Implement promotion in multiplication (#141)

Fixes #109

Author: timholy

README.md

@@ -83,7 +83,7 @@ RGBRGB{Float64}(
 
 Note that `c1⋅c2 = (c1.r*c2.r + c1.g*c2.g + c1.b*c2.b)/3`, where the division by 3 ensures the equivalence `norm(x) == norm(Gray(x)) == norm(RGB(x, x, x))`.
 
-It is designed to not support the ordinary multiplication operation `*` because it is not obvious which one of these should be the default option.
+Ordinary multiplication `*` is not supported because it is not obvious which one of these should be the default option.
 
 However, `*` is defined for grayscale since all these three multiplication operations (i.e., `⋅`, `⊙` and `⊗`) are equivalent in the 1D vector space.
 

src/ColorVectorSpace.jl

@@ -180,8 +180,9 @@ end
 
 # New multiplication operators
 (⋅)(x::AbstractRGB, y::AbstractRGB)  = (T = acctype(eltype(x), eltype(y)); T(red(x))*T(red(y)) + T(green(x))*T(green(y)) + T(blue(x))*T(blue(y)))/3
+(⋅)(x::Union{AbstractRGB,AbstractGray}, y::Union{AbstractRGB,AbstractGray})  = ⋅(promote(x, y)...)
 (⊙)(x::C, y::C) where C<:AbstractRGB = base_color_type(C)(red(x)*red(y), green(x)*green(y), blue(x)*blue(y))
-(⊙)(x::AbstractRGB, y::AbstractRGB)  = ⊙(promote(x, y)...)
+(⊙)(x::Union{AbstractRGB,AbstractGray}, y::Union{AbstractRGB,AbstractGray})  = ⊙(promote(x, y)...)
 # ⊗ defined below
 
 isfinite(c::Colorant{T}) where {T<:Normed} = true
@@ -362,6 +363,7 @@ function ⊗(a::AbstractRGB, b::AbstractRGB)
     agbr, abbg, arbb, abbr, arbg, agbb = ag*br, ab*bg, ar*bb, ab*br, ar*bg, ag*bb
     return RGBRGB(ar*br, agbr, abbr, arbg, ag*bg, abbg, arbb, agbb, ab*bb)
 end
+⊗(a::Union{AbstractRGB,AbstractGray}, b::Union{AbstractRGB,AbstractGray}) = ⊗(promote(a, b)...)
 
 """
     varmult(op, itr; corrected::Bool=true, mean=Statistics.mean(itr), dims=:)

test/runtests.jl

@@ -466,6 +466,12 @@ ColorTypes.blue(c::RatRGB)  = c.b
         @test RGB24(0.4, 0.6, 0.5) - AGray32(0.4, 0.2) === ARGB32(0, 0.2, 0.1, 0.8)
         @test ARGB32(0.4, 0, 0.2, 0.5) + Gray24(0.4)   === ARGB32(0.8, 0.4, 0.6, 0.5N0f8+1N0f8)
         @test ARGB32(0.4, 0, 0.2, 0.5) + AGray32(0.4, 0.2) === ARGB32(0.8, 0.4, 0.6, 0.5N0f8+0.2N0f8)
+
+        g, rgb = Gray(0.2), RGB(0.1, 0.2, 0.3)
+        @test g ⋅ rgb == rgb ⋅ g ≈ 0.2*(0.1 + 0.2 + 0.3)/3
+        @test g ⊙ rgb == rgb ⊙ g ≈ RGB(0.2*0.1, 0.2^2, 0.2*0.3)
+        @test g ⊗ rgb == RGB(g) ⊗ rgb
+        @test rgb ⊗ g == rgb ⊗ RGB(g)
     end
 
     @testset "Custom RGB arithmetic" begin