Images: change alpha settings to support channel packing

This also replaces the Use Alpha setting. We now have these alpha modes:

* Straight: store RGB and alpha channels separately with alpha acting as a
  mask, also known as unassociated alpha.
* Premultiplied: transparent RGB pixels are multiplied by the alpha channel.
  The natural format for renders.
* Channel Packed: different images are packed in the RGB and alpha channels,
  and they should not influence each other. Channel packing is commonly used
  by game engines to save memory.
* None: ignore alpha channel from the file and make image fully opaque.

Cycles OSL does not correctly support Channel Packed and None yet, we are
missing fine control over the OpenImageIO texture cache to do that.

Fixes T53672

14 files changed, 115 insertions, 52 deletions

diff --git a/source/blender/blenkernel/BKE_blender_version.h b/source/blender/blenkernel/BKE_blender_version.h
index 5d659d63e27..077bbce2264 100644
--- a/source/blender/blenkernel/BKE_blender_version.h
+++ b/source/blender/blenkernel/BKE_blender_version.h
@@ -27,7 +27,7 @@
  * \note Use #STRINGIFY() rather than defining with quotes.
  */
 #define BLENDER_VERSION 280
-#define BLENDER_SUBVERSION 69
+#define BLENDER_SUBVERSION 70
 /** Several breakages with 280, e.g. collections vs layers. */
 #define BLENDER_MINVERSION 280
 #define BLENDER_MINSUBVERSION 0
diff --git a/source/blender/blenkernel/intern/image.c b/source/blender/blenkernel/intern/image.c
index d59ead25396..9960994400f 100644
--- a/source/blender/blenkernel/intern/image.c
+++ b/source/blender/blenkernel/intern/image.c
@@ -502,6 +502,12 @@ static void image_init_color_management(Image *ima)
     if (ibuf->flags & IB_alphamode_premul) {
       ima->alpha_mode = IMA_ALPHA_PREMUL;
     }
+    else if (ibuf->flags & IB_alphamode_channel_packed) {
+      ima->alpha_mode = IMA_ALPHA_CHANNEL_PACKED;
+    }
+    else if (ibuf->flags & IB_alphamode_ignore) {
+      ima->alpha_mode = IMA_ALPHA_IGNORE;
+    }
     else {
       ima->alpha_mode = IMA_ALPHA_STRAIGHT;
     }
@@ -3592,16 +3598,18 @@ static void image_initialize_after_load(Image *ima, ImBuf *UNUSED(ibuf))
 
 static int imbuf_alpha_flags_for_image(Image *ima)
 {
-  int flag = 0;
-
-  if (ima->flag & IMA_IGNORE_ALPHA) {
-    flag |= IB_ignore_alpha;
-  }
-  else if (ima->alpha_mode == IMA_ALPHA_PREMUL) {
-    flag |= IB_alphamode_premul;
+  switch (ima->alpha_mode) {
+    case IMA_ALPHA_STRAIGHT:
+      return 0;
+    case IMA_ALPHA_PREMUL:
+      return IB_alphamode_premul;
+    case IMA_ALPHA_CHANNEL_PACKED:
+      return IB_alphamode_channel_packed;
+    case IMA_ALPHA_IGNORE:
+      return IB_alphamode_ignore;
   }
 
-  return flag;
+  return 0;
 }
 
 /* the number of files will vary according to the stereo format */
diff --git a/source/blender/blenloader/intern/versioning_260.c b/source/blender/blenloader/intern/versioning_260.c
index 77ef27182f2..7b0aab99aea 100644
--- a/source/blender/blenloader/intern/versioning_260.c
+++ b/source/blender/blenloader/intern/versioning_260.c
@@ -1808,6 +1808,7 @@ void blo_do_versions_260(FileData *fd, Library *UNUSED(lib), Main *bmain)
         Image *image = blo_do_versions_newlibadr(fd, tex->id.lib, tex->ima);
 
         if (image && (image->flag & IMA_DO_PREMUL) == 0) {
+          const int IMA_IGNORE_ALPHA = (1 << 12);
           image->flag |= IMA_IGNORE_ALPHA;
         }
       }
diff --git a/source/blender/blenloader/intern/versioning_280.c b/source/blender/blenloader/intern/versioning_280.c
index 7ba0eb7b791..13ccc374073 100644
--- a/source/blender/blenloader/intern/versioning_280.c
+++ b/source/blender/blenloader/intern/versioning_280.c
@@ -3450,6 +3450,17 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
     }
   }
 
+  if (!MAIN_VERSION_ATLEAST(bmain, 280, 70)) {
+    /* New image alpha modes. */
+    LISTBASE_FOREACH (Image *, image, &bmain->images) {
+      const int IMA_IGNORE_ALPHA = (1 << 12);
+      if (image->flag & IMA_IGNORE_ALPHA) {
+        image->alpha_mode = IMA_ALPHA_IGNORE;
+        image->flag &= ~IMA_IGNORE_ALPHA;
+      }
+    }
+  }
+
   {
     /* Versioning code until next subversion bump goes here. */
   }
diff --git a/source/blender/editors/space_image/image_buttons.c b/source/blender/editors/space_image/image_buttons.c
index 68af854e367..9f429329597 100644
--- a/source/blender/editors/space_image/image_buttons.c
+++ b/source/blender/editors/space_image/image_buttons.c
@@ -42,6 +42,7 @@
 
 #include "RE_pipeline.h"
 
+#include "IMB_colormanagement.h"
 #include "IMB_imbuf.h"
 #include "IMB_imbuf_types.h"
 
@@ -995,10 +996,11 @@ void uiTemplateImage(uiLayout *layout,
 
           if (has_alpha) {
             col = uiLayoutColumn(layout, false);
-            uiItemR(col, &imaptr, "use_alpha", 0, NULL, ICON_NONE);
-            row = uiLayoutRow(col, false);
-            uiLayoutSetActive(row, RNA_boolean_get(&imaptr, "use_alpha"));
-            uiItemR(row, &imaptr, "alpha_mode", 0, IFACE_("Alpha"), ICON_NONE);
+            uiItemR(col, &imaptr, "alpha_mode", 0, IFACE_("Alpha"), ICON_NONE);
+
+            /* Alpha mode has no effect for non-color data. */
+            bool is_data = IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name);
+            uiLayoutSetActive(col, !is_data);
           }
 
           if (ima->source == IMA_SRC_MOVIE) {
diff --git a/source/blender/imbuf/IMB_imbuf.h b/source/blender/imbuf/IMB_imbuf.h
index 674dc61cd9e..8574f33bce6 100644
--- a/source/blender/imbuf/IMB_imbuf.h
+++ b/source/blender/imbuf/IMB_imbuf.h
@@ -476,6 +476,9 @@ int imb_get_anim_type(const char *name);
  */
 bool IMB_isfloat(struct ImBuf *ibuf);
 
+/* Do byte/float and colorspace conversions need to take alpha into account? */
+bool IMB_alpha_affects_rgb(const struct ImBuf *ibuf);
+
 /* create char buffer, color corrected if necessary, for ImBufs that lack one */
 void IMB_rect_from_float(struct ImBuf *ibuf);
 void IMB_float_from_rect(struct ImBuf *ibuf);
diff --git a/source/blender/imbuf/IMB_imbuf_types.h b/source/blender/imbuf/IMB_imbuf_types.h
index 0f2529e261a..61aa1f401a0 100644
--- a/source/blender/imbuf/IMB_imbuf_types.h
+++ b/source/blender/imbuf/IMB_imbuf_types.h
@@ -285,10 +285,12 @@ enum {
   IB_alphamode_premul = 1 << 12,
   /** if this flag is set, alpha mode would be guessed from file */
   IB_alphamode_detect = 1 << 13,
+  /* alpha channel is unrelated to RGB and should not affect it */
+  IB_alphamode_channel_packed = 1 << 14,
   /** ignore alpha on load and substitute it with 1.0f */
-  IB_ignore_alpha = 1 << 14,
-  IB_thumbnail = 1 << 15,
-  IB_multiview = 1 << 16,
+  IB_alphamode_ignore = 1 << 15,
+  IB_thumbnail = 1 << 16,
+  IB_multiview = 1 << 17,
 };
 
 /** \} */
diff --git a/source/blender/imbuf/intern/colormanagement.c b/source/blender/imbuf/intern/colormanagement.c
index ba0f10446a1..8a10af7e184 100644
--- a/source/blender/imbuf/intern/colormanagement.c
+++ b/source/blender/imbuf/intern/colormanagement.c
@@ -1057,13 +1057,19 @@ void colormanage_imbuf_make_linear(ImBuf *ibuf, const char *from_colorspace)
 
   if (ibuf->rect_float) {
     const char *to_colorspace = global_role_scene_linear;
+    const bool predivide = IMB_alpha_affects_rgb(ibuf);
 
     if (ibuf->rect) {
       imb_freerectImBuf(ibuf);
     }
 
-    IMB_colormanagement_transform(
-        ibuf->rect_float, ibuf->x, ibuf->y, ibuf->channels, from_colorspace, to_colorspace, true);
+    IMB_colormanagement_transform(ibuf->rect_float,
+                                  ibuf->x,
+                                  ibuf->y,
+                                  ibuf->channels,
+                                  from_colorspace,
+                                  to_colorspace,
+                                  predivide);
   }
 }
 
@@ -1405,6 +1411,7 @@ typedef struct DisplayBufferThread {
   int channels;
   float dither;
   bool is_data;
+  bool predivide;
 
   const char *byte_colorspace;
   const char *float_colorspace;
@@ -1469,6 +1476,7 @@ static void display_buffer_init_handle(void *handle_v,
   handle->channels = channels;
   handle->dither = dither;
   handle->is_data = is_data;
+  handle->predivide = IMB_alpha_affects_rgb(ibuf);
 
   handle->byte_colorspace = init_data->byte_colorspace;
   handle->float_colorspace = init_data->float_colorspace;
@@ -1486,6 +1494,7 @@ static void display_buffer_apply_get_linear_buffer(DisplayBufferThread *handle,
 
   bool is_data = handle->is_data;
   bool is_data_display = handle->cm_processor->is_data_result;
+  bool predivide = handle->predivide;
 
   if (!handle->buffer) {
     unsigned char *byte_buffer = handle->byte_buffer;
@@ -1534,7 +1543,7 @@ static void display_buffer_apply_get_linear_buffer(DisplayBufferThread *handle,
 
     if (!is_data && !is_data_display) {
       IMB_colormanagement_transform(
-          linear_buffer, width, height, channels, from_colorspace, to_colorspace, true);
+          linear_buffer, width, height, channels, from_colorspace, to_colorspace, predivide);
     }
 
     *is_straight_alpha = false;
@@ -1590,13 +1599,13 @@ static void *do_display_buffer_apply_thread(void *handle_v)
     }
   }
   else {
-    bool is_straight_alpha, predivide;
+    bool is_straight_alpha;
     float *linear_buffer = MEM_mallocN(((size_t)channels) * width * height * sizeof(float),
                                        "color conversion linear buffer");
 
     display_buffer_apply_get_linear_buffer(handle, height, linear_buffer, &is_straight_alpha);
 
-    predivide = is_straight_alpha == false;
+    bool predivide = handle->predivide && (is_straight_alpha == false);
 
     if (is_data) {
       /* special case for data buffers - no color space conversions,
@@ -2178,6 +2187,7 @@ void IMB_colormanagement_imbuf_to_srgb_texture(unsigned char *out_buffer,
 
   /* TODO(brecht): make this multithreaded, or at least process in batches. */
   const unsigned char *in_buffer = (unsigned char *)ibuf->rect;
+  const bool use_premultiply = IMB_alpha_affects_rgb(ibuf);
 
   for (int y = 0; y < height; y++) {
     const size_t in_offset = (offset_y + y) * ibuf->x + offset_x;
@@ -2192,11 +2202,13 @@ void IMB_colormanagement_imbuf_to_srgb_texture(unsigned char *out_buffer,
         rgba_uchar_to_float(pixel, in);
         OCIO_processorApplyRGB(processor, pixel);
         linearrgb_to_srgb_v3_v3(pixel, pixel);
-        mul_v3_fl(pixel, pixel[3]);
+        if (use_premultiply) {
+          mul_v3_fl(pixel, pixel[3]);
+        }
         rgba_float_to_uchar(out, pixel);
       }
     }
-    else {
+    else if (use_premultiply) {
       /* Premultiply only. */
       for (int x = 0; x < width; x++, in += 4, out += 4) {
         out[0] = (in[0] * in[3]) >> 8;
@@ -2205,6 +2217,15 @@ void IMB_colormanagement_imbuf_to_srgb_texture(unsigned char *out_buffer,
         out[3] = in[3];
       }
     }
+    else {
+      /* Copy only. */
+      for (int x = 0; x < width; x++, in += 4, out += 4) {
+        out[0] = in[0];
+        out[1] = in[1];
+        out[2] = in[2];
+        out[3] = in[3];
+      }
+    }
   }
 }
 
diff --git a/source/blender/imbuf/intern/divers.c b/source/blender/imbuf/intern/divers.c
index 1e16f0975e2..aa49453d68c 100644
--- a/source/blender/imbuf/intern/divers.c
+++ b/source/blender/imbuf/intern/divers.c
@@ -96,6 +96,12 @@ MINLINE void float_to_byte_dither_v4(
   b[3] = unit_float_to_uchar_clamp(f[3]);
 }
 
+/* Test if colorspace conversions of pixels in buffer need to take into account alpha. */
+bool IMB_alpha_affects_rgb(const ImBuf *ibuf)
+{
+  return (ibuf->flags & IB_alphamode_channel_packed) == 0;
+}
+
 /* float to byte pixels, output 4-channel RGBA */
 void IMB_buffer_byte_from_float(uchar *rect_to,
                                 const float *rect_from,
@@ -728,16 +734,19 @@ void IMB_rect_from_float(ImBuf *ibuf)
   buffer = MEM_dupallocN(ibuf->rect_float);
 
   /* first make float buffer in byte space */
+  const bool predivide = IMB_alpha_affects_rgb(ibuf);
   IMB_colormanagement_transform(buffer,
                                 ibuf->x,
                                 ibuf->y,
                                 ibuf->channels,
                                 from_colorspace,
                                 ibuf->rect_colorspace->name,
-                                true);
+                                predivide);
 
   /* convert from float's premul alpha to byte's straight alpha */
-  IMB_unpremultiply_rect_float(buffer, ibuf->channels, ibuf->x, ibuf->y);
+  if (IMB_alpha_affects_rgb(ibuf)) {
+    IMB_unpremultiply_rect_float(buffer, ibuf->channels, ibuf->x, ibuf->y);
+  }
 
   /* convert float to byte */
   IMB_buffer_byte_from_float((unsigned char *)ibuf->rect,
@@ -802,7 +811,9 @@ void IMB_float_from_rect(ImBuf *ibuf)
       rect_float, ibuf->x, ibuf->y, ibuf->channels, ibuf->rect_colorspace, false);
 
   /* byte buffer is straight alpha, float should always be premul */
-  IMB_premultiply_rect_float(rect_float, ibuf->channels, ibuf->x, ibuf->y);
+  if (IMB_alpha_affects_rgb(ibuf)) {
+    IMB_premultiply_rect_float(rect_float, ibuf->channels, ibuf->x, ibuf->y);
+  }
 
   if (ibuf->rect_float == NULL) {
     ibuf->rect_float = rect_float;
diff --git a/source/blender/imbuf/intern/readimage.c b/source/blender/imbuf/intern/readimage.c
index 9297227bc87..d9f3c7ec6bb 100644
--- a/source/blender/imbuf/intern/readimage.c
+++ b/source/blender/imbuf/intern/readimage.c
@@ -61,20 +61,16 @@ static void imb_handle_alpha(ImBuf *ibuf,
   }
 
   bool is_data = (colorspace && IMB_colormanagement_space_name_is_data(colorspace));
-  int alpha_flags;
+  int alpha_flags = (flags & IB_alphamode_detect) ? ibuf->flags : flags;
 
-  if (flags & IB_alphamode_detect) {
-    alpha_flags = ibuf->flags & IB_alphamode_premul;
-  }
-  else {
-    alpha_flags = flags & IB_alphamode_premul;
-  }
-
-  if (is_data) {
+  if (is_data || (flags & IB_alphamode_channel_packed)) {
     /* Don't touch alpha. */
+    ibuf->flags |= IB_alphamode_channel_packed;
   }
-  else if (flags & IB_ignore_alpha) {
+  else if (flags & IB_alphamode_ignore) {
+    /* Make opaque. */
     IMB_rectfill_alpha(ibuf, 1.0f);
+    ibuf->flags |= IB_alphamode_ignore;
   }
   else {
     if (alpha_flags & IB_alphamode_premul) {
diff --git a/source/blender/makesdna/DNA_image_types.h b/source/blender/makesdna/DNA_image_types.h
index 3e42aa17492..6141472125f 100644
--- a/source/blender/makesdna/DNA_image_types.h
+++ b/source/blender/makesdna/DNA_image_types.h
@@ -178,7 +178,7 @@ enum {
   /** For image user, but these flags are mixed. */
   IMA_USER_FRAME_IN_RANGE = (1 << 10),
   IMA_VIEW_AS_RENDER = (1 << 11),
-  IMA_IGNORE_ALPHA = (1 << 12),
+  IMA_FLAG_UNUSED_12 = (1 << 12), /* cleared */
   IMA_DEINTERLACE = (1 << 13),
   IMA_USE_VIEWS = (1 << 14),
   IMA_FLAG_UNUSED_15 = (1 << 15), /* cleared */
@@ -233,6 +233,8 @@ enum {
 enum {
   IMA_ALPHA_STRAIGHT = 0,
   IMA_ALPHA_PREMUL = 1,
+  IMA_ALPHA_CHANNEL_PACKED = 2,
+  IMA_ALPHA_IGNORE = 3,
 };
 
 #endif
diff --git a/source/blender/makesrna/intern/rna_image.c b/source/blender/makesrna/intern/rna_image.c
index 40c8c7e1582..c033e393625 100644
--- a/source/blender/makesrna/intern/rna_image.c
+++ b/source/blender/makesrna/intern/rna_image.c
@@ -680,12 +680,26 @@ static void rna_def_image(BlenderRNA *brna)
        "STRAIGHT",
        0,
        "Straight",
-       "Transparent RGB and alpha pixels are unmodified"},
+       "Store RGB and alpha channels separately with alpha acting as a mask, also known as "
+       "unassociated alpha. Commonly used by image editing applications and file formats like "
+       "PNG"},
       {IMA_ALPHA_PREMUL,
        "PREMUL",
        0,
        "Premultiplied",
-       "Transparent RGB pixels are multiplied by the alpha channel"},
+       "Store RGB channels with alpha multipled in, also known as associated alpha. The natural "
+       "format for renders and used by file formats like OpenEXR"},
+      {IMA_ALPHA_CHANNEL_PACKED,
+       "CHANNEL_PACKED",
+       0,
+       "Channel Packed",
+       "Different images are packed in the RGB and alpha channels, and they should not "
+       "affect each other. Channel packing is commonly used by game engines to save memory"},
+      {IMA_ALPHA_IGNORE,
+       "NONE",
+       0,
+       "None",
+       "Ignore alpha channel from the file and make image fully opaque"},
       {0, NULL, 0, NULL, NULL},
   };
 
@@ -744,15 +758,6 @@ static void rna_def_image(BlenderRNA *brna)
       "Apply render part of display transformation when displaying this image on the screen");
   RNA_def_property_update(prop, NC_IMAGE | ND_DISPLAY, NULL);
 
-  prop = RNA_def_property(srna, "use_alpha", PROP_BOOLEAN, PROP_NONE);
-  RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_STATIC);
-  RNA_def_property_boolean_negative_sdna(prop, NULL, "flag", IMA_IGNORE_ALPHA);
-  RNA_def_property_ui_text(
-      prop,
-      "Use Alpha",
-      "Use the alpha channel information from the image or make image fully opaque");
-  RNA_def_property_update(prop, NC_IMAGE | ND_DISPLAY, "rna_Image_colormanage_update");
-
   prop = RNA_def_property(srna, "use_deinterlace", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_STATIC);
   RNA_def_property_boolean_sdna(prop, NULL, "flag", IMA_DEINTERLACE);
diff --git a/source/blender/nodes/shader/nodes/node_shader_tex_image.c b/source/blender/nodes/shader/nodes/node_shader_tex_image.c
index 8086bb2e15d..786386bb63e 100644
--- a/source/blender/nodes/shader/nodes/node_shader_tex_image.c
+++ b/source/blender/nodes/shader/nodes/node_shader_tex_image.c
@@ -184,7 +184,8 @@ static int node_shader_gpu_tex_image(GPUMaterial *mat,
      * that if we blend the color with a transparent shader using alpha as
      * a factor, we don't multiply alpha into the color twice. */
     if (out[1].hasoutput &&
-        !IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name)) {
+        !(ELEM(ima->alpha_mode, IMA_ALPHA_IGNORE, IMA_ALPHA_CHANNEL_PACKED) ||
+          IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name))) {
       GPU_link(mat, "tex_color_alpha_unpremultiply", out[0].link, &out[0].link);
     }
     else {
diff --git a/source/blender/render/intern/source/imagetexture.c b/source/blender/render/intern/source/imagetexture.c
index c147794dae7..1aa16dc1019 100644
--- a/source/blender/render/intern/source/imagetexture.c
+++ b/source/blender/render/intern/source/imagetexture.c
@@ -233,7 +233,7 @@ int imagewrap(Tex *tex,
 
   /* keep this before interpolation [#29761] */
   if (ima) {
-    if ((tex->imaflag & TEX_USEALPHA) && (ima->flag & IMA_IGNORE_ALPHA) == 0) {
+    if ((tex->imaflag & TEX_USEALPHA) && (ima->alpha_mode != IMA_ALPHA_IGNORE)) {
       if ((tex->imaflag & TEX_CALCALPHA) == 0) {
         texres->talpha = true;
       }
@@ -1056,7 +1056,7 @@ static int imagewraposa_aniso(Tex *tex,
   image_mipmap_test(tex, ibuf);
 
   if (ima) {
-    if ((tex->imaflag & TEX_USEALPHA) && (ima->flag & IMA_IGNORE_ALPHA) == 0) {
+    if ((tex->imaflag & TEX_USEALPHA) && (ima->alpha_mode != IMA_ALPHA_IGNORE)) {
       if ((tex->imaflag & TEX_CALCALPHA) == 0) {
         texres->talpha = 1;
       }
@@ -1512,7 +1512,7 @@ int imagewraposa(Tex *tex,
   image_mipmap_test(tex, ibuf);
 
   if (ima) {
-    if ((tex->imaflag & TEX_USEALPHA) && (ima->flag & IMA_IGNORE_ALPHA) == 0) {
+    if ((tex->imaflag & TEX_USEALPHA) && (ima->alpha_mode != IMA_ALPHA_IGNORE)) {
       if ((tex->imaflag & TEX_CALCALPHA) == 0) {
         texres->talpha = true;
       }