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
Using 'r' to name the third component is problematic because that is commonly used to represent the red component of a vector representing a color. Under GLSL this is awkward because the texture swizzling for a vector uses a single letter for each component and the names for colors, textures and positions are synonymous. GLSL works around this by naming the components of the texture s, t, p and q. Cogl already effectively already exposes this naming because it exposes GLSL so it makes sense to use that naming consistently. Another alternative could be u, v and w. This is what Blender and Direct3D use. However the w component conflicts with the w component of a position vertex.