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

1 files changed, 14 insertions, 3 deletions

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;