pack several .png files into an .ico flie in Julia.

pack_ico.jl

function pack_ico(io::IO, pngs::Bytes...)
    # all values are in little endian

    # header
    write(io, 0x0000) # reserved to be 0
    write(io, 0x0001) # 1 for .ico, 2 for .cur
    write(io, length(pngs) % UInt16) # number of images

    # image directory
    c = 6 + 16length(pngs) # offsets of current image, initial value is the header length
    for png in pngs
        write(io, png[20], png[24]) # width, height, big-endian
        write(io, 0x0000) # color platte and reserved
        write(io, 0x0001) # color planes
        write(io, 0x0020) # bits per pixel
        write(io, length(png) % UInt32) # size
        write(io, c % UInt32)
        c += length(png)
    end

    # content
    for png in pngs
        write(io, png)
    end
end

pack_ico(pngs::Bytes...) = let io = IOBuffer()
    pack_ico(io, pngs)
    take!(io)
end