Conversion from Lab -> RGB -> Lab is wrong for some inputs about colors.jl

[rdeits]

This started happening with ColorTypes v0.6.1 because that version updated gamutmax(::Lab) and gamutmin(::Lab), which caused rand(Lab{Float32}) to start drawing from a larger space, which caused QuartzImageIO to encounter this pre-existing issue.

[rdeits]

I suspect the problem is with

Colors.jl/src/conversions.jl

Line 303 in 7ce73e6

function cnvt(::Type{XYZ{T}}, c::Lab, wp::XYZ) where T

returning negative XYZ components for this particular input, but I don't know enough about color spaces to understand what's going on.

[rdeits]

From comparison against https://github.com/THEjoezack/ColorMine/blob/master/ColorMine/ColorSpaces/Conversions/LabConverter.cs it appears that Colors.jl uses a default white-point of (0.95, 1.00, 1.08), but that ColorMine uses (95, 100, 108), which makes all of the XYZ results produced by ColorMine 100x larger than those from Colors.jl (but all of the magic constants used in the Lab->XYZ conversion are the same for both libraries). So I suspect the conversion above should be adjusted to reflect the fact that Colors.jl is using a different XYZ gamut than ColorMine

[rdeits]

Okay, I finally understand what's going on. That particular Lab value is outside the range that can be represented in RGB (with values between 0 and 1), so information is necessarily lost in the conversion. I will update the QuartzImageIO tests accordingly.

[KelSolaar]

outside the range that can be represented in RGB

RGB here is sRGB, that value might possibly be encoded by another RGB colourspace with a larger gamut, e.g. Adobe RGB 1998, DCI-P3, BT.2020 or ACES2065-1.

In that particular case it cannot be encoded by ACES2065-1 which encompasses the whole visible spectrum, so it is not a colour :)

sRGB (Linear)       :  [-0.04445121  0.0303619  -0.04719857]
ACES 2065-1 (Linear):  [-0.01621534  0.01624061 -0.03871465]

[rdeits]

Ah, cool, thanks! I've still got some catching up to do ;-)

[timholy]

Thanks for figuring this out!