add GenericImage and GenericRGBImage alias

[johnnychen94]

Cont. #76

GenericImage is more general than AbstractArray{<:Colorant}, the latter doesn't catch AbstractArray{<:Number}

• PixelLike
• GenericImage
• GenericRGBImage
• RGB2dImage

[codecov]

Codecov Report

Merging #80 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   66.17%   66.17%           
=======================================
  Files           9        9           
  Lines         408      408           
=======================================
  Hits          270      270           
  Misses        138      138

Impacted Files	Coverage Δ
src/ImageCore.jl	61.11% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #80 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #80   +/-   ##
=======================================
  Coverage   66.17%   66.17%           
=======================================
  Files           9        9           
  Lines         408      408           
=======================================
  Hits          270      270           
  Misses        138      138

Impacted Files	Coverage Δ
src/ImageCore.jl	61.11% <ø> (ø)	⬆️

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[timholy]

I'm a little bothered by an explosion of names that do not yet have obvious value. Perhaps some illustrations showing what they make better?

[johnnychen94]

These three seems quite redundant according to my use experiences

const RealLike{T<:Real} = NumberLike{T}
const FloatLike{T<:AbstractFloat} = RealLike{T}
const FractionalLike{T<:Union{FixedPoint, AbstractFloat}} = RealLike{T}

They appear to be some kind of short alias in the beginning, for example, in ImageDistances

function colwise(dist::PreMetric,
                 a::AbstractMatrix{<:GenericImage},
                 b::AbstractMatrix{<:GenericImage})
    n = get_common_ncols(a, b)
    r = Vector{result_type(dist, a, b)}(undef, n)
    colwise!(r, dist, a, b)
end

This annotation is more readable than

colwise(dist::PreMetric,
                 a::AbstractMatrix{<:AbstractArray{Union{<:Colorant, <:Number}}},
                 b::AbstractMatrix{<:AbstractArray{Union{<:Colorant, <:Number}}})

Another advantage I didn't check yet, is to provide a similar power of traits. Hypothetically speaking,

# ImageMesh.jl
struct Pixel{T<:Point, CT<:Colorant}
    pos::T
    value::CT
end

# AnotherPackage.jl
using ImageCore
using ImageMesh

const GenericImage = Union{ImageCore.GenericImage, AbstractArray{<:Pixel}}

foo(img::GenericImage) # extend the GenericImage

---

However, I'm not very certain of it.

[timholy]

These three seems quite redundant according to my use experiences

That's perhaps the point: why add types that are not actually necessary for anything? The complexity of a project is not entirely independent of length(names(PackageName)). Prior to #76, it looks like NumberLike was widely used, FloatLike and FractionalLike weren't used at all, and RealLike existed only in a couple of isolated places in Images.jl and probably could have used NumberLike instead. That doesn't seem to have changed since they were added, arguing that perhaps the new types weren't necessary.

This annotation is more readable than

colwise(dist::PreMetric,
                 a::AbstractMatrix{<:AbstractArray{Union{<:Colorant, <:Number}}},
                 b::AbstractMatrix{<:AbstractArray{Union{<:Colorant, <:Number}}})

I see value in these:

const NumberLike = Union{Number,AbstractGray}
const Pixel = Union{Number,Colorant}
const GenericGrayImage{T<:NumberLike,N} = AbstractArray{T,N}
const GenericImage{T<:Pixel,N} = AbstractArray{T,N}

I just don't understand why we can't stop there. (I can also live with Generic2dImage, though note it's only 3 characters shorter than GenericImage{T,2}. I know we have AbstractVector and AbstractMatrix, but in linear algebra those correspond to important concepts, whereas in image processing dimensionality is only rarely important from a conceptual standpoint.) I really don't want to get to Generic2dAbstractRGBImageWithN0f8OrN0f16ElementType.

In general I'd add new names on a need-to-have basis.