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

2 files changed, 44 insertions, 2 deletions

diff --git a/source/blender/draw/modes/shaders/object_empty_image_frag.glsl b/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
index 386d05636f9..dbc403dc39b 100644
--- a/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
+++ b/source/blender/draw/modes/shaders/object_empty_image_frag.glsl
@@ -14,12 +14,33 @@ uniform sampler2D image;
 uniform int depthMode;
 uniform bool useAlphaTest;
 
+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;
+  }
+}
+
+vec4 texture_read_as_srgb(sampler2D tex, vec2 co)
+{
+  /* By convention image textures return scene linear colors, but
+   * overlays still assume srgb. */
+  vec4 color = texture2D(tex, co);
+  color.r = linearrgb_to_srgb(color.r);
+  color.g = linearrgb_to_srgb(color.g);
+  color.b = linearrgb_to_srgb(color.b);
+  return color;
+}
+
 void main()
 {
 #ifdef USE_WIRE
   fragColor = finalColor;
 #else
-  vec4 tex_col = texture(image, texCoord_interp);
+  vec4 tex_col = texture_read_as_srgb(image, texCoord_interp);
   fragColor = finalColor * tex_col;
 
   if (useAlphaTest) {
diff --git a/source/blender/draw/modes/shaders/paint_texture_frag.glsl b/source/blender/draw/modes/shaders/paint_texture_frag.glsl
index c7e110122c5..5d74213a445 100644
--- a/source/blender/draw/modes/shaders/paint_texture_frag.glsl
+++ b/source/blender/draw/modes/shaders/paint_texture_frag.glsl
@@ -16,6 +16,27 @@ uniform vec3 maskingColor;
 uniform bool maskingInvertStencil;
 #endif
 
+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;
+  }
+}
+
+vec4 texture_read_as_srgb(sampler2D tex, vec2 co)
+{
+  /* By convention image textures return scene linear colors, but
+   * overlays still assume srgb. */
+  vec4 color = texture2D(tex, co);
+  color.r = linearrgb_to_srgb(color.r);
+  color.g = linearrgb_to_srgb(color.g);
+  color.b = linearrgb_to_srgb(color.b);
+  return color;
+}
+
 void main()
 {
   vec2 uv = uv_interp;
@@ -24,7 +45,7 @@ void main()
     uv = (floor(uv_interp * tex_size) + 0.5) / tex_size;
   }
 
-  vec4 color = texture(image, uv);
+  vec4 color = texture_read_as_srgb(image, uv);
   color.a *= alpha;
 
 #ifdef TEXTURE_PAINT_MASK