Code to downsample

Downsampling.jl

module Downsampling

using TiledIteration, BenchmarkTools
I = rand(UInt16, 10000, 10000);
D = zeros(size(I).÷2...);

function avgNxN!(I, D, ::Val{N}) where N
    for c = 1:size(D,2), bc = 1-N:0, r = 1:size(D,1), br = 1-N:0
        @inbounds D[r,c] += I[r*N+br, c*N+bc]/N/N
    end
end
avg2x2!(I, D) = avgNxN!(I, D, Val(2))
avg4x4!(I, D) = avgNxN!(I, D, Val(4))
avg2x2(I) = avg2x2!(I, zeros(size(I).÷2...))
avg4x4(I) = avg4x4!(I, zeros(size(I).÷4...))
        
precompile(avg2x2!, (typeof(I), typeof(D)) )
precompile(avg4x4!, (typeof(I), typeof(D)) )
precompile(avg2x2, (typeof(I),) )
precompile(avg4x4, (typeof(I),) )

function tileavg!(I, D, factor, avgfun)
    tiled_I = collect(TileIterator(axes(I), size(I).÷factor))
    tiled_D = collect(TileIterator(axes(D), size(D).÷factor))
    Threads.@threads for i = eachindex(tiled_I)
        @views avgfun(I[tiled_I[i]...], D[tiled_D[i]...])
    end
end 
tileavg2x2!(I, D, factor=100) = tileavg!(I, D, factor, avg2x2!)
tileavg4x4!(I, D, factor=50) = tileavg!(I, D, factor, avg4x4!)
tileavg2x2(I, factor=100) = tileavg2x2!(I, zeros(size(I).÷2...), factor)
tileavg4x4(I, factor=50) = tileavg2x2!(I, zeros(size(I).÷4...), factor)

precompile(tileavg2x2!, (typeof(I), typeof(D)) )
precompile(tileavg4x4!, (typeof(I), typeof(D)) )
precompile(tileavg2x2, (typeof(I),) )
precompile(tileavg4x4, (typeof(I),) )

function demo(I)
    D = zeros(size(I).÷2...);
    avg2x2!(I, D)
    @info "Testing avg2x2!" sum(float(I)) sum(D)*4 sum(float(I)) == sum(D)*4 
    @info "Benchmarking avg2x2!"
    display(@benchmark avg2x2!($I, $D))
    @info "Benchmarking avg2x2"
    display(@benchmark avg2x2($I))
    @info "Benchmarking tileavg2x2!"
    display(@benchmark tileavg2x2!($I, $D))
    @info "Benchmarking tileavg2x2"
    display(@benchmark tileavg2x2($I))
        
    D = zeros(size(I).÷4...);
    avg4x4!(I, D)
    @info "Testing avg4x4!" sum(float(I)) sum(D)*16 sum(float(I)) == sum(D)*16

    @info "Benchmarking avg4x4!"
    display(@benchmark avg4x4!($I, $D))
    @info "Benchmarking avg4x4"
    display(@benchmark avg4x4($I))
    @info "Benchmarking tileavg4x4!"
    display(@benchmark tileavg4x4!($I, $D))
    @info "Benchmarking tileavg4x4"
    display(@benchmark tileavg4x4($I))
end
    
end