lbarray);
 	while (a--) {
 		ListBase *lb = lbarray[a];
@@ -1166,6 +1171,74 @@ void BKE_main_unlock(struct Main *bmain)
 	BLI_spin_unlock((SpinLock *) bmain->lock);
 }
 
+/**
+ * Generates a raw .blend file thumbnail data from given image.
+ *
+ * \param bmain If not NULL, also store generated data in this Main.
+ * \param img ImBuf image to generate thumbnail data from.
+ * \return The generated .blend file raw thumbnail data.
+ */
+BlendThumbnail *BKE_main_thumbnail_from_imbuf(Main *bmain, ImBuf *img)
+{
+	BlendThumbnail *data = NULL;
+
+	if (bmain) {
+		MEM_SAFE_FREE(bmain->blen_thumb);
+	}
+
+	if (img) {
+		const size_t sz = BLEN_THUMB_MEMSIZE(img->x, img->y);
+		data = MEM_mallocN(sz, __func__);
+
+		IMB_rect_from_float(img);  /* Just in case... */
+		data->width = img->x;
+		data->height = img->y;
+		memcpy(data->rect, img->rect, sz - sizeof(*data));
+	}
+
+	if (bmain) {
+		bmain->blen_thumb = data;
+	}
+	return data;
+}
+
+/**
+ * Generates an image from raw .blend file thumbnail \a data.
+ *
+ * \param bmain Use this bmain->blen_thumb data if given \a data is NULL.
+ * \param data Raw .blend file thumbnail data.
+ * \return An ImBuf from given data, or NULL if invalid.
+ */
+ImBuf *BKE_main_thumbnail_to_imbuf(Main *bmain, BlendThumbnail *data)
+{
+	ImBuf *img = NULL;
+
+	if (!data && bmain) {
+		data = bmain->blen_thumb;
+	}
+
+	if (data) {
+		/* Note: we cannot use IMB_allocFromBuffer(), since it tries to dupalloc passed buffer, which will fail
+		 *       here (we do not want to pass the first two ints!). */
+		img = IMB_allocImBuf((unsigned int)data->width, (unsigned int)data->height, 32, IB_rect | IB_metadata);
+		memcpy(img->rect, data->rect, BLEN_THUMB_MEMSIZE(data->width, data->height) - sizeof(*data));
+	}
+
+	return img;
+}
+
+/**
+ * Generates an empty (black) thumbnail for given Main.
+ */
+void BKE_main_thumbnail_create(struct Main *bmain)
+{
+	MEM_SAFE_FREE(bmain->blen_thumb);
+
+	bmain->blen_thumb = MEM_callocN(BLEN_THUMB_MEMSIZE(BLEN_THUMB_SIZE, BLEN_THUMB_SIZE), __func__);
+	bmain->blen_thumb->width = BLEN_THUMB_SIZE;
+	bmain->blen_thumb->height = BLEN_THUMB_SIZE;
+}
+
 /* ***************** ID ************************ */
 ID *BKE_libblock_find_name_ex(struct Main *bmain, const short type, const char *name)
 {
diff --git a/source/blender/blenloader/BLO_blend_defs.h b/source/blender/blenloader/BLO_blend_defs.h
index 44f0fa9aa53..a6b06a080cc 100644
--- a/source/blender/blenloader/BLO_blend_defs.h
+++ b/source/blender/blenloader/BLO_blend_defs.h
@@ -75,4 +75,6 @@ enum {
 	ENDB = BLEND_MAKE_ID('E', 'N', 'D', 'B'),
 };
 
+#define BLEN_THUMB_MEMSIZE_FILE(_x, _y) (sizeof(int) * (size_t)(2 + (_x) * (_y)))
+
 #endif  /* __BLO_BLEND_DEFS_H__ */
diff --git a/source/blender/blenloader/BLO_readfile.h b/source/blender/blenloader/BLO_readfile.h
index 5f881c0855c..77bdb99b54b 100644
--- a/source/blender/blenloader/BLO_readfile.h
+++ b/source/blender/blenloader/BLO_readfile.h
@@ -36,6 +36,7 @@
 extern "C" {
 #endif
 
+struct BlendThumbnail;
 struct bScreen;
 struct LinkNode;
 struct Main;
@@ -278,6 +279,15 @@ void BLO_expand_main(void *fdhandle, struct Main *mainvar);
 void BLO_update_defaults_userpref_blend(void);
 void BLO_update_defaults_startup_blend(struct Main *mainvar);
 
+/**
+ * Does a very light reading of given .blend file to extract its stored thumbnail.
+ *
+ * \param filepath The path of the file to extract thumbnail from.
+ * \return The raw thumbnail
+ *         (MEM-allocated, as stored in file, use BKE_main_thumbnail_to_imbuf() to convert it to ImBuf image).
+ */
+struct BlendThumbnail *BLO_thumbnail_from_file(const char *filepath);
+
 #ifdef __cplusplus
 } 
 #endif
diff --git a/source/blender/blenloader/BLO_writefile.h b/source/blender/blenloader/BLO_writefile.h
index 7a8429afec0..0d66eb743aa 100644
--- a/source/blender/blenloader/BLO_writefile.h
+++ b/source/blender/blenloader/BLO_writefile.h
@@ -33,14 +33,14 @@
  *  \brief external writefile function prototypes.
  */
 
+struct BlendThumbnail;
 struct MemFile;
 struct Main;
 struct ReportList;
 
-extern int BLO_write_file(struct Main *mainvar, const char *filepath, int write_flags, struct ReportList *reports, const int *thumb);
+extern int BLO_write_file(struct Main *mainvar, const char *filepath, int write_flags,
+        struct ReportList *reports, const struct BlendThumbnail *thumb);
 extern int BLO_write_file_mem(struct Main *mainvar, struct MemFile *compare, struct MemFile *current, int write_flags);
 
-#define BLEN_THUMB_SIZE 128
-
 #endif
 
diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index 6581fb3f3b7..ee07f2b09cf 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -917,6 +917,41 @@ static int read_file_dna(FileData *fd)
 	return 0;
 }
 
+static int *read_file_thumbnail(FileData *fd)
+{
+	BHead *bhead;
+	int *blend_thumb = NULL;
+
+	for (bhead = blo_firstbhead(fd); bhead; bhead = blo_nextbhead(fd, bhead)) {
+		if (bhead->code == TEST) {
+			const bool do_endian_swap = (fd->flags & FD_FLAGS_SWITCH_ENDIAN) != 0;
+			int *data = (int *)(bhead + 1);
+
+			if (bhead->len < (2 * sizeof(int))) {
+				break;
+			}
+
+			if (do_endian_swap) {
+				BLI_endian_switch_int32(&data[0]);
+				BLI_endian_switch_int32(&data[1]);
+			}
+
+			if (bhead->len < BLEN_THUMB_MEMSIZE_FILE(data[0], data[1])) {
+				break;
+			}
+
+			blend_thumb = data;
+			break;
+		}
+		else if (bhead->code != REND) {
+			/* Thumbnail is stored in TEST immediately after first REND... */
+			break;
+		}
+	}
+
+	return blend_thumb;
+}
+
 static int fd_read_from_file(FileData *filedata, void *buffer, unsigned int size)
 {
 	int readsize = read(filedata->filedes, buffer, size);
@@ -1080,6 +1115,33 @@ FileData *blo_openblenderfile(const char *filepath, ReportList *reports)
 	}
 }
 
+/**
+ * Same as blo_openblenderfile(), but does not reads DNA data, only header. Use it for light access
+ * (e.g. thumbnail reading).
+ */
+static FileData *blo_openblenderfile_minimal(const char *filepath)
+{
+	gzFile gzfile;
+	errno = 0;
+	gzfile = BLI_gzopen(filepath, "rb");
+
+	if (gzfile != (gzFile)Z_NULL) {
+		FileData *fd = filedata_new();
+		fd->gzfiledes = gzfile;
+		fd->read = fd_read_gzip_from_file;
+
+		decode_blender_header(fd);
+
+		if (fd->flags & FD_FLAGS_FILE_OK) {
+			return fd;
+		}
+
+		blo_freefiledata(fd);
+	}
+
+	return NULL;
+}
+
 static int fd_read_gzip_from_memory(FileData *filedata, void *buffer, unsigned int size)
 {
 	int err;
@@ -1290,6 +1352,33 @@ bool BLO_library_path_explode(const char *path, char *r_dir, char **r_group, cha
 	return true;
 }
 
+BlendThumbnail *BLO_thumbnail_from_file(const char *filepath)
+{
+	FileData *fd;
+	BlendThumbnail *data;
+	int *fd_data;
+
+	fd = blo_openblenderfile_minimal(filepath);
+	fd_data = fd ? read_file_thumbnail(fd) : NULL;
+
+	if (fd_data) {
+		const size_t sz = BLEN_THUMB_MEMSIZE(fd_data[0], fd_data[1]);
+		data = MEM_mallocN(sz, __func__);
+
+		BLI_assert((sz - sizeof(*data)) == (BLEN_THUMB_MEMSIZE_FILE(fd_data[0], fd_data[1]) - (sizeof(*fd_data) * 2)));
+		data->width = fd_data[0];
+		data->height = fd_data[1];
+		memcpy(data->rect, &fd_data[2], sz - sizeof(*data));
+	}
+	else {
+		data = NULL;
+	}
+
+	blo_freefiledata(fd);
+
+	return data;
+}
+
 /* ************** OLD POINTERS ******************* */
 
 static void *newdataadr(FileData *fd, void *adr)		/* only direct databocks */
@@ -8174,6 +8263,24 @@ BlendFileData *blo_read_file_internal(FileData *fd, const char *filepath)
 	bfd->type = BLENFILETYPE_BLEND;
 	BLI_strncpy(bfd->main->name, filepath, sizeof(bfd->main->name));
 
+	if (G.background) {
+		/* We only read & store .blend thumbnail in background mode
+		 * (because we cannot re-generate it, no OpenGL available).
+		 */
+		const int *data = read_file_thumbnail(fd);
+
+		if (data) {
+			const size_t sz = BLEN_THUMB_MEMSIZE(data[0], data[1]);
+			bfd->main->blen_thumb = MEM_mallocN(sz, __func__);
+
+			BLI_assert((sz - sizeof(*bfd->main->blen_thumb)) ==
+			           (BLEN_THUMB_MEMSIZE_FILE(data[0], data[1]) - (sizeof(*data) * 2)));
+			bfd->main->blen_thumb->width = data[0];
+			bfd->main->blen_thumb->height = data[1];
+			memcpy(bfd->main->blen_thumb->rect, &data[2], sz - sizeof(*bfd->main->blen_thumb));
+		}
+	}
+
 	while (bhead) {
 		switch (bhead->code) {
 		case DATA:
diff --git a/source/blender/blenloader/intern/writefile.c b/source/blender/blenloader/intern/writefile.c
index 2dfb100cc62..94f3237d919 100644
--- a/source/blender/blenloader/intern/writefile.c
+++ b/source/blender/blenloader/intern/writefile.c
@@ -3688,10 +3688,18 @@ static void write_global(WriteData *wd, int fileflags, Main *mainvar)
  * second are an RGBA image (unsigned char)
  * note, this uses 'TEST' since new types will segfault on file load for older blender versions.
  */
-static void write_thumb(WriteData *wd, const int *img)
+static void write_thumb(WriteData *wd, const BlendThumbnail *thumb)
 {
-	if (img)
-		writedata(wd, TEST, (2 + img[0] * img[1]) * sizeof(int), img);
+	if (thumb) {
+		size_t sz = BLEN_THUMB_MEMSIZE_FILE(thumb->width, thumb->height);
+		int *img = alloca(sz);
+
+		BLI_assert((sz - (sizeof(*img) * 2)) == (BLEN_THUMB_MEMSIZE(thumb->width, thumb->height) - sizeof(thumb)));
+		img[0] = thumb->width;
+		img[1] = thumb->height;
+		memcpy(&img[2], thumb->rect, sz - (sizeof(*img) * 2));
+		writedata(wd, TEST, sz, img);
+	}
 }
 
 /* if MemFile * there's filesave to memory */
@@ -3699,7 +3707,7 @@ static int write_file_handle(
         Main *mainvar,
         WriteWrap *ww,
         MemFile *compare, MemFile *current,
-        int write_user_block, int write_flags, const int *thumb)
+        int write_user_block, int write_flags, const BlendThumbnail *thumb)
 {
 	BHead bhead;
 	ListBase mainlist;
@@ -3831,7 +3839,8 @@ static bool do_history(const char *name, ReportList *reports)
 }
 
 /* return: success (1) */
-int BLO_write_file(Main *mainvar, const char *filepath, int write_flags, ReportList *reports, const int *thumb)
+int BLO_write_file(
+        Main *mainvar, const char *filepath, int write_flags, ReportList *reports, const BlendThumbnail *thumb)
 {
 	char tempname[FILE_MAX+1];
 	int err, write_user_block;
diff --git a/source/blender/imbuf/intern/thumbs_blend.c b/source/blender/imbuf/intern/thumbs_blend.c
index 17d9f3d0735..b5c6deb6b01 100644
--- a/source/blender/imbuf/intern/thumbs_blend.c
+++ b/source/blender/imbuf/intern/thumbs_blend.c
@@ -28,8 +28,6 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include "zlib.h"
-
 #include "BLI_utildefines.h"
 #include "BLI_endian_switch.h"
 #include "BLI_fileops.h"
@@ -41,6 +39,8 @@
 #include "BKE_global.h"
 #include "BKE_idcode.h"
 #include "BKE_icons.h"
+#include "BKE_library.h"
+#include "BKE_main.h"
 
 #include "DNA_ID.h"  /* For preview images... */
 
@@ -48,97 +48,20 @@
 #include "IMB_imbuf.h"
 #include "IMB_thumbs.h"
 
-/* extracts the thumbnail from between the 'REND' and the 'GLOB'
- * chunks of the header, don't use typical blend loader because its too slow */
-
-static ImBuf *loadblend_thumb(gzFile gzfile)
-{
-	char buf[12];
-	int bhead[24 / sizeof(int)]; /* max size on 64bit */
-	char endian, pointer_size;
-	char endian_switch;
-	int sizeof_bhead;
-
-	/* read the blend file header */
-	if (gzread(gzfile, buf, 12) != 12)
-		return NULL;
-	if (!STREQLEN(buf, "BLENDER", 7))
-		return NULL;
-
-	if (buf[7] == '-')
-		pointer_size = 8;
-	else if (buf[7] == '_')
-		pointer_size = 4;
-	else
-		return NULL;
-
-	sizeof_bhead = 16 + pointer_size;
-
-	if (buf[8] == 'V')
-		endian = B_ENDIAN;  /* big: PPC */
-	else if (buf[8] == 'v')
-		endian = L_ENDIAN;  /* little: x86 */
-	else
-		return NULL;
-
-	endian_switch = ((ENDIAN_ORDER != endian)) ? 1 : 0;
-
-	while (gzread(gzfile, bhead, sizeof_bhead) == sizeof_bhead) {
-		if (endian_switch)
-			BLI_endian_switch_int32(&bhead[1]);  /* length */
-
-		if (bhead[0] == REND) {
-			gzseek(gzfile, bhead[1], SEEK_CUR); /* skip to the next */
-		}
-		else {
-			break;
-		}
-	}
-
-	/* using 'TEST' since new names segfault when loading in old blenders */
-	if (bhead[0] == TEST) {
-		ImBuf *img = NULL;
-		int size[2];
-
-		if (gzread(gzfile, size, sizeof(size)) != sizeof(size))
-			return NULL;
-
-		if (endian_switch) {
-			BLI_endian_switch_int32(&size[0]);
-			BLI_endian_switch_int32(&size[1]);
-		}
-		/* length */
-		bhead[1] -= sizeof(int) * 2;
-
-		/* inconsistent image size, quit early */
-		if (bhead[1] != size[0] * size[1] * sizeof(int))
-			return NULL;
-	
-		/* finally malloc and read the data */
-		img = IMB_allocImBuf(size[0], size[1], 32, IB_rect | IB_metadata);
-	
-		if (gzread(gzfile, img->rect, bhead[1]) != bhead[1]) {
-			IMB_freeImBuf(img);
-			img = NULL;
-		}
-	
-		return img;
-	}
-	
-	return NULL;
-}
+#include "MEM_guardedalloc.h"
 
 ImBuf *IMB_thumb_load_blend(const char *blen_path, const char *blen_group, const char *blen_id)
 {
+	ImBuf *ima = NULL;
+
 	if (blen_group && blen_id) {
 		LinkNode *ln, *names, *lp, *previews = NULL;
 		struct BlendHandle *libfiledata = BLO_blendhandle_from_file(blen_path, NULL);
-		ImBuf *ima = NULL;
 		int idcode = BKE_idcode_from_name(blen_group);
 		int i, nprevs, nnames;
 
 		if (libfiledata == NULL) {
-			return NULL;
+			return ima;
 		}
 
 		/* Note: we should handle all previews for a same group at once, would avoid reopening .blend file
@@ -180,25 +103,19 @@ ImBuf *IMB_thumb_load_blend(const char *blen_path, const char *blen_group, const
 
 		BLI_linklist_free(previews, BKE_previewimg_freefunc);
 		BLI_linklist_free(names, free);
-		return ima;
 	}
 	else {
-		gzFile gzfile;
-		/* not necessarily a gzip */
-		gzfile = BLI_gzopen(blen_path, "rb");
+		BlendThumbnail *data;
 
-		if (NULL == gzfile) {
-			return NULL;
-		}
-		else {
-			ImBuf *img = loadblend_thumb(gzfile);
-
-			/* read ok! */
-			gzclose(gzfile);
+		data = BLO_thumbnail_from_file(blen_path);
+		ima = BKE_main_thumbnail_to_imbuf(NULL, data);
 
-			return img;
+		if (data) {
+			MEM_freeN(data);
 		}
 	}
+
+	return ima;
 }
 
 /* add a fake passepartout overlay to a byte buffer, use for blend file thumbnails */
diff --git a/source/blender/windowmanager/intern/wm_files.c b/source/blender/windowmanager/intern/wm_files.c
index a4f69b24351..6927e2dbc14 100644
--- a/source/blender/windowmanager/intern/wm_files.c
+++ b/source/blender/windowmanager/intern/wm_files.c
@@ -76,6 +76,7 @@
 #include "BKE_context.h"
 #include "BKE_depsgraph.h"
 #include "BKE_global.h"
+#include "BKE_library.h"
 #include "BKE_main.h"
 #include "BKE_packedFile.h"
 #include "BKE_report.h"
@@ -864,11 +865,11 @@ static void wm_history_file_update(void)
 
 
 /* screen can be NULL */
-static ImBuf *blend_file_thumb(Scene *scene, bScreen *screen, int **thumb_pt)
+static ImBuf *blend_file_thumb(Scene *scene, bScreen *screen, BlendThumbnail **thumb_pt)
 {
 	/* will be scaled down, but gives some nice oversampling */
 	ImBuf *ibuf;
-	int *thumb;
+	BlendThumbnail *thumb;
 	char err_out[256] = "unknown";
 
 	/* screen if no camera found */
@@ -876,7 +877,11 @@ static ImBuf *blend_file_thumb(Scene *scene, bScreen *screen, int **thumb_pt)
 	ARegion *ar = NULL;
 	View3D *v3d = NULL;
 
-	*thumb_pt = NULL;
+	/* In case we are given a valid thumbnail data, just generate image from it. */
+	if (*thumb_pt) {
+		thumb = *thumb_pt;
+		return BKE_main_thumbnail_to_imbuf(NULL, thumb);
+	}
 
 	/* scene can be NULL if running a script at startup and calling the save operator */
 	if (G.background || scene == NULL)
@@ -914,13 +919,7 @@ static ImBuf *blend_file_thumb(Scene *scene, bScreen *screen, int **thumb_pt)
 		/* add pretty overlay */
 		IMB_thumb_overlay_blend(ibuf->rect, ibuf->x, ibuf->y, aspect);
 		
-		/* first write into thumb buffer */
-		thumb = MEM_mallocN(((2 + (BLEN_THUMB_SIZE * BLEN_THUMB_SIZE))) * sizeof(int), "write_file thumb");
-
-		thumb[0] = BLEN_THUMB_SIZE;
-		thumb[1] = BLEN_THUMB_SIZE;
-
-		memcpy(thumb + 2, ibuf->rect, BLEN_THUMB_SIZE * BLEN_THUMB_SIZE * sizeof(int));
+		thumb = BKE_main_thumbnail_from_imbuf(NULL, ibuf);
 	}
 	else {
 		/* '*thumb_pt' needs to stay NULL to prevent a bad thumbnail from being handled */
@@ -959,25 +958,26 @@ int wm_file_write(bContext *C, const char *filepath, int fileflags, ReportList *
 {
 	Library *li;
 	int len;
-	int *thumb = NULL;
+	int ret = -1;
+	BlendThumbnail *thumb, *main_thumb;
 	ImBuf *ibuf_thumb = NULL;
 
 	len = strlen(filepath);
 	
 	if (len == 0) {
 		BKE_report(reports, RPT_ERROR, "Path is empty, cannot save");
-		return -1;
+		return ret;
 	}
 
 	if (len >= FILE_MAX) {
 		BKE_report(reports, RPT_ERROR, "Path too long, cannot save");
-		return -1;
+		return ret;
 	}
 	
 	/* Check if file write permission is ok */
 	if (BLI_exists(filepath) && !BLI_file_is_writable(filepath)) {
 		BKE_reportf(reports, RPT_ERROR, "Cannot save blend file, path '%s' is not writable", filepath);
-		return -1;
+		return ret;
 	}
  
 	/* note: used to replace the file extension (to ensure '.blend'),
@@ -988,18 +988,21 @@ int wm_file_write(bContext *C, const char *filepath, int fileflags, ReportList *
 	for (li = G.main->library.first; li; li = li->id.next) {
 		if (BLI_path_cmp(li->filepath, filepath) == 0) {
 			BKE_reportf(reports, RPT_ERROR, "Cannot overwrite used library '%.240s'", filepath);
-			return -1;
+			return ret;
 		}
 	}
 
+	/* Call pre-save callbacks befores writing preview, that way you can generate custom file thumbnail... */
+	BLI_callback_exec(G.main, NULL, BLI_CB_EVT_SAVE_PRE);
+
 	/* blend file thumbnail */
 	/* save before exit_editmode, otherwise derivedmeshes for shared data corrupt #27765) */
+	/* Main now can store a .blend thumbnail, usefull for background mode or thumbnail customization. */
+	main_thumb = thumb = CTX_data_main(C)->blen_thumb;
 	if ((U.flag & USER_SAVE_PREVIEWS) && BLI_thread_is_main()) {
 		ibuf_thumb = blend_file_thumb(CTX_data_scene(C), CTX_wm_screen(C), &thumb);
 	}
 
-	BLI_callback_exec(G.main, NULL, BLI_CB_EVT_SAVE_PRE);
-
 	/* operator now handles overwrite checks */
 
 	if (G.fileflags & G_AUTOPACK) {
@@ -1044,22 +1047,21 @@ int wm_file_write(bContext *C, const char *filepath, int fileflags, ReportList *
 		if (ibuf_thumb) {
 			IMB_thumb_delete(filepath, THB_FAIL); /* without this a failed thumb overrides */
 			ibuf_thumb = IMB_thumb_create(filepath, THB_LARGE, THB_SOURCE_BLEND, ibuf_thumb);
-			IMB_freeImBuf(ibuf_thumb);
 		}
 
-		if (thumb) MEM_freeN(thumb);
+		ret = 0;  /* Success. */
 	}
-	else {
-		if (ibuf_thumb) IMB_freeImBuf(ibuf_thumb);
-		if (thumb) MEM_freeN(thumb);
-		
-		WM_cursor_wait(0);
-		return -1;
+
+	if (ibuf_thumb) {
+		IMB_freeImBuf(ibuf_thumb);
+	}
+	if (thumb && thumb != main_thumb) {
+		MEM_freeN(thumb);
 	}
 
 	WM_cursor_wait(0);
-	
-	return 0;
+
+	return ret;
 }
 
 /**