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, 15 insertions, 32 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_material.glsl b/source/blender/gpu/shaders/gpu_shader_material.glsl
index 2f80bc95a0c..5596940cbf6 100644
--- a/source/blender/gpu/shaders/gpu_shader_material.glsl
+++ b/source/blender/gpu/shaders/gpu_shader_material.glsl
@@ -117,38 +117,6 @@ void hsv_to_rgb(vec4 hsv, out vec4 outcol)
   outcol = vec4(rgb, hsv.w);
 }
 
-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);
-}
-
-float linearrgb_to_srgb(float c)
-{
-  if (c < 0.0031308)
-    return (c < 0.0) ? 0.0 : c * 12.92;
-  else
-    return 1.055 * pow(c, 1.0 / 2.4) - 0.055;
-}
-
-void srgb_to_linearrgb(vec4 col_from, out 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;
-}
-
-void linearrgb_to_srgb(vec4 col_from, out vec4 col_to)
-{
-  col_to.r = linearrgb_to_srgb(col_from.r);
-  col_to.g = linearrgb_to_srgb(col_from.g);
-  col_to.b = linearrgb_to_srgb(col_from.b);
-  col_to.a = col_from.a;
-}
-
 void color_to_normal_new_shading(vec3 color, out vec3 normal)
 {
   normal = vec3(2.0) * color - vec3(1.0);
@@ -2330,6 +2298,21 @@ void node_tex_environment_empty(vec3 co, out vec4 color)
 /* 16bits floats limits. Higher/Lower values produce +/-inf. */
 #define safe_color(a) (clamp(a, -65520.0, 65520.0))
 
+void tex_color_alpha_clear(vec4 color, out vec4 result)
+{
+  result = vec4(color.rgb, 1.0);
+}
+
+void tex_color_alpha_unpremultiply(vec4 color, out vec4 result)
+{
+  if (color.a == 0.0 || color.a == 1.0) {
+    result = vec4(color.rgb, 1.0);
+  }
+  else {
+    result = vec4(color.rgb / color.a, 1.0);
+  }
+}
+
 void node_tex_image_linear(vec3 co, sampler2D ima, out vec4 color, out float alpha)
 {
   color = safe_color(texture(ima, co.xy));