yMax := r.Max.Y - dst.Rect.Min.Y

	y := r.Min.Y - dst.Rect.Min.Y
	sy := sp.Y - src.Rect.Min.Y
	for ; y != yMax; y, sy = y+1, sy+1 {
		dpix := dst.Pix[y*dst.Stride:]
		spix := src.Pix[sy*src.Stride:]

		for i, si := i0, si0; i < i1; i, si = i+4, si+1 {
			p := spix[si]
			d := dpix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
			d[0] = p
			d[1] = p
			d[2] = p
			d[3] = 255

		b.WriteString(paletteStr)
		// Write an image with bounds 2x1 but tc.nPix pixels. If tc.nPix != 2
		// then this should result in an invalid GIF image. First, write a
		// magic 0x2c (image descriptor) byte, bounds=(0,0)-(2,1), a flags
		// byte, and 2-bit LZW literals.
		b.WriteString("\x2c\x00\x00\x00\x00\x02\x00\x01\x00\x00\x02")
		if tc.nPix > 0 {
			enc := lzwEncode(make([]byte, tc.nPix))
			if len(enc)+tc.extraExisting > 0xff {

func uninterlace(m *image.Paletted) {
	var nPix []uint8
	dx := m.Bounds().Dx()
	dy := m.Bounds().Dy()
	nPix = make([]uint8, dx*dy)
	offset := 0 // steps through the input by sequential scan lines.
	for _, pass := range interlacing {
		nOffset := pass.start * dx // steps through the output as defined by pass.
		for y := pass.start; y < dy; y += pass.skip {
			copy(nPix[nOffset:nOffset+dx], m.Pix[offset:offset+dx])
			offset += dx

			} else if nrgba != nil {
				stride, pix = nrgba.Stride, nrgba.Pix
			}
			if stride != 0 {
				j0 := (y - b.Min.Y) * stride
				j1 := j0 + b.Dx()*4
				for j := j0; j < j1; j += 4 {
					cr0[i+0] = pix[j+0]
					cr0[i+1] = pix[j+1]
					cr0[i+2] = pix[j+2]
					i += 3
				}
			} else {
				for x := b.Min.X; x < b.Max.X; x++ {
					r, g, b, _ := m.At(x, y).RGBA()
					cr0[i+0] = uint8(r >> 8)

			if err := check(m0.Bounds(), m0.Cr, m1.Cr, m0.CStride, m1.CStride); err != nil {
				t.Errorf("%s (Cr): %v", tc, err)
				continue
			}
		case *image.Gray:
			m1 := m1.(*image.Gray)
			if err := check(m0.Bounds(), m0.Pix, m1.Pix, m0.Stride, m1.Stride); err != nil {
				t.Errorf("%s: %v", tc, err)
				continue
			}
		default:
			t.Errorf("%s: unexpected image type %T", tc, m0)
			continue
		}
	}
}

	bw.setup()
	lzww := lzw.NewWriter(bw, lzw.LSB, litWidth)
	if dx := b.Dx(); dx == pm.Stride {
		_, e.err = lzww.Write(pm.Pix[:dx*b.Dy()])
		if e.err != nil {
			lzww.Close()
			return
		}
	} else {
		for i, y := 0, b.Min.Y; y < b.Max.Y; i, y = i+pm.Stride, y+1 {
			_, e.err = lzww.Write(pm.Pix[i : i+dx])
			if e.err != nil {
				lzww.Close()
				return
			}
		}
	}
	lzww.Close() // flush to bw

	}
}

func BenchmarkEncodeRandomPaletted(b *testing.B) {
	paletted := image.NewPaletted(image.Rect(0, 0, 640, 480), palette.Plan9)
	rnd := rand.New(rand.NewSource(123))
	for i := range paletted.Pix {
		paletted.Pix[i] = uint8(rnd.Intn(256))
	}

	b.SetBytes(640 * 480 * 1)
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		Encode(io.Discard, paletted, nil)
	}
}

	qt := &d.quant[d.comp[compIndex].tq]
	for zig := 0; zig < blockSize; zig++ {
		b[unzig[zig]] *= qt[zig]
	}
	idct(b)
	dst, stride := []byte(nil), 0
	if d.nComp == 1 {
		dst, stride = d.img1.Pix[8*(by*d.img1.Stride+bx):], d.img1.Stride
	} else {
		switch compIndex {
		case 0:
			dst, stride = d.img3.Y[8*(by*d.img3.YStride+bx):], d.img3.YStride
		case 1:

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		{"(YCbCrSubsampleRatio).String", Method, 0},
		{"Alpha", Type, 0},
		{"Alpha.Pix", Field, 0},
		{"Alpha.Rect", Field, 0},
		{"Alpha.Stride", Field, 0},
		{"Alpha16", Type, 0},
		{"Alpha16.Pix", Field, 0},
		{"Alpha16.Rect", Field, 0},
		{"Alpha16.Stride", Field, 0},
		{"Black", Var, 0},
		{"CMYK", Type, 5},
		{"CMYK.Pix", Field, 5},
		{"CMYK.Rect", Field, 5},
		{"CMYK.Stride", Field, 5},