Function applyPredictTransform

transform.go:29–122

(pixels []color.NRGBA, width, height, transBits int)

)

func applyPredictTransform(pixels []color.NRGBA, width, height, transBits int) (int, int, []color.NRGBA) {
    bw := (width + (1 << transBits) - 1) >> transBits
    bh := (height + (1 << transBits) - 1) >> transBits

    blocks := make([]color.NRGBA, bw * bh)
    deltas := make([]color.NRGBA, width * height)
    
    accum := [][]int{
        make([]int, 256),
        make([]int, 256),
        make([]int, 256),
        make([]int, 256),
        make([]int, 40),
    }

    histos := make([][]int, len(accum))
    for i := range accum {
        histos[i] = make([]int, len(accum[i]))
    }

    for y := 0; y < bh; y++ {
        for x := 0; x < bw; x++ {
            mx := min((x + 1) << transBits, width)
            my := min((y + 1) << transBits, height)

            var best int
            var bestEntropy float64
            for i := 0; i < 14; i++ {
                for j := range accum {
                    copy(histos[j], accum[j])
                }

                for tx := x << transBits; tx < mx; tx++ {
                    for ty := y << transBits; ty < my; ty++ {
                        d := applyFilter(pixels, width, tx, ty, i)

                        off := ty * width + tx
                        histos[0][int(uint8(pixels[off].R - d.R))]++
                        histos[1][int(uint8(pixels[off].G - d.G))]++
                        histos[2][int(uint8(pixels[off].B - d.B))]++
                        histos[3][int(uint8(pixels[off].A - d.A))]++
                    }
                }

                var total float64
                for _, histo := range histos {
                    sum := 0
                    sumSquares := 0
                
                    for _, count := range histo {
                        sum += count
                        sumSquares += count * count
                    }
                
                    if sum == 0 {
                        continue
                    }