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, 2 insertions, 1 deletions

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 {