internal/png/stdlibreader.go
func (d *decoder) readImagePass(r io.Reader, pass int, allocateOnly bool) (image.Image, error) {
	bitsPerPixel := 0
	pixOffset := 0
	var (
		gray     *image.Gray
		rgba     *image.RGBA
		paletted *image.Paletted
		nrgba    *image.NRGBA
		gray16   *image.Gray16
		rgba64   *image.RGBA64
		nrgba64  *image.NRGBA64
		img      image.Image
	)
	width, height := d.width, d.height
	if d.interlace == itAdam7 && !allocateOnly {
		p := interlacing[pass]
		// Add the multiplication factor and subtract one, effectively rounding up.
		width = (width - p.xOffset + p.xFactor - 1) / p.xFactor
		height = (height - p.yOffset + p.yFactor - 1) / p.yFactor
		// A PNG image can't have zero width or height, but for an interlaced
		// image, an individual pass might have zero width or height. If so, we
		// shouldn't even read a per-row filter type byte, so return early.
		if width == 0 || height == 0 {
			return nil, nil
		}
	}
	switch d.cb {
	case cbG1, cbG2, cbG4, cbG8:
		bitsPerPixel = d.depth
		if d.useTransparent {
			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
			img = nrgba
		} else {
			gray = image.NewGray(image.Rect(0, 0, width, height))
			img = gray
		}
	case cbGA8:
		bitsPerPixel = 16
		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
		img = nrgba
	case cbTC8:
		bitsPerPixel = 24
		if d.useTransparent {
			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
			img = nrgba
		} else {
			rgba = image.NewRGBA(image.Rect(0, 0, width, height))
			img = rgba
		}
	case cbP1, cbP2, cbP4, cbP8:
		bitsPerPixel = d.depth
		paletted = image.NewPaletted(image.Rect(0, 0, width, height), d.palette)
		img = paletted
	case cbTCA8:
		bitsPerPixel = 32
		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
		img = nrgba
	case cbG16:
		bitsPerPixel = 16
		if d.useTransparent {
			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
			img = nrgba64
		} else {
			gray16 = image.NewGray16(image.Rect(0, 0, width, height))
			img = gray16
		}
	case cbGA16:
		bitsPerPixel = 32
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
		img = nrgba64
	case cbTC16:
		bitsPerPixel = 48
		if d.useTransparent {
			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
			img = nrgba64
		} else {
			rgba64 = image.NewRGBA64(image.Rect(0, 0, width, height))
			img = rgba64
		}
	case cbTCA16:
		bitsPerPixel = 64
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
		img = nrgba64
	}
	if allocateOnly {
		return img, nil
	}
	bytesPerPixel := (bitsPerPixel + 7) / 8

	// The +1 is for the per-row filter type, which is at cr[0].
	rowSize := 1 + (bitsPerPixel*width+7)/8
	// cr and pr are the bytes for the current and previous row.
	cr := make([]uint8, rowSize)
	pr := make([]uint8, rowSize)

	for y := 0; y < height; y++ {
		// Read the decompressed bytes.
		_, err := io.ReadFull(r, cr)
		if err != nil {
			if err == io.EOF || err == io.ErrUnexpectedEOF {
				return nil, FormatError("not enough pixel data")
			}
			return nil, err
		}

		// Apply the filter.
		cdat := cr[1:]
		pdat := pr[1:]
		switch cr[0] {
		case ftNone:
			// No-op.
		case ftSub:
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += cdat[i-bytesPerPixel]
			}
		case ftUp:
			for i, p := range pdat {
				cdat[i] += p
			}
		case ftAverage:
			// The first column has no column to the left of it, so it is a
			// special case. We know that the first column exists because we
			// check above that width != 0, and so len(cdat) != 0.
			for i := 0; i < bytesPerPixel; i++ {
				cdat[i] += pdat[i] / 2
			}
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
			}
		case ftPaeth:
			filterPaeth(cdat, pdat, bytesPerPixel)
		default:
			return nil, FormatError("bad filter type")
		}

		// Convert from bytes to colors.
		switch d.cb {
		case cbG1:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 8 {
					b := cdat[x/8]
					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
						ycol := (b >> 7) * 0xff
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 1
					}
				}
			} else {
				for x := 0; x < width; x += 8 {
					b := cdat[x/8]
					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 7) * 0xff})
						b <<= 1
					}
				}
			}
		case cbG2:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 4 {
					b := cdat[x/4]
					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
						ycol := (b >> 6) * 0x55
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 2
					}
				}
			} else {
				for x := 0; x < width; x += 4 {
					b := cdat[x/4]
					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 6) * 0x55})
						b <<= 2
					}
				}
			}
		case cbG4:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 2 {
					b := cdat[x/2]
					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
						ycol := (b >> 4) * 0x11
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 4
					}
				}
			} else {
				for x := 0; x < width; x += 2 {
					b := cdat[x/2]
					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 4) * 0x11})
						b <<= 4
					}
				}
			}
		case cbG8:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x++ {
					ycol := cdat[x]
					acol := uint8(0xff)
					if ycol == ty {
						acol = 0x00
					}
					nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, acol})
				}
			} else {
				copy(gray.Pix[pixOffset:], cdat)
				pixOffset += gray.Stride
			}
		case cbGA8:
			for x := 0; x < width; x++ {
				ycol := cdat[2*x+0]
				nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, cdat[2*x+1]})
			}
		case cbTC8:
			if d.useTransparent {
				pix, i, j := nrgba.Pix, pixOffset, 0
				tr, tg, tb := d.transparent[1], d.transparent[3], d.transparent[5]
				for x := 0; x < width; x++ {
					r := cdat[j+0]
					g := cdat[j+1]
					b := cdat[j+2]
					a := uint8(0xff)
					if r == tr && g == tg && b == tb {
						a = 0x00
					}
					pix[i+0] = r
					pix[i+1] = g
					pix[i+2] = b
					pix[i+3] = a
					i += 4
					j += 3
				}
				pixOffset += nrgba.Stride
			} else {
				pix, i, j := rgba.Pix, pixOffset, 0
				for x := 0; x < width; x++ {
					pix[i+0] = cdat[j+0]
					pix[i+1] = cdat[j+1]
					pix[i+2] = cdat[j+2]
					pix[i+3] = 0xff
					i += 4
					j += 3
				}
				pixOffset += rgba.Stride
			}
		case cbP1:
			for x := 0; x < width; x += 8 {
				b := cdat[x/8]
				for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
					idx := b >> 7
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 1
				}
			}
		case cbP2:
			for x := 0; x < width; x += 4 {
				b := cdat[x/4]
				for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
					idx := b >> 6
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 2
				}
			}
		case cbP4:
			for x := 0; x < width; x += 2 {
				b := cdat[x/2]
				for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
					idx := b >> 4
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 4
				}
			}
		case cbP8:
			if len(paletted.Palette) != 255 {
				for x := 0; x < width; x++ {
					if len(paletted.Palette) <= int(cdat[x]) {
						paletted.Palette = paletted.Palette[:int(cdat[x])+1]
					}
				}
			}
			copy(paletted.Pix[pixOffset:], cdat)
			pixOffset += paletted.Stride
		case cbTCA8:
			copy(nrgba.Pix[pixOffset:], cdat)
			pixOffset += nrgba.Stride
		case cbG16:
			if d.useTransparent {
				ty := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
				for x := 0; x < width; x++ {
					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
					acol := uint16(0xffff)
					if ycol == ty {
						acol = 0x0000
					}
					nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
				}
			} else {
				for x := 0; x < width; x++ {
					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
					gray16.SetGray16(x, y, color.Gray16{ycol})
				}
			}
		case cbGA16:
			for x := 0; x < width; x++ {
				ycol := uint16(cdat[4*x+0])<<8 | uint16(cdat[4*x+1])
				acol := uint16(cdat[4*x+2])<<8 | uint16(cdat[4*x+3])
				nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
			}
		case cbTC16:
			if d.useTransparent {
				tr := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
				tg := uint16(d.transparent[2])<<8 | uint16(d.transparent[3])
				tb := uint16(d.transparent[4])<<8 | uint16(d.transparent[5])
				for x := 0; x < width; x++ {
					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
					acol := uint16(0xffff)
					if rcol == tr && gcol == tg && bcol == tb {
						acol = 0x0000
					}
					nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
				}
			} else {
				for x := 0; x < width; x++ {
					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
					rgba64.SetRGBA64(x, y, color.RGBA64{rcol, gcol, bcol, 0xffff})
				}
			}
		case cbTCA16:
			for x := 0; x < width; x++ {
				rcol := uint16(cdat[8*x+0])<<8 | uint16(cdat[8*x+1])
				gcol := uint16(cdat[8*x+2])<<8 | uint16(cdat[8*x+3])
				bcol := uint16(cdat[8*x+4])<<8 | uint16(cdat[8*x+5])
				acol := uint16(cdat[8*x+6])<<8 | uint16(cdat[8*x+7])
				nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
			}
		}

		// The current row for y is the previous row for y+1.
		pr, cr = cr, pr
	}

	return img, nil
}
func (d *decoder) readImagePass(r io.Reader, pass int, allocateOnly bool) (image.Image, error) {
	bitsPerPixel := 0
	pixOffset := 0
	var (
		gray     *image.Gray
		rgba     *image.RGBA
		paletted *image.Paletted
		nrgba    *image.NRGBA
		gray16   *image.Gray16
		rgba64   *image.RGBA64
		nrgba64  *image.NRGBA64
		img      image.Image
	)
	width, height := d.width, d.height
	if d.interlace == itAdam7 && !allocateOnly {
		p := interlacing[pass]
		// Add the multiplication factor and subtract one, effectively rounding up.
		width = (width - p.xOffset + p.xFactor - 1) / p.xFactor
		height = (height - p.yOffset + p.yFactor - 1) / p.yFactor
		// A PNG image can't have zero width or height, but for an interlaced
		// image, an individual pass might have zero width or height. If so, we
		// shouldn't even read a per-row filter type byte, so return early.
		if width == 0 || height == 0 {
			return nil, nil
		}
	}
	switch d.cb {
	case cbG1, cbG2, cbG4, cbG8:
		bitsPerPixel = d.depth
		if d.useTransparent {
			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
			img = nrgba
		} else {
			gray = image.NewGray(image.Rect(0, 0, width, height))
			img = gray
		}
	case cbGA8:
		bitsPerPixel = 16
		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
		img = nrgba
	case cbTC8:
		bitsPerPixel = 24
		if d.useTransparent {
			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
			img = nrgba
		} else {
			rgba = image.NewRGBA(image.Rect(0, 0, width, height))
			img = rgba
		}
	case cbP1, cbP2, cbP4, cbP8:
		bitsPerPixel = d.depth
		paletted = image.NewPaletted(image.Rect(0, 0, width, height), d.palette)
		img = paletted
	case cbTCA8:
		bitsPerPixel = 32
		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
		img = nrgba
	case cbG16:
		bitsPerPixel = 16
		if d.useTransparent {
			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
			img = nrgba64
		} else {
			gray16 = image.NewGray16(image.Rect(0, 0, width, height))
			img = gray16
		}
	case cbGA16:
		bitsPerPixel = 32
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
		img = nrgba64
	case cbTC16:
		bitsPerPixel = 48
		if d.useTransparent {
			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
			img = nrgba64
		} else {
			rgba64 = image.NewRGBA64(image.Rect(0, 0, width, height))
			img = rgba64
		}
	case cbTCA16:
		bitsPerPixel = 64
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
		img = nrgba64
	}
	if allocateOnly {
		return img, nil
	}
	bytesPerPixel := (bitsPerPixel + 7) / 8

	// The +1 is for the per-row filter type, which is at cr[0].
	rowSize := 1 + (bitsPerPixel*width+7)/8
	// cr and pr are the bytes for the current and previous row.
	cr := make([]uint8, rowSize)
	pr := make([]uint8, rowSize)

	for y := 0; y < height; y++ {
		// Read the decompressed bytes.
		_, err := io.ReadFull(r, cr)
		if err != nil {
			if err == io.EOF || err == io.ErrUnexpectedEOF {
				return nil, FormatError("not enough pixel data")
			}
			return nil, err
		}

		// Apply the filter.
		cdat := cr[1:]
		pdat := pr[1:]
		switch cr[0] {
		case ftNone:
			// No-op.
		case ftSub:
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += cdat[i-bytesPerPixel]
			}
		case ftUp:
			for i, p := range pdat {
				cdat[i] += p
			}
		case ftAverage:
			// The first column has no column to the left of it, so it is a
			// special case. We know that the first column exists because we
			// check above that width != 0, and so len(cdat) != 0.
			for i := 0; i < bytesPerPixel; i++ {
				cdat[i] += pdat[i] / 2
			}
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
			}
		case ftPaeth:
			filterPaeth(cdat, pdat, bytesPerPixel)
		default:
			return nil, FormatError("bad filter type")
		}

		// Convert from bytes to colors.
		switch d.cb {
		case cbG1:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 8 {
					b := cdat[x/8]
					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
						ycol := (b >> 7) * 0xff
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 1
					}
				}
			} else {
				for x := 0; x < width; x += 8 {
					b := cdat[x/8]
					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 7) * 0xff})
						b <<= 1
					}
				}
			}
		case cbG2:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 4 {
					b := cdat[x/4]
					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
						ycol := (b >> 6) * 0x55
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 2
					}
				}
			} else {
				for x := 0; x < width; x += 4 {
					b := cdat[x/4]
					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 6) * 0x55})
						b <<= 2
					}
				}
			}
		case cbG4:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x += 2 {
					b := cdat[x/2]
					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
						ycol := (b >> 4) * 0x11
						acol := uint8(0xff)
						if ycol == ty {
							acol = 0x00
						}
						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
						b <<= 4
					}
				}
			} else {
				for x := 0; x < width; x += 2 {
					b := cdat[x/2]
					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
						gray.SetGray(x+x2, y, color.Gray{(b >> 4) * 0x11})
						b <<= 4
					}
				}
			}
		case cbG8:
			if d.useTransparent {
				ty := d.transparent[1]
				for x := 0; x < width; x++ {
					ycol := cdat[x]
					acol := uint8(0xff)
					if ycol == ty {
						acol = 0x00
					}
					nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, acol})
				}
			} else {
				copy(gray.Pix[pixOffset:], cdat)
				pixOffset += gray.Stride
			}
		case cbGA8:
			for x := 0; x < width; x++ {
				ycol := cdat[2*x+0]
				nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, cdat[2*x+1]})
			}
		case cbTC8:
			if d.useTransparent {
				pix, i, j := nrgba.Pix, pixOffset, 0
				tr, tg, tb := d.transparent[1], d.transparent[3], d.transparent[5]
				for x := 0; x < width; x++ {
					r := cdat[j+0]
					g := cdat[j+1]
					b := cdat[j+2]
					a := uint8(0xff)
					if r == tr && g == tg && b == tb {
						a = 0x00
					}
					pix[i+0] = r
					pix[i+1] = g
					pix[i+2] = b
					pix[i+3] = a
					i += 4
					j += 3
				}
				pixOffset += nrgba.Stride
			} else {
				pix, i, j := rgba.Pix, pixOffset, 0
				for x := 0; x < width; x++ {
					pix[i+0] = cdat[j+0]
					pix[i+1] = cdat[j+1]
					pix[i+2] = cdat[j+2]
					pix[i+3] = 0xff
					i += 4
					j += 3
				}
				pixOffset += rgba.Stride
			}
		case cbP1:
			for x := 0; x < width; x += 8 {
				b := cdat[x/8]
				for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
					idx := b >> 7
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 1
				}
			}
		case cbP2:
			for x := 0; x < width; x += 4 {
				b := cdat[x/4]
				for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
					idx := b >> 6
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 2
				}
			}
		case cbP4:
			for x := 0; x < width; x += 2 {
				b := cdat[x/2]
				for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
					idx := b >> 4
					if len(paletted.Palette) <= int(idx) {
						paletted.Palette = paletted.Palette[:int(idx)+1]
					}
					paletted.SetColorIndex(x+x2, y, idx)
					b <<= 4
				}
			}
		case cbP8:
			if len(paletted.Palette) != 255 {
				for x := 0; x < width; x++ {
					if len(paletted.Palette) <= int(cdat[x]) {
						paletted.Palette = paletted.Palette[:int(cdat[x])+1]
					}
				}
			}
			copy(paletted.Pix[pixOffset:], cdat)
			pixOffset += paletted.Stride
		case cbTCA8:
			copy(nrgba.Pix[pixOffset:], cdat)
			pixOffset += nrgba.Stride
		case cbG16:
			if d.useTransparent {
				ty := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
				for x := 0; x < width; x++ {
					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
					acol := uint16(0xffff)
					if ycol == ty {
						acol = 0x0000
					}
					nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
				}
			} else {
				for x := 0; x < width; x++ {
					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
					gray16.SetGray16(x, y, color.Gray16{ycol})
				}
			}
		case cbGA16:
			for x := 0; x < width; x++ {
				ycol := uint16(cdat[4*x+0])<<8 | uint16(cdat[4*x+1])
				acol := uint16(cdat[4*x+2])<<8 | uint16(cdat[4*x+3])
				nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
			}
		case cbTC16:
			if d.useTransparent {
				tr := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
				tg := uint16(d.transparent[2])<<8 | uint16(d.transparent[3])
				tb := uint16(d.transparent[4])<<8 | uint16(d.transparent[5])
				for x := 0; x < width; x++ {
					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
					acol := uint16(0xffff)
					if rcol == tr && gcol == tg && bcol == tb {
						acol = 0x0000
					}
					nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
				}
			} else {
				for x := 0; x < width; x++ {
					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
					rgba64.SetRGBA64(x, y, color.RGBA64{rcol, gcol, bcol, 0xffff})
				}
			}
		case cbTCA16:
			for x := 0; x < width; x++ {
				rcol := uint16(cdat[8*x+0])<<8 | uint16(cdat[8*x+1])
				gcol := uint16(cdat[8*x+2])<<8 | uint16(cdat[8*x+3])
				bcol := uint16(cdat[8*x+4])<<8 | uint16(cdat[8*x+5])
				acol := uint16(cdat[8*x+6])<<8 | uint16(cdat[8*x+7])
				nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
			}
		}

		// The current row for y is the previous row for y+1.
		pr, cr = cr, pr
	}

	return img, nil
}
// readImagePass reads a single image pass, sized according to the pass number.