Necessity of `color_rettype`

[kimikage]

The color_rettype has been around since the initial working commit of ColorVectorSpace, but now, I'm not sure what the point of it is.

ColorVectorSpace.jl/src/ColorVectorSpace.jl

Lines 76 to 82 in 85b8759

	# Scalar binary RGB operations require the same RGB type for each element,
	# otherwise we don't know which to return
	color_rettype(::Type{A}, ::Type{B}) where {A<:AbstractRGB,B<:AbstractRGB} = _color_rettype(base_colorant_type(A), base_colorant_type(B))
	color_rettype(::Type{A}, ::Type{B}) where {A<:AbstractGray,B<:AbstractGray} = _color_rettype(base_colorant_type(A), base_colorant_type(B))
	color_rettype(::Type{A}, ::Type{B}) where {A<:TransparentRGB,B<:TransparentRGB} = _color_rettype(base_colorant_type(A), base_colorant_type(B))
	color_rettype(::Type{A}, ::Type{B}) where {A<:TransparentGray,B<:TransparentGray} = _color_rettype(base_colorant_type(A), base_colorant_type(B))
	_color_rettype(::Type{C}, ::Type{C}) where {C<:Colorant} = C

The revamp of the color promotion rules is relatively recent (i.e. it was in ColorTypes v0.10.0), but I think there is less ambiguity now. As you know, ColorVectorSpace v0.9 requires ColorTypes v0.10.

julia> RGB24(0.1, 0.2, 0.3) + RGB{Float32}(0.4, 0.5, 0.6)
ERROR: MethodError: no method matching _color_rettype(::Type{RGB24}, ::Type{RGB})
In binary operation with Type{RGB24} and Type{RGB}, the return type is ambiguous

julia> BGR(0.1, 0.2, 0.3) + RGB{N0f8}(0.4, 0.5, 0.6)
ERROR: MethodError: no method matching _color_rettype(::Type{BGR}, ::Type{RGB})
In binary operation with Type{BGR} and Type{RGB}, the return type is ambiguous

julia> promote_type(RGB24, RGB)
RGB

julia> promote_type(BGR, RGB)
RGB

[kimikage]

If we keep the color_rettype, we can generalize it as follows.

color_rettype(::Type{C1}, ::Type{C2}) where {C1, C2} = base_colorant_type(promote_type(C1, C2))
color_rettype(c1::Colorant, c2::Colorant) = color_rettype(typeof(c1), typeof(c2))

[kimikage]

Here is another example of inconsistency in this:

julia> Gray24(1) * Gray{N0f8}(1)
ERROR: MethodError: no method matching _color_rettype(::Type{Gray24}, ::Type{Gray})
In binary operation with Type{Gray24} and Type{Gray}, the return type is ambiguous

julia> Gray24(1) ⊙ Gray{N0f8}(1)
Gray{N0f8}(1.0)