qoi_decoder.sf

#!/usr/bin/ruby

# Implementation of the QOI decoder (generating a PNG file).

# See also:
#   https://qoiformat.org/
#   https://github.com/phoboslab/qoi

require('Imager')

func qoi_decoder(bytes) {

    func decode32(a,b,c,d) {
        (a << 8*3) | (b << 8*2) | (c << 8*1) | (d << 8*0)
    }

    func invalid() {
        die "Not a QOIF image"
    }

    var index = 0
    [bytes[index..index+3]] == %b'qoif' || invalid()
    index += 4

    var width  = decode32(bytes[index..index+3])
    index += 4
    var height = decode32(bytes[index..index+3])
    index += 4

    var channels   = bytes[index++]
    var colorspace = bytes[index++]

    width>0 && height>0 || invalid()
    channels   ~~ 1..4  || invalid()
    colorspace ~~ [0,1] || invalid()

    bytes.pop == 0x01 || invalid()
    7.times { bytes.pop == 0x00 || invalid() }

    say [width, height, channels, colorspace]

    var img = %O<Imager>.new(
        xsize    => width,
        ysize    => height,
        channels => channels,
    )

    var run = 0
    var (R, G, B, A) = (0, 0, 0, 255)

    var pixels = []
    var colors = 64.of { [0,0,0,0] }

    loop {

        if (run > 0) {
            --run
        }
        else {
            var byte = (bytes[index++] \\ break)

            if (byte == 0b_11_11_11_10) {   # OP RGB
                R = bytes[index++]
                G = bytes[index++]
                B = bytes[index++]
            }
            elsif (byte == 0b_11_11_11_11) { # OP RGBA
                R = bytes[index++]
                G = bytes[index++]
                B = bytes[index++]
                A = bytes[index++]
            }
            elsif (byte >> 6 == 0b00) {     # OP INDEX
                (R, G, B, A) = colors[byte]...
            }
            elsif (byte >> 6 == 0b01) {     # OP DIFF
                var dr = (byte & 0b00_11_00_00 >> 4)-2
                var dg = (byte & 0b00_00_11_00 >> 2)-2
                var db = (byte & 0b00_00_00_11 >> 0)-2

                R.addmod!(dr, 256)
                G.addmod!(dg, 256)
                B.addmod!(db, 256)
            }
            elsif (byte >> 6 == 0b10) {   # OP LUMA
                var byte2 = bytes[index++]

                var dg    = (byte  & 0b00_111_111)-32
                var dr_dg = (byte2 >> 4)-8
                var db_dg = (byte2 & 0b0000_1111)-8

                var dr = (dr_dg+dg)
                var db = (db_dg+dg)

                R.addmod!(dr, 256)
                G.addmod!(dg, 256)
                B.addmod!(db, 256)
            }
            elsif (byte >> 6 == 0b11) {     # OP RUN
                run = (byte & 0b00_111_111)
            }

            colors[sum(3*R, 5*G, 7*B, 11*A)%64] = [R, G, B, A]
        }

        pixels << (R, G, B, A)
    }

    for row in (^height) {
        var line = pixels.splice(0, 4*width)
        img.setscanline(y => row, pixels => line.pack('C*'))
    }

    return img
}

ARGV || do {
    STDERR << "usage: #{File(__MAIN__).basename} [input.qoi] [output.png]\n"
    Sys.exit(2)
}

var in_file  = File(ARGV[0])
var out_file = File(ARGV[1] \\ (in_file - /\.qoi\z/i + '.png'))
var bytes = in_file.read(:raw).bytes

var img = qoi_decoder(bytes)
img.write(file => out_file, type => 'png')