move storage type conversion trait from ImageCore to Colors

[johnnychen94]

It looks like the most appropriate place for these traits is here:

https://github.com/JuliaImages/ImageCore.jl/blob/4fb132187d68054be7413ec229071115c14f72bd/src/convert_reinterpret.jl#L61-L107

[kimikage]

I have no objection to "defining" them in Colors or ColorTypes, but I am not comfortable exporting them. 😕

I can fully understand why you would prefer to have them defined in Colors (or ColorTypes), but I don't see what the trouble is with having them in ImageCore.

The same goes for clamp01 and clamp01nan.(cf. #469 (comment))

[kimikage]

Also, as you know, there is the inconsistency problem with float other than for FixedPoint/AbstractFloat.

[johnnychen94]

I have no objection to "defining" them in Colors or ColorTypes, but I am not comfortable exporting them.

I'm okay with this since it's mainly the package developer that needs to take care of the storage type.

My main consideration when proposing this move is that I want to give better support for them on broader color types. Currently,

julia> n0f8(RGB)
RGB{N0f8}

julia> n0f8(RGB{Float32})
RGB{N0f8}

julia> n0f8(RGB24)
ERROR: TypeError: in Type{...} expression, expected UnionAll, got Type{RGB24}
Stacktrace:
 [1] n0f8(#unused#::Type{RGB24})
   @ ImageCore ~/.julia/packages/ImageCore/iXG0W/src/convert_reinterpret.jl:82
 [2] top-level scope
   @ REPL[21]:1

This need arises when I tried to convert all colorant images to N0f8/UInt8 byte sequences:

canonical_colorant_type(::Type{CT}) where CT<:Colorant = n0f8(CT)
canonical_colorant_type(::Type{T}) where T<:Real = Gray{N0f8}

but I don't see what the trouble is with having them in ImageCore.

The only reason that I feel it better to live in Colors is that they are pixel-level operations. 😄

[kimikage]

Even if we do the migration, I don't see any reason not to fix n0f8(RGB24) etc. in the current ImageCore. (I'm not sure whether n0f8(RGB24) should return RGB{N0f8} or RGB24, though.)

[kimikage]

I think it's straightforward to define n0f8 and so on in FixedPointNumbers and implement n0f8(::Colorant) and so on in ColorTypes. However, I'm not sure that it's really a good idea for FixedPointNumbers to provide such non-generic functions.
Also, I don't think float32 and float64 are functions that should be exported by the low-level packages.

However, I think the following should be solved within ColorTypes.

julia> convert(Colorant{Float32}, RGB24(1))
ERROR: nonparametric type RGB24 has ambiguous destination Colorant{Float32, N} where N

(Of course, this is a known issue:
https://github.com/JuliaGraphics/ColorTypes.jl/blob/81297fff41e28dc64ace72a37c090ee22363ba71/test/conversions.jl#L446)

[johnnychen94]

Makes sense to me; either move or not are good. We can keep this issue open until we decide the 1.0 status.