Implement a default offset option for all higher dimensions, to support a more complete centered

[BioTurboNick]

Fixes #339 , but requires support from Julia Base to call the right axes methods. The API for this is probably not right, and obviously needs tests and docs. But would like feedback first.

[jishnub]

One concern that I see is that a lot of code uses Base.require_one_based_indexing to shield against offset axes, but this does not check the axes beyond the ndims of an array. Perhaps we need better support from Base for offset trailing axes. IIUC, this PR would mean that the transpose of a one-based OffsetVector would end up being offset if the default offset is non-zero.

[BioTurboNick]

IUC, this PR would mean that the transpose of a one-based OffsetVector would end up being offset if the default offset is non-zero.

Yes, exactly.

One concern that I see is that a lot of code uses Base.require_one_based_indexing to shield against offset axes, but this does not check the axes beyond the ndims of an array.

Though couldn't OffsetArrays provide an implementation of that?

[BioTurboNick]

require_one_based_indexing relies on has_offset_axes, which I provided an implementation of here.

[BioTurboNick]

Seems I have a StackOverflow issue in one of the constructors. Would it be better as a kwarg?

[jishnub]

I'm a bit unsure of this PR's implications, e.g.

julia> v = OffsetVector(1:2, (0,), 3)
1:2 with indices 1:2

julia> v'
1×2 adjoint(OffsetArray(::UnitRange{Int64}, 1:2)) with eltype Int64 with indices 4:4×1:2:
 1  2

julia> view(v,:)'
1×2 adjoint(view(OffsetArray(::UnitRange{Int64}, 1:2), :)) with eltype Int64 with indices Base.OneTo(1)×OffsetArrays.IdOffsetRange(values=1:2, indices=1:2):
 1  2

julia> v == view(v,:)
true

julia> v' == view(v,:)'
false

so wrappers will behave differently as they are not inheriting the trailing axes of the parent

[BioTurboNick]

Hmm, so it's using the axes(A::AbstractArray{T,N}, d) method, which returns hardcoded OneTo(1) for trailing dimensions.

So there could be an axes(AA::SubArray, d) that passes through to the parent.

Although views of OffsetArrays seem to behave inconsisently?

julia> v = OffsetVector(1:2, (-1,))
1:2 with indices 0:1

julia> axes(view(v, :))
(Base.IdentityUnitRange(OffsetArrays.IdOffsetRange(values=0:1, indices=0:1)),)

julia> axes(view(v, 0:1))
(Base.OneTo(2),)

Why does : preserve parent indexing while specifying the range resets them to 1:n?

[jishnub]

This is because the axes of an indexing operation are the axes of the indices, and a : is transformed to a Base.Slice internally, which has the same indices as its values. On the other hand, the axes of a UnitRange are 1-based, and are unrelated to the starting point of the range. This is one of the fundamental (and less intuitive) aspects of offset indexing, and I had this same question answered by Tim Holy three years back

[BioTurboNick]

Oy. Ok.

Given how tangled this is... maybe the easiest solution is to take defensive measures in the ImageFiltering functions.

[jishnub]

Closing, as this doesn't seem like something that we'll do here.