Cycles/Eevee: unified and improved texture image color space handling

Cycles now uses the color space on the image datablock, and uses OpenColorIO
to convert to scene linear as needed. Byte images do not take extra memory,
they are compressed in scene linear + sRGB transfer function which in common
cases is a no-op.

Eevee and workbench were changed to work similar. Float images are stored as
scene linear. Byte images are compressed as scene linear + sRGB and stored in
a GL_SRGB8_ALPHA8 texture. From the GLSL shader side this means they are read
as scene linear, simplifying the code and taking advantage of hardware support.

Further, OpenGL image textures are now all stored with premultiplied alpha.
Eevee texture sampling looks a little different now because interpolation
happens premultiplied and in scene linear space.

Overlays and grease pencil work in sRGB space so those now have an extra
conversion to sRGB after reading from image textures. This is not particularly
elegant but as long as engines use different conventions, one or the other
needs to do conversion.

This change breaks compatibility for cases where multiple image texture nodes
were using the same image with different color space node settings. However it
gives more predictable behavior for baking and texture painting if save, load
and image editing operations have a single color space to handle.

Differential Revision: https://developer.blender.org/D4807

1 files changed, 8 insertions, 22 deletions

diff --git a/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl b/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
index c76ad8c1d7b..96f8f6e4c7a 100644
--- a/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
+++ b/source/blender/draw/engines/workbench/shaders/workbench_common_lib.glsl
@@ -140,32 +140,18 @@ vec2 matcap_uv_compute(vec3 I, vec3 N, bool flipped)
   return matcap_uv * 0.496 + 0.5;
 }
 
-float srgb_to_linearrgb(float c)
-{
-  if (c < 0.04045) {
-    return (c < 0.0) ? 0.0 : c * (1.0 / 12.92);
-  }
-  else {
-    return pow((c + 0.055) * (1.0 / 1.055), 2.4);
-  }
-}
-
-vec4 srgb_to_linearrgb(vec4 col_from)
-{
-  vec4 col_to;
-  col_to.r = srgb_to_linearrgb(col_from.r);
-  col_to.g = srgb_to_linearrgb(col_from.g);
-  col_to.b = srgb_to_linearrgb(col_from.b);
-  col_to.a = col_from.a;
-  return col_to;
-}
-
-vec4 workbench_sample_texture(sampler2D image, vec2 coord, bool srgb, bool nearest_sampling)
+vec4 workbench_sample_texture(sampler2D image, vec2 coord, bool nearest_sampling)
 {
   vec2 tex_size = vec2(textureSize(image, 0).xy);
   /* TODO(fclem) We could do the same with sampler objects.
    * But this is a quick workaround instead of messing with the GPUTexture itself. */
   vec2 uv = nearest_sampling ? (floor(coord * tex_size) + 0.5) / tex_size : coord;
   vec4 color = texture(image, uv);
-  return (srgb) ? srgb_to_linearrgb(color) : color;
+
+  /* Unpremultiply, ideally shaders would be added so this is not needed. */
+  if (!(color.a == 0.0 || color.a == 1.0)) {
+    color.rgb = color.rgb / color.a;
+  }
+
+  return color;
 }