Blenlib: Explicit Colors.

Colors are often thought of as being 4 values that make up that can make any color.
But that is of course too limited. In C we didn’t spend time to annotate what we meant
when using colors.

Recently `BLI_color.hh` was made to facilitate color structures in CPP. CPP has possibilities to
enforce annotating structures during compilation and can adds conversions between them using
function overloading and explicit constructors.

The storage structs can hold 4 channels (r, g, b and a).

Usage:

Convert a theme byte color to a linearrgb premultiplied.
```
ColorTheme4b theme_color;
ColorSceneLinear4f<eAlpha::Premultiplied> linearrgb_color =
    BLI_color_convert_to_scene_linear(theme_color).premultiply_alpha();
```

The API is structured to make most use of inlining. Most notable are space
conversions done via `BLI_color_convert_to*` functions.

- Conversions between spaces (theme <=> scene linear) should always be done by
  invoking the `BLI_color_convert_to*` methods.
- Encoding colors (compressing to store colors inside a less precision storage)
  should be done by invoking the `encode` and `decode` methods.
- Changing alpha association should be done by invoking `premultiply_alpha` or
  `unpremultiply_alpha` methods.

# Encoding.

Color encoding is used to store colors with less precision as in using `uint8_t` in
stead of `float`. This encoding is supported for `eSpace::SceneLinear`.
To make this clear to the developer the `eSpace::SceneLinearByteEncoded`
space is added.

# Precision

Colors can be stored using `uint8_t` or `float` colors. The conversion
between the two precisions are available as methods. (`to_4b` and
`to_4f`).

# Alpha conversion

Alpha conversion is only supported in SceneLinear space.

Extending:
- This file can be extended with `ColorHex/Hsl/Hsv` for different representations
  of rgb based colors. `ColorHsl4f<eSpace::SceneLinear, eAlpha::Premultiplied>`
- Add non RGB spaces/storages ColorXyz.

Reviewed By: JacquesLucke, brecht

Differential Revision: https://developer.blender.org/D10978

1 files changed, 11 insertions, 9 deletions

diff --git a/source/blender/blenkernel/intern/geometry_component_mesh.cc b/source/blender/blenkernel/intern/geometry_component_mesh.cc
index 9e622ab2cdf..42f3a854aec 100644
--- a/source/blender/blenkernel/intern/geometry_component_mesh.cc
+++ b/source/blender/blenkernel/intern/geometry_component_mesh.cc
@@ -773,18 +773,20 @@ static void set_loop_uv(MLoopUV &uv, float2 co)
   copy_v2_v2(uv.uv, co);
 }
 
-static Color4f get_loop_color(const MLoopCol &col)
+static ColorGeometry4f get_loop_color(const MLoopCol &col)
 {
-  Color4f srgb_color;
-  rgba_uchar_to_float(srgb_color, &col.r);
-  Color4f linear_color;
-  srgb_to_linearrgb_v4(linear_color, srgb_color);
+  ColorGeometry4b encoded_color = ColorGeometry4b(col.r, col.g, col.b, col.a);
+  ColorGeometry4f linear_color = encoded_color.decode();
   return linear_color;
 }
 
-static void set_loop_color(MLoopCol &col, Color4f linear_color)
+static void set_loop_color(MLoopCol &col, ColorGeometry4f linear_color)
 {
-  linearrgb_to_srgb_uchar4(&col.r, linear_color);
+  ColorGeometry4b encoded_color = linear_color.encode();
+  col.r = encoded_color.r;
+  col.g = encoded_color.g;
+  col.b = encoded_color.b;
+  col.a = encoded_color.a;
 }
 
 static float get_crease(const MEdge &edge)
@@ -1121,8 +1123,8 @@ static ComponentAttributeProviders create_attribute_providers_for_mesh()
       CD_PROP_COLOR,
       CD_MLOOPCOL,
       corner_access,
-      make_derived_read_attribute<MLoopCol, Color4f, get_loop_color>,
-      make_derived_write_attribute<MLoopCol, Color4f, get_loop_color, set_loop_color>);
+      make_derived_read_attribute<MLoopCol, ColorGeometry4f, get_loop_color>,
+      make_derived_write_attribute<MLoopCol, ColorGeometry4f, get_loop_color, set_loop_color>);
 
   static VertexGroupsAttributeProvider vertex_groups;
   static CustomDataAttributeProvider corner_custom_data(ATTR_DOMAIN_CORNER, corner_access);