image_png_decode.go

// decode decodes the IDAT data into an image.
func (d *decoder) decode() (image.Image, error) {
	r, err := zlib.NewReader(d)
	if err != nil {
		return nil, err
	}
	defer r.Close()
	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
	)
	switch d.cb {
	case cbG1, cbG2, cbG4, cbG8:
		bitsPerPixel = d.depth
		gray = image.NewGray(image.Rect(0, 0, d.width, d.height))
		img = gray
	case cbGA8:
		bitsPerPixel = 16
		nrgba = image.NewNRGBA(image.Rect(0, 0, d.width, d.height))
		img = nrgba
	case cbTC8:
		bitsPerPixel = 24
		rgba = image.NewRGBA(image.Rect(0, 0, d.width, d.height))
		img = rgba
	case cbP1, cbP2, cbP4, cbP8:
		bitsPerPixel = d.depth
		paletted = image.NewPaletted(image.Rect(0, 0, d.width, d.height), d.palette)
		img = paletted
	case cbTCA8:
		bitsPerPixel = 32
		nrgba = image.NewNRGBA(image.Rect(0, 0, d.width, d.height))
		img = nrgba
	case cbG16:
		bitsPerPixel = 16
		gray16 = image.NewGray16(image.Rect(0, 0, d.width, d.height))
		img = gray16
	case cbGA16:
		bitsPerPixel = 32
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, d.width, d.height))
		img = nrgba64
	case cbTC16:
		bitsPerPixel = 48
		rgba64 = image.NewRGBA64(image.Rect(0, 0, d.width, d.height))
		img = rgba64
	case cbTCA16:
		bitsPerPixel = 64
		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, d.width, d.height))
		img = nrgba64
	}
	bytesPerPixel := (bitsPerPixel + 7) / 8

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

	for y := 0; y < d.height; y++ {
		// Read the decompressed bytes.
		_, err := io.ReadFull(r, cr)
		if err != nil {
			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:
			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:
			for x := 0; x < d.width; x += 8 {
				b := cdat[x/8]
				for x2 := 0; x2 < 8 && x+x2 < d.width; x2++ {
					gray.SetGray(x+x2, y, color.Gray{(b >> 7) * 0xff})
					b <<= 1
				}
			}
		case cbG2:
			for x := 0; x < d.width; x += 4 {
				b := cdat[x/4]
				for x2 := 0; x2 < 4 && x+x2 < d.width; x2++ {
					gray.SetGray(x+x2, y, color.Gray{(b >> 6) * 0x55})
					b <<= 2
				}
			}
		case cbG4:
			for x := 0; x < d.width; x += 2 {
				b := cdat[x/2]
				for x2 := 0; x2 < 2 && x+x2 < d.width; x2++ {
					gray.SetGray(x+x2, y, color.Gray{(b >> 4) * 0x11})
					b <<= 4
				}
			}
		case cbG8:
			copy(gray.Pix[pixOffset:], cdat)
			pixOffset += gray.Stride
		case cbGA8:
			for x := 0; x < d.width; x++ {
				ycol := cdat[2*x+0]
				nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, cdat[2*x+1]})
			}
		case cbTC8:
			pix, i, j := rgba.Pix, pixOffset, 0
			for x := 0; x < d.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 < d.width; x += 8 {
				b := cdat[x/8]
				for x2 := 0; x2 < 8 && x+x2 < d.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 < d.width; x += 4 {
				b := cdat[x/4]
				for x2 := 0; x2 < 4 && x+x2 < d.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 < d.width; x += 2 {
				b := cdat[x/2]
				for x2 := 0; x2 < 2 && x+x2 < d.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 < d.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:
			for x := 0; x < d.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 < d.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:
			for x := 0; x < d.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 < d.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
	}

	// Check for EOF, to verify the zlib checksum.
	n := 0
	for i := 0; n == 0 && err == nil; i++ {
		if i == 100 {
			return nil, io.ErrNoProgress
		}
		n, err = r.Read(pr[:1])
	}
	if err != nil && err != io.EOF {
		return nil, FormatError(err.Error())
	}
	if n != 0 || d.idatLength != 0 {
		return nil, FormatError("too much pixel data")
	}

	return img, nil
}