Overflow in ContrastStretching

[Tim Holy]

adjust_histogram(img, ContrastStretching()) is suggested as a deprecation for imstretch; with --depwarn=yes, we get

julia> using Images

julia> img = Gray{N0f8}.([1 0; 0 1])
2×2 Array{Gray{N0f8},2} with eltype Gray{N0f8}:
 Gray{N0f8}(1.0)  Gray{N0f8}(0.0)
 Gray{N0f8}(0.0)  Gray{N0f8}(1.0)

julia> imgs = imstretch(img, 0.3, 0.4)
┌ Warning: `imstretch` will be removed in a future release, please use `adjust_histogram(img, ContrastStretching())` instead.
│   caller = top-level scope at REPL[3]:1
└ @ Core REPL[3]:1
2×2 Array{Gray{Float64},2} with eltype Gray{Float64}:
 Gray{Float64}(0.618123)    Gray{Float64}(0.00274462)
 Gray{Float64}(0.00274462)  Gray{Float64}(0.618123)

(As pointed out in JuliaImages/Images.jl#865 (comment), it would have been nice if this had suggested "please use adjust_histogram(img, ContrastStretching(; t=0.3, slope=0.4)) instead so that it was copy/pastable.)

But if we follow the suggestion, we get:

julia> adjust_histogram(img, ContrastStretching(t=0.3, slope=0.4))
2×2 Array{Gray{N0f8},2} with eltype Gray{N0f8}:
 Gray{N0f8}(0.0)    Gray{N0f8}(0.149)
 Gray{N0f8}(0.149)  Gray{N0f8}(0.0)

which is very different. However:

julia> adjust_histogram(float.(img), ContrastStretching(t=0.3, slope=0.4))
2×2 Array{Gray{Float32},2} with eltype Gray{Float32}:
 Gray{Float32}(0.618123)    Gray{Float32}(0.00274462)
 Gray{Float32}(0.00274462)  Gray{Float32}(0.618123)

indicating that it's overflow at fault.