gtsocial-umbx

reader.go (19462B)

---

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Package tiff implements a TIFF image decoder and encoder.
 //
 // The TIFF specification is at http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf
 package tiff // import "golang.org/x/image/tiff"
 
 import (
 	"compress/zlib"
 	"encoding/binary"
 	"fmt"
 	"image"
 	"image/color"
 	"io"
 	"io/ioutil"
 	"math"
 
 	"golang.org/x/image/ccitt"
 	"golang.org/x/image/tiff/lzw"
 )
 
 // A FormatError reports that the input is not a valid TIFF image.
 type FormatError string
 
 func (e FormatError) Error() string {
 	return "tiff: invalid format: " + string(e)
 }
 
 // An UnsupportedError reports that the input uses a valid but
 // unimplemented feature.
 type UnsupportedError string
 
 func (e UnsupportedError) Error() string {
 	return "tiff: unsupported feature: " + string(e)
 }
 
 var errNoPixels = FormatError("not enough pixel data")
 
 const maxChunkSize = 10 << 20 // 10M
 
 // safeReadtAt is a verbatim copy of internal/saferio.ReadDataAt from the
 // standard library, which is used to read data from a reader using a length
 // provided by untrusted data, without allocating the entire slice ahead of time
 // if it is large (>maxChunkSize). This allows us to avoid allocating giant
 // slices before learning that we can't actually read that much data from the
 // reader.
 func safeReadAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
 	if int64(n) < 0 || n != uint64(int(n)) {
 		// n is too large to fit in int, so we can't allocate
 		// a buffer large enough. Treat this as a read failure.
 		return nil, io.ErrUnexpectedEOF
 	}
 
 	if n < maxChunkSize {
 		buf := make([]byte, n)
 		_, err := r.ReadAt(buf, off)
 		if err != nil {
 			// io.SectionReader can return EOF for n == 0,
 			// but for our purposes that is a success.
 			if err != io.EOF || n > 0 {
 				return nil, err
 			}
 		}
 		return buf, nil
 	}
 
 	var buf []byte
 	buf1 := make([]byte, maxChunkSize)
 	for n > 0 {
 		next := n
 		if next > maxChunkSize {
 			next = maxChunkSize
 		}
 		_, err := r.ReadAt(buf1[:next], off)
 		if err != nil {
 			return nil, err
 		}
 		buf = append(buf, buf1[:next]...)
 		n -= next
 		off += int64(next)
 	}
 	return buf, nil
 }
 
 type decoder struct {
 	r         io.ReaderAt
 	byteOrder binary.ByteOrder
 	config    image.Config
 	mode      imageMode
 	bpp       uint
 	features  map[int][]uint
 	palette   []color.Color
 
 	buf   []byte
 	off   int    // Current offset in buf.
 	v     uint32 // Buffer value for reading with arbitrary bit depths.
 	nbits uint   // Remaining number of bits in v.
 }
 
 // firstVal returns the first uint of the features entry with the given tag,
 // or 0 if the tag does not exist.
 func (d *decoder) firstVal(tag int) uint {
 	f := d.features[tag]
 	if len(f) == 0 {
 		return 0
 	}
 	return f[0]
 }
 
 // ifdUint decodes the IFD entry in p, which must be of the Byte, Short
 // or Long type, and returns the decoded uint values.
 func (d *decoder) ifdUint(p []byte) (u []uint, err error) {
 	var raw []byte
 	if len(p) < ifdLen {
 		return nil, FormatError("bad IFD entry")
 	}
 
 	datatype := d.byteOrder.Uint16(p[2:4])
 	if dt := int(datatype); dt <= 0 || dt >= len(lengths) {
 		return nil, UnsupportedError("IFD entry datatype")
 	}
 
 	count := d.byteOrder.Uint32(p[4:8])
 	if count > math.MaxInt32/lengths[datatype] {
 		return nil, FormatError("IFD data too large")
 	}
 	if datalen := lengths[datatype] * count; datalen > 4 {
 		// The IFD contains a pointer to the real value.
 		raw, err = safeReadAt(d.r, uint64(datalen), int64(d.byteOrder.Uint32(p[8:12])))
 	} else {
 		raw = p[8 : 8+datalen]
 	}
 	if err != nil {
 		return nil, err
 	}
 
 	u = make([]uint, count)
 	switch datatype {
 	case dtByte:
 		for i := uint32(0); i < count; i++ {
 			u[i] = uint(raw[i])
 		}
 	case dtShort:
 		for i := uint32(0); i < count; i++ {
 			u[i] = uint(d.byteOrder.Uint16(raw[2*i : 2*(i+1)]))
 		}
 	case dtLong:
 		for i := uint32(0); i < count; i++ {
 			u[i] = uint(d.byteOrder.Uint32(raw[4*i : 4*(i+1)]))
 		}
 	default:
 		return nil, UnsupportedError("data type")
 	}
 	return u, nil
 }
 
 // parseIFD decides whether the IFD entry in p is "interesting" and
 // stows away the data in the decoder. It returns the tag number of the
 // entry and an error, if any.
 func (d *decoder) parseIFD(p []byte) (int, error) {
 	tag := d.byteOrder.Uint16(p[0:2])
 	switch tag {
 	case tBitsPerSample,
 		tExtraSamples,
 		tPhotometricInterpretation,
 		tCompression,
 		tPredictor,
 		tStripOffsets,
 		tStripByteCounts,
 		tRowsPerStrip,
 		tTileWidth,
 		tTileLength,
 		tTileOffsets,
 		tTileByteCounts,
 		tImageLength,
 		tImageWidth,
 		tFillOrder,
 		tT4Options,
 		tT6Options:
 		val, err := d.ifdUint(p)
 		if err != nil {
 			return 0, err
 		}
 		d.features[int(tag)] = val
 	case tColorMap:
 		val, err := d.ifdUint(p)
 		if err != nil {
 			return 0, err
 		}
 		numcolors := len(val) / 3
 		if len(val)%3 != 0 || numcolors <= 0 || numcolors > 256 {
 			return 0, FormatError("bad ColorMap length")
 		}
 		d.palette = make([]color.Color, numcolors)
 		for i := 0; i < numcolors; i++ {
 			d.palette[i] = color.RGBA64{
 				uint16(val[i]),
 				uint16(val[i+numcolors]),
 				uint16(val[i+2*numcolors]),
 				0xffff,
 			}
 		}
 	case tSampleFormat:
 		// Page 27 of the spec: If the SampleFormat is present and
 		// the value is not 1 [= unsigned integer data], a Baseline
 		// TIFF reader that cannot handle the SampleFormat value
 		// must terminate the import process gracefully.
 		val, err := d.ifdUint(p)
 		if err != nil {
 			return 0, err
 		}
 		for _, v := range val {
 			if v != 1 {
 				return 0, UnsupportedError("sample format")
 			}
 		}
 	}
 	return int(tag), nil
 }
 
 // readBits reads n bits from the internal buffer starting at the current offset.
 func (d *decoder) readBits(n uint) (v uint32, ok bool) {
 	for d.nbits < n {
 		d.v <<= 8
 		if d.off >= len(d.buf) {
 			return 0, false
 		}
 		d.v |= uint32(d.buf[d.off])
 		d.off++
 		d.nbits += 8
 	}
 	d.nbits -= n
 	rv := d.v >> d.nbits
 	d.v &^= rv << d.nbits
 	return rv, true
 }
 
 // flushBits discards the unread bits in the buffer used by readBits.
 // It is used at the end of a line.
 func (d *decoder) flushBits() {
 	d.v = 0
 	d.nbits = 0
 }
 
 // minInt returns the smaller of x or y.
 func minInt(a, b int) int {
 	if a <= b {
 		return a
 	}
 	return b
 }
 
 // decode decodes the raw data of an image.
 // It reads from d.buf and writes the strip or tile into dst.
 func (d *decoder) decode(dst image.Image, xmin, ymin, xmax, ymax int) error {
 	d.off = 0
 
 	// Apply horizontal predictor if necessary.
 	// In this case, p contains the color difference to the preceding pixel.
 	// See page 64-65 of the spec.
 	if d.firstVal(tPredictor) == prHorizontal {
 		switch d.bpp {
 		case 16:
 			var off int
 			n := 2 * len(d.features[tBitsPerSample]) // bytes per sample times samples per pixel
 			for y := ymin; y < ymax; y++ {
 				off += n
 				for x := 0; x < (xmax-xmin-1)*n; x += 2 {
 					if off+2 > len(d.buf) {
 						return errNoPixels
 					}
 					v0 := d.byteOrder.Uint16(d.buf[off-n : off-n+2])
 					v1 := d.byteOrder.Uint16(d.buf[off : off+2])
 					d.byteOrder.PutUint16(d.buf[off:off+2], v1+v0)
 					off += 2
 				}
 			}
 		case 8:
 			var off int
 			n := 1 * len(d.features[tBitsPerSample]) // bytes per sample times samples per pixel
 			for y := ymin; y < ymax; y++ {
 				off += n
 				for x := 0; x < (xmax-xmin-1)*n; x++ {
 					if off >= len(d.buf) {
 						return errNoPixels
 					}
 					d.buf[off] += d.buf[off-n]
 					off++
 				}
 			}
 		case 1:
 			return UnsupportedError("horizontal predictor with 1 BitsPerSample")
 		}
 	}
 
 	rMaxX := minInt(xmax, dst.Bounds().Max.X)
 	rMaxY := minInt(ymax, dst.Bounds().Max.Y)
 	switch d.mode {
 	case mGray, mGrayInvert:
 		if d.bpp == 16 {
 			img := dst.(*image.Gray16)
 			for y := ymin; y < rMaxY; y++ {
 				for x := xmin; x < rMaxX; x++ {
 					if d.off+2 > len(d.buf) {
 						return errNoPixels
 					}
 					v := d.byteOrder.Uint16(d.buf[d.off : d.off+2])
 					d.off += 2
 					if d.mode == mGrayInvert {
 						v = 0xffff - v
 					}
 					img.SetGray16(x, y, color.Gray16{v})
 				}
 				if rMaxX == img.Bounds().Max.X {
 					d.off += 2 * (xmax - img.Bounds().Max.X)
 				}
 			}
 		} else {
 			img := dst.(*image.Gray)
 			max := uint32((1 << d.bpp) - 1)
 			for y := ymin; y < rMaxY; y++ {
 				for x := xmin; x < rMaxX; x++ {
 					v, ok := d.readBits(d.bpp)
 					if !ok {
 						return errNoPixels
 					}
 					v = v * 0xff / max
 					if d.mode == mGrayInvert {
 						v = 0xff - v
 					}
 					img.SetGray(x, y, color.Gray{uint8(v)})
 				}
 				d.flushBits()
 			}
 		}
 	case mPaletted:
 		img := dst.(*image.Paletted)
 		for y := ymin; y < rMaxY; y++ {
 			for x := xmin; x < rMaxX; x++ {
 				v, ok := d.readBits(d.bpp)
 				if !ok {
 					return errNoPixels
 				}
 				img.SetColorIndex(x, y, uint8(v))
 			}
 			d.flushBits()
 		}
 	case mRGB:
 		if d.bpp == 16 {
 			img := dst.(*image.RGBA64)
 			for y := ymin; y < rMaxY; y++ {
 				for x := xmin; x < rMaxX; x++ {
 					if d.off+6 > len(d.buf) {
 						return errNoPixels
 					}
 					r := d.byteOrder.Uint16(d.buf[d.off+0 : d.off+2])
 					g := d.byteOrder.Uint16(d.buf[d.off+2 : d.off+4])
 					b := d.byteOrder.Uint16(d.buf[d.off+4 : d.off+6])
 					d.off += 6
 					img.SetRGBA64(x, y, color.RGBA64{r, g, b, 0xffff})
 				}
 			}
 		} else {
 			img := dst.(*image.RGBA)
 			for y := ymin; y < rMaxY; y++ {
 				min := img.PixOffset(xmin, y)
 				max := img.PixOffset(rMaxX, y)
 				off := (y - ymin) * (xmax - xmin) * 3
 				for i := min; i < max; i += 4 {
 					if off+3 > len(d.buf) {
 						return errNoPixels
 					}
 					img.Pix[i+0] = d.buf[off+0]
 					img.Pix[i+1] = d.buf[off+1]
 					img.Pix[i+2] = d.buf[off+2]
 					img.Pix[i+3] = 0xff
 					off += 3
 				}
 			}
 		}
 	case mNRGBA:
 		if d.bpp == 16 {
 			img := dst.(*image.NRGBA64)
 			for y := ymin; y < rMaxY; y++ {
 				for x := xmin; x < rMaxX; x++ {
 					if d.off+8 > len(d.buf) {
 						return errNoPixels
 					}
 					r := d.byteOrder.Uint16(d.buf[d.off+0 : d.off+2])
 					g := d.byteOrder.Uint16(d.buf[d.off+2 : d.off+4])
 					b := d.byteOrder.Uint16(d.buf[d.off+4 : d.off+6])
 					a := d.byteOrder.Uint16(d.buf[d.off+6 : d.off+8])
 					d.off += 8
 					img.SetNRGBA64(x, y, color.NRGBA64{r, g, b, a})
 				}
 			}
 		} else {
 			img := dst.(*image.NRGBA)
 			for y := ymin; y < rMaxY; y++ {
 				min := img.PixOffset(xmin, y)
 				max := img.PixOffset(rMaxX, y)
 				i0, i1 := (y-ymin)*(xmax-xmin)*4, (y-ymin+1)*(xmax-xmin)*4
 				if i1 > len(d.buf) {
 					return errNoPixels
 				}
 				copy(img.Pix[min:max], d.buf[i0:i1])
 			}
 		}
 	case mRGBA:
 		if d.bpp == 16 {
 			img := dst.(*image.RGBA64)
 			for y := ymin; y < rMaxY; y++ {
 				for x := xmin; x < rMaxX; x++ {
 					if d.off+8 > len(d.buf) {
 						return errNoPixels
 					}
 					r := d.byteOrder.Uint16(d.buf[d.off+0 : d.off+2])
 					g := d.byteOrder.Uint16(d.buf[d.off+2 : d.off+4])
 					b := d.byteOrder.Uint16(d.buf[d.off+4 : d.off+6])
 					a := d.byteOrder.Uint16(d.buf[d.off+6 : d.off+8])
 					d.off += 8
 					img.SetRGBA64(x, y, color.RGBA64{r, g, b, a})
 				}
 			}
 		} else {
 			img := dst.(*image.RGBA)
 			for y := ymin; y < rMaxY; y++ {
 				min := img.PixOffset(xmin, y)
 				max := img.PixOffset(rMaxX, y)
 				i0, i1 := (y-ymin)*(xmax-xmin)*4, (y-ymin+1)*(xmax-xmin)*4
 				if i1 > len(d.buf) {
 					return errNoPixels
 				}
 				copy(img.Pix[min:max], d.buf[i0:i1])
 			}
 		}
 	}
 
 	return nil
 }
 
 func newDecoder(r io.Reader) (*decoder, error) {
 	d := &decoder{
 		r:        newReaderAt(r),
 		features: make(map[int][]uint),
 	}
 
 	p := make([]byte, 8)
 	if _, err := d.r.ReadAt(p, 0); err != nil {
 		if err == io.EOF {
 			err = io.ErrUnexpectedEOF
 		}
 		return nil, err
 	}
 	switch string(p[0:4]) {
 	case leHeader:
 		d.byteOrder = binary.LittleEndian
 	case beHeader:
 		d.byteOrder = binary.BigEndian
 	default:
 		return nil, FormatError("malformed header")
 	}
 
 	ifdOffset := int64(d.byteOrder.Uint32(p[4:8]))
 
 	// The first two bytes contain the number of entries (12 bytes each).
 	if _, err := d.r.ReadAt(p[0:2], ifdOffset); err != nil {
 		return nil, err
 	}
 	numItems := int(d.byteOrder.Uint16(p[0:2]))
 
 	// All IFD entries are read in one chunk.
 	var err error
 	p, err = safeReadAt(d.r, uint64(ifdLen*numItems), ifdOffset+2)
 	if err != nil {
 		return nil, err
 	}
 
 	prevTag := -1
 	for i := 0; i < len(p); i += ifdLen {
 		tag, err := d.parseIFD(p[i : i+ifdLen])
 		if err != nil {
 			return nil, err
 		}
 		if tag <= prevTag {
 			return nil, FormatError("tags are not sorted in ascending order")
 		}
 		prevTag = tag
 	}
 
 	d.config.Width = int(d.firstVal(tImageWidth))
 	d.config.Height = int(d.firstVal(tImageLength))
 
 	if _, ok := d.features[tBitsPerSample]; !ok {
 		// Default is 1 per specification.
 		d.features[tBitsPerSample] = []uint{1}
 	}
 	d.bpp = d.firstVal(tBitsPerSample)
 	switch d.bpp {
 	case 0:
 		return nil, FormatError("BitsPerSample must not be 0")
 	case 1, 8, 16:
 		// Nothing to do, these are accepted by this implementation.
 	default:
 		return nil, UnsupportedError(fmt.Sprintf("BitsPerSample of %v", d.bpp))
 	}
 
 	// Determine the image mode.
 	switch d.firstVal(tPhotometricInterpretation) {
 	case pRGB:
 		if d.bpp == 16 {
 			for _, b := range d.features[tBitsPerSample] {
 				if b != 16 {
 					return nil, FormatError("wrong number of samples for 16bit RGB")
 				}
 			}
 		} else {
 			for _, b := range d.features[tBitsPerSample] {
 				if b != 8 {
 					return nil, FormatError("wrong number of samples for 8bit RGB")
 				}
 			}
 		}
 		// RGB images normally have 3 samples per pixel.
 		// If there are more, ExtraSamples (p. 31-32 of the spec)
 		// gives their meaning (usually an alpha channel).
 		//
 		// This implementation does not support extra samples
 		// of an unspecified type.
 		switch len(d.features[tBitsPerSample]) {
 		case 3:
 			d.mode = mRGB
 			if d.bpp == 16 {
 				d.config.ColorModel = color.RGBA64Model
 			} else {
 				d.config.ColorModel = color.RGBAModel
 			}
 		case 4:
 			switch d.firstVal(tExtraSamples) {
 			case 1:
 				d.mode = mRGBA
 				if d.bpp == 16 {
 					d.config.ColorModel = color.RGBA64Model
 				} else {
 					d.config.ColorModel = color.RGBAModel
 				}
 			case 2:
 				d.mode = mNRGBA
 				if d.bpp == 16 {
 					d.config.ColorModel = color.NRGBA64Model
 				} else {
 					d.config.ColorModel = color.NRGBAModel
 				}
 			default:
 				return nil, FormatError("wrong number of samples for RGB")
 			}
 		default:
 			return nil, FormatError("wrong number of samples for RGB")
 		}
 	case pPaletted:
 		d.mode = mPaletted
 		d.config.ColorModel = color.Palette(d.palette)
 	case pWhiteIsZero:
 		d.mode = mGrayInvert
 		if d.bpp == 16 {
 			d.config.ColorModel = color.Gray16Model
 		} else {
 			d.config.ColorModel = color.GrayModel
 		}
 	case pBlackIsZero:
 		d.mode = mGray
 		if d.bpp == 16 {
 			d.config.ColorModel = color.Gray16Model
 		} else {
 			d.config.ColorModel = color.GrayModel
 		}
 	default:
 		return nil, UnsupportedError("color model")
 	}
 
 	return d, nil
 }
 
 // DecodeConfig returns the color model and dimensions of a TIFF image without
 // decoding the entire image.
 func DecodeConfig(r io.Reader) (image.Config, error) {
 	d, err := newDecoder(r)
 	if err != nil {
 		return image.Config{}, err
 	}
 	return d.config, nil
 }
 
 func ccittFillOrder(tiffFillOrder uint) ccitt.Order {
 	if tiffFillOrder == 2 {
 		return ccitt.LSB
 	}
 	return ccitt.MSB
 }
 
 // Decode reads a TIFF image from r and returns it as an image.Image.
 // The type of Image returned depends on the contents of the TIFF.
 func Decode(r io.Reader) (img image.Image, err error) {
 	d, err := newDecoder(r)
 	if err != nil {
 		return
 	}
 
 	blockPadding := false
 	blockWidth := d.config.Width
 	blockHeight := d.config.Height
 	blocksAcross := 1
 	blocksDown := 1
 
 	if d.config.Width == 0 {
 		blocksAcross = 0
 	}
 	if d.config.Height == 0 {
 		blocksDown = 0
 	}
 
 	var blockOffsets, blockCounts []uint
 
 	if int(d.firstVal(tTileWidth)) != 0 {
 		blockPadding = true
 
 		blockWidth = int(d.firstVal(tTileWidth))
 		blockHeight = int(d.firstVal(tTileLength))
 
 		if blockWidth != 0 {
 			blocksAcross = (d.config.Width + blockWidth - 1) / blockWidth
 		}
 		if blockHeight != 0 {
 			blocksDown = (d.config.Height + blockHeight - 1) / blockHeight
 		}
 
 		blockCounts = d.features[tTileByteCounts]
 		blockOffsets = d.features[tTileOffsets]
 
 	} else {
 		if int(d.firstVal(tRowsPerStrip)) != 0 {
 			blockHeight = int(d.firstVal(tRowsPerStrip))
 		}
 
 		if blockHeight != 0 {
 			blocksDown = (d.config.Height + blockHeight - 1) / blockHeight
 		}
 
 		blockOffsets = d.features[tStripOffsets]
 		blockCounts = d.features[tStripByteCounts]
 	}
 
 	// Check if we have the right number of strips/tiles, offsets and counts.
 	if n := blocksAcross * blocksDown; len(blockOffsets) < n || len(blockCounts) < n {
 		return nil, FormatError("inconsistent header")
 	}
 
 	imgRect := image.Rect(0, 0, d.config.Width, d.config.Height)
 	switch d.mode {
 	case mGray, mGrayInvert:
 		if d.bpp == 16 {
 			img = image.NewGray16(imgRect)
 		} else {
 			img = image.NewGray(imgRect)
 		}
 	case mPaletted:
 		img = image.NewPaletted(imgRect, d.palette)
 	case mNRGBA:
 		if d.bpp == 16 {
 			img = image.NewNRGBA64(imgRect)
 		} else {
 			img = image.NewNRGBA(imgRect)
 		}
 	case mRGB, mRGBA:
 		if d.bpp == 16 {
 			img = image.NewRGBA64(imgRect)
 		} else {
 			img = image.NewRGBA(imgRect)
 		}
 	}
 
 	for i := 0; i < blocksAcross; i++ {
 		blkW := blockWidth
 		if !blockPadding && i == blocksAcross-1 && d.config.Width%blockWidth != 0 {
 			blkW = d.config.Width % blockWidth
 		}
 		for j := 0; j < blocksDown; j++ {
 			blkH := blockHeight
 			if !blockPadding && j == blocksDown-1 && d.config.Height%blockHeight != 0 {
 				blkH = d.config.Height % blockHeight
 			}
 			offset := int64(blockOffsets[j*blocksAcross+i])
 			n := int64(blockCounts[j*blocksAcross+i])
 			switch d.firstVal(tCompression) {
 
 			// According to the spec, Compression does not have a default value,
 			// but some tools interpret a missing Compression value as none so we do
 			// the same.
 			case cNone, 0:
 				if b, ok := d.r.(*buffer); ok {
 					d.buf, err = b.Slice(int(offset), int(n))
 				} else {
 					d.buf, err = safeReadAt(d.r, uint64(n), offset)
 				}
 			case cG3:
 				inv := d.firstVal(tPhotometricInterpretation) == pWhiteIsZero
 				order := ccittFillOrder(d.firstVal(tFillOrder))
 				r := ccitt.NewReader(io.NewSectionReader(d.r, offset, n), order, ccitt.Group3, blkW, blkH, &ccitt.Options{Invert: inv, Align: false})
 				d.buf, err = ioutil.ReadAll(r)
 			case cG4:
 				inv := d.firstVal(tPhotometricInterpretation) == pWhiteIsZero
 				order := ccittFillOrder(d.firstVal(tFillOrder))
 				r := ccitt.NewReader(io.NewSectionReader(d.r, offset, n), order, ccitt.Group4, blkW, blkH, &ccitt.Options{Invert: inv, Align: false})
 				d.buf, err = ioutil.ReadAll(r)
 			case cLZW:
 				r := lzw.NewReader(io.NewSectionReader(d.r, offset, n), lzw.MSB, 8)
 				d.buf, err = ioutil.ReadAll(r)
 				r.Close()
 			case cDeflate, cDeflateOld:
 				var r io.ReadCloser
 				r, err = zlib.NewReader(io.NewSectionReader(d.r, offset, n))
 				if err != nil {
 					return nil, err
 				}
 				d.buf, err = ioutil.ReadAll(r)
 				r.Close()
 			case cPackBits:
 				d.buf, err = unpackBits(io.NewSectionReader(d.r, offset, n))
 			default:
 				err = UnsupportedError(fmt.Sprintf("compression value %d", d.firstVal(tCompression)))
 			}
 			if err != nil {
 				return nil, err
 			}
 
 			xmin := i * blockWidth
 			ymin := j * blockHeight
 			xmax := xmin + blkW
 			ymax := ymin + blkH
 			err = d.decode(img, xmin, ymin, xmax, ymax)
 			if err != nil {
 				return nil, err
 			}
 		}
 	}
 	return
 }
 
 func init() {
 	image.RegisterFormat("tiff", leHeader, Decode, DecodeConfig)
 	image.RegisterFormat("tiff", beHeader, Decode, DecodeConfig)
 }