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, 5 insertions, 3 deletions

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,
 };
 
 /** \} */