writer.go

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package gif
     6  
     7  import (
     8  	"bufio"
     9  	"bytes"
    10  	"compress/lzw"
    11  	"errors"
    12  	"image"
    13  	"image/color"
    14  	"image/color/palette"
    15  	"image/draw"
    16  	"internal/byteorder"
    17  	"io"
    18  	"math/bits"
    19  )
    20  
    21  // Graphic control extension fields.
    22  const (
    23  	gcLabel     = 0xF9
    24  	gcBlockSize = 0x04
    25  )
    26  
    27  func log2(x int) int {
    28  	if x < 2 {
    29  		return 0
    30  	}
    31  	return bits.Len(uint(x-1)) - 1
    32  }
    33  
    34  // writer is a buffered writer.
    35  type writer interface {
    36  	Flush() error
    37  	io.Writer
    38  	io.ByteWriter
    39  }
    40  
    41  // encoder encodes an image to the GIF format.
    42  type encoder struct {
    43  	// w is the writer to write to. err is the first error encountered during
    44  	// writing. All attempted writes after the first error become no-ops.
    45  	w   writer
    46  	err error
    47  	// g is a reference to the data that is being encoded.
    48  	g GIF
    49  	// globalCT is the size in bytes of the global color table.
    50  	globalCT int
    51  	// buf is a scratch buffer. It must be at least 256 for the blockWriter.
    52  	buf              [256]byte
    53  	globalColorTable [3 * 256]byte
    54  	localColorTable  [3 * 256]byte
    55  }
    56  
    57  // blockWriter writes the block structure of GIF image data, which
    58  // comprises (n, (n bytes)) blocks, with 1 <= n <= 255. It is the
    59  // writer given to the LZW encoder, which is thus immune to the
    60  // blocking.
    61  type blockWriter struct {
    62  	e *encoder
    63  }
    64  
    65  func (b blockWriter) setup() {
    66  	b.e.buf[0] = 0
    67  }
    68  
    69  func (b blockWriter) Flush() error {
    70  	return b.e.err
    71  }
    72  
    73  func (b blockWriter) WriteByte(c byte) error {
    74  	if b.e.err != nil {
    75  		return b.e.err
    76  	}
    77  
    78  	// Append c to buffered sub-block.
    79  	b.e.buf[0]++
    80  	b.e.buf[b.e.buf[0]] = c
    81  	if b.e.buf[0] < 255 {
    82  		return nil
    83  	}
    84  
    85  	// Flush block
    86  	b.e.write(b.e.buf[:256])
    87  	b.e.buf[0] = 0
    88  	return b.e.err
    89  }
    90  
    91  // blockWriter must be an io.Writer for lzw.NewWriter, but this is never
    92  // actually called.
    93  func (b blockWriter) Write(data []byte) (int, error) {
    94  	for i, c := range data {
    95  		if err := b.WriteByte(c); err != nil {
    96  			return i, err
    97  		}
    98  	}
    99  	return len(data), nil
   100  }
   101  
   102  func (b blockWriter) close() {
   103  	// Write the block terminator (0x00), either by itself, or along with a
   104  	// pending sub-block.
   105  	if b.e.buf[0] == 0 {
   106  		b.e.writeByte(0)
   107  	} else {
   108  		n := uint(b.e.buf[0])
   109  		b.e.buf[n+1] = 0
   110  		b.e.write(b.e.buf[:n+2])
   111  	}
   112  	b.e.flush()
   113  }
   114  
   115  func (e *encoder) flush() {
   116  	if e.err != nil {
   117  		return
   118  	}
   119  	e.err = e.w.Flush()
   120  }
   121  
   122  func (e *encoder) write(p []byte) {
   123  	if e.err != nil {
   124  		return
   125  	}
   126  	_, e.err = e.w.Write(p)
   127  }
   128  
   129  func (e *encoder) writeByte(b byte) {
   130  	if e.err != nil {
   131  		return
   132  	}
   133  	e.err = e.w.WriteByte(b)
   134  }
   135  
   136  func (e *encoder) writeHeader() {
   137  	if e.err != nil {
   138  		return
   139  	}
   140  	_, e.err = io.WriteString(e.w, "GIF89a")
   141  	if e.err != nil {
   142  		return
   143  	}
   144  
   145  	// Logical screen width and height.
   146  	byteorder.LEPutUint16(e.buf[0:2], uint16(e.g.Config.Width))
   147  	byteorder.LEPutUint16(e.buf[2:4], uint16(e.g.Config.Height))
   148  	e.write(e.buf[:4])
   149  
   150  	if p, ok := e.g.Config.ColorModel.(color.Palette); ok && len(p) > 0 {
   151  		paddedSize := log2(len(p)) // Size of Global Color Table: 2^(1+n).
   152  		e.buf[0] = fColorTable | uint8(paddedSize)
   153  		e.buf[1] = e.g.BackgroundIndex
   154  		e.buf[2] = 0x00 // Pixel Aspect Ratio.
   155  		e.write(e.buf[:3])
   156  		var err error
   157  		e.globalCT, err = encodeColorTable(e.globalColorTable[:], p, paddedSize)
   158  		if err != nil && e.err == nil {
   159  			e.err = err
   160  			return
   161  		}
   162  		e.write(e.globalColorTable[:e.globalCT])
   163  	} else {
   164  		// All frames have a local color table, so a global color table
   165  		// is not needed.
   166  		e.buf[0] = 0x00
   167  		e.buf[1] = 0x00 // Background Color Index.
   168  		e.buf[2] = 0x00 // Pixel Aspect Ratio.
   169  		e.write(e.buf[:3])
   170  	}
   171  
   172  	// Add animation info if necessary.
   173  	if len(e.g.Image) > 1 && e.g.LoopCount >= 0 {
   174  		e.buf[0] = 0x21 // Extension Introducer.
   175  		e.buf[1] = 0xff // Application Label.
   176  		e.buf[2] = 0x0b // Block Size.
   177  		e.write(e.buf[:3])
   178  		_, err := io.WriteString(e.w, "NETSCAPE2.0") // Application Identifier.
   179  		if err != nil && e.err == nil {
   180  			e.err = err
   181  			return
   182  		}
   183  		e.buf[0] = 0x03 // Block Size.
   184  		e.buf[1] = 0x01 // Sub-block Index.
   185  		byteorder.LEPutUint16(e.buf[2:4], uint16(e.g.LoopCount))
   186  		e.buf[4] = 0x00 // Block Terminator.
   187  		e.write(e.buf[:5])
   188  	}
   189  }
   190  
   191  func encodeColorTable(dst []byte, p color.Palette, size int) (int, error) {
   192  	if uint(size) >= 8 {
   193  		return 0, errors.New("gif: cannot encode color table with more than 256 entries")
   194  	}
   195  	for i, c := range p {
   196  		if c == nil {
   197  			return 0, errors.New("gif: cannot encode color table with nil entries")
   198  		}
   199  		var r, g, b uint8
   200  		// It is most likely that the palette is full of color.RGBAs, so they
   201  		// get a fast path.
   202  		if rgba, ok := c.(color.RGBA); ok {
   203  			r, g, b = rgba.R, rgba.G, rgba.B
   204  		} else {
   205  			rr, gg, bb, _ := c.RGBA()
   206  			r, g, b = uint8(rr>>8), uint8(gg>>8), uint8(bb>>8)
   207  		}
   208  		dst[3*i+0] = r
   209  		dst[3*i+1] = g
   210  		dst[3*i+2] = b
   211  	}
   212  	n := 1 << (size + 1)
   213  	if n > len(p) {
   214  		// Pad with black.
   215  		clear(dst[3*len(p) : 3*n])
   216  	}
   217  	return 3 * n, nil
   218  }
   219  
   220  func (e *encoder) colorTablesMatch(localLen, transparentIndex int) bool {
   221  	localSize := 3 * localLen
   222  	if transparentIndex >= 0 {
   223  		trOff := 3 * transparentIndex
   224  		return bytes.Equal(e.globalColorTable[:trOff], e.localColorTable[:trOff]) &&
   225  			bytes.Equal(e.globalColorTable[trOff+3:localSize], e.localColorTable[trOff+3:localSize])
   226  	}
   227  	return bytes.Equal(e.globalColorTable[:localSize], e.localColorTable[:localSize])
   228  }
   229  
   230  func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte) {
   231  	if e.err != nil {
   232  		return
   233  	}
   234  
   235  	if len(pm.Palette) == 0 {
   236  		e.err = errors.New("gif: cannot encode image block with empty palette")
   237  		return
   238  	}
   239  
   240  	b := pm.Bounds()
   241  	if b.Min.X < 0 || b.Max.X >= 1<<16 || b.Min.Y < 0 || b.Max.Y >= 1<<16 {
   242  		e.err = errors.New("gif: image block is too large to encode")
   243  		return
   244  	}
   245  	if !b.In(image.Rectangle{Max: image.Point{e.g.Config.Width, e.g.Config.Height}}) {
   246  		e.err = errors.New("gif: image block is out of bounds")
   247  		return
   248  	}
   249  
   250  	transparentIndex := -1
   251  	for i, c := range pm.Palette {
   252  		if c == nil {
   253  			e.err = errors.New("gif: cannot encode color table with nil entries")
   254  			return
   255  		}
   256  		if _, _, _, a := c.RGBA(); a == 0 {
   257  			transparentIndex = i
   258  			break
   259  		}
   260  	}
   261  
   262  	if delay > 0 || disposal != 0 || transparentIndex != -1 {
   263  		e.buf[0] = sExtension  // Extension Introducer.
   264  		e.buf[1] = gcLabel     // Graphic Control Label.
   265  		e.buf[2] = gcBlockSize // Block Size.
   266  		if transparentIndex != -1 {
   267  			e.buf[3] = 0x01 | disposal<<2
   268  		} else {
   269  			e.buf[3] = 0x00 | disposal<<2
   270  		}
   271  		byteorder.LEPutUint16(e.buf[4:6], uint16(delay)) // Delay Time (1/100ths of a second)
   272  
   273  		// Transparent color index.
   274  		if transparentIndex != -1 {
   275  			e.buf[6] = uint8(transparentIndex)
   276  		} else {
   277  			e.buf[6] = 0x00
   278  		}
   279  		e.buf[7] = 0x00 // Block Terminator.
   280  		e.write(e.buf[:8])
   281  	}
   282  	e.buf[0] = sImageDescriptor
   283  	byteorder.LEPutUint16(e.buf[1:3], uint16(b.Min.X))
   284  	byteorder.LEPutUint16(e.buf[3:5], uint16(b.Min.Y))
   285  	byteorder.LEPutUint16(e.buf[5:7], uint16(b.Dx()))
   286  	byteorder.LEPutUint16(e.buf[7:9], uint16(b.Dy()))
   287  	e.write(e.buf[:9])
   288  
   289  	// To determine whether or not this frame's palette is the same as the
   290  	// global palette, we can check a couple things. First, do they actually
   291  	// point to the same []color.Color? If so, they are equal so long as the
   292  	// frame's palette is not longer than the global palette...
   293  	paddedSize := log2(len(pm.Palette)) // Size of Local Color Table: 2^(1+n).
   294  	if gp, ok := e.g.Config.ColorModel.(color.Palette); ok && len(pm.Palette) <= len(gp) && &gp[0] == &pm.Palette[0] {
   295  		e.writeByte(0) // Use the global color table.
   296  	} else {
   297  		ct, err := encodeColorTable(e.localColorTable[:], pm.Palette, paddedSize)
   298  		if err != nil {
   299  			if e.err == nil {
   300  				e.err = err
   301  			}
   302  			return
   303  		}
   304  		// This frame's palette is not the very same slice as the global
   305  		// palette, but it might be a copy, possibly with one value turned into
   306  		// transparency by DecodeAll.
   307  		if ct <= e.globalCT && e.colorTablesMatch(len(pm.Palette), transparentIndex) {
   308  			e.writeByte(0) // Use the global color table.
   309  		} else {
   310  			// Use a local color table.
   311  			e.writeByte(fColorTable | uint8(paddedSize))
   312  			e.write(e.localColorTable[:ct])
   313  		}
   314  	}
   315  
   316  	litWidth := paddedSize + 1
   317  	if litWidth < 2 {
   318  		litWidth = 2
   319  	}
   320  	e.writeByte(uint8(litWidth)) // LZW Minimum Code Size.
   321  
   322  	bw := blockWriter{e: e}
   323  	bw.setup()
   324  	lzww := lzw.NewWriter(bw, lzw.LSB, litWidth)
   325  	if dx := b.Dx(); dx == pm.Stride {
   326  		_, e.err = lzww.Write(pm.Pix[:dx*b.Dy()])
   327  		if e.err != nil {
   328  			lzww.Close()
   329  			return
   330  		}
   331  	} else {
   332  		for i, y := 0, b.Min.Y; y < b.Max.Y; i, y = i+pm.Stride, y+1 {
   333  			_, e.err = lzww.Write(pm.Pix[i : i+dx])
   334  			if e.err != nil {
   335  				lzww.Close()
   336  				return
   337  			}
   338  		}
   339  	}
   340  	lzww.Close() // flush to bw
   341  	bw.close()   // flush to e.w
   342  }
   343  
   344  // Options are the encoding parameters.
   345  type Options struct {
   346  	// NumColors is the maximum number of colors used in the image.
   347  	// It ranges from 1 to 256.
   348  	NumColors int
   349  
   350  	// Quantizer is used to produce a palette with size NumColors.
   351  	// palette.Plan9 is used in place of a nil Quantizer.
   352  	Quantizer draw.Quantizer
   353  
   354  	// Drawer is used to convert the source image to the desired palette.
   355  	// draw.FloydSteinberg is used in place of a nil Drawer.
   356  	Drawer draw.Drawer
   357  }
   358  
   359  // EncodeAll writes the images in g to w in GIF format with the
   360  // given loop count and delay between frames.
   361  func EncodeAll(w io.Writer, g *GIF) error {
   362  	if len(g.Image) == 0 {
   363  		return errors.New("gif: must provide at least one image")
   364  	}
   365  
   366  	if len(g.Image) != len(g.Delay) {
   367  		return errors.New("gif: mismatched image and delay lengths")
   368  	}
   369  
   370  	e := encoder{g: *g}
   371  	// The GIF.Disposal, GIF.Config and GIF.BackgroundIndex fields were added
   372  	// in Go 1.5. Valid Go 1.4 code, such as when the Disposal field is omitted
   373  	// in a GIF struct literal, should still produce valid GIFs.
   374  	if e.g.Disposal != nil && len(e.g.Image) != len(e.g.Disposal) {
   375  		return errors.New("gif: mismatched image and disposal lengths")
   376  	}
   377  	if e.g.Config == (image.Config{}) {
   378  		p := g.Image[0].Bounds().Max
   379  		e.g.Config.Width = p.X
   380  		e.g.Config.Height = p.Y
   381  	} else if e.g.Config.ColorModel != nil {
   382  		if _, ok := e.g.Config.ColorModel.(color.Palette); !ok {
   383  			return errors.New("gif: GIF color model must be a color.Palette")
   384  		}
   385  	}
   386  
   387  	if ww, ok := w.(writer); ok {
   388  		e.w = ww
   389  	} else {
   390  		e.w = bufio.NewWriter(w)
   391  	}
   392  
   393  	e.writeHeader()
   394  	for i, pm := range g.Image {
   395  		disposal := uint8(0)
   396  		if g.Disposal != nil {
   397  			disposal = g.Disposal[i]
   398  		}
   399  		e.writeImageBlock(pm, g.Delay[i], disposal)
   400  	}
   401  	e.writeByte(sTrailer)
   402  	e.flush()
   403  	return e.err
   404  }
   405  
   406  // Encode writes the Image m to w in GIF format.
   407  func Encode(w io.Writer, m image.Image, o *Options) error {
   408  	// Check for bounds and size restrictions.
   409  	b := m.Bounds()
   410  	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
   411  		return errors.New("gif: image is too large to encode")
   412  	}
   413  
   414  	opts := Options{}
   415  	if o != nil {
   416  		opts = *o
   417  	}
   418  	if opts.NumColors < 1 || 256 < opts.NumColors {
   419  		opts.NumColors = 256
   420  	}
   421  	if opts.Drawer == nil {
   422  		opts.Drawer = draw.FloydSteinberg
   423  	}
   424  
   425  	pm, _ := m.(*image.Paletted)
   426  	if pm == nil {
   427  		if cp, ok := m.ColorModel().(color.Palette); ok {
   428  			pm = image.NewPaletted(b, cp)
   429  			for y := b.Min.Y; y < b.Max.Y; y++ {
   430  				for x := b.Min.X; x < b.Max.X; x++ {
   431  					pm.Set(x, y, cp.Convert(m.At(x, y)))
   432  				}
   433  			}
   434  		}
   435  	}
   436  	if pm == nil || len(pm.Palette) > opts.NumColors {
   437  		// Set pm to be a palettedized copy of m, including its bounds, which
   438  		// might not start at (0, 0).
   439  		//
   440  		// TODO: Pick a better sub-sample of the Plan 9 palette.
   441  		pm = image.NewPaletted(b, palette.Plan9[:opts.NumColors])
   442  		if opts.Quantizer != nil {
   443  			pm.Palette = opts.Quantizer.Quantize(make(color.Palette, 0, opts.NumColors), m)
   444  		}
   445  		opts.Drawer.Draw(pm, b, m, b.Min)
   446  	}
   447  
   448  	// When calling Encode instead of EncodeAll, the single-frame image is
   449  	// translated such that its top-left corner is (0, 0), so that the single
   450  	// frame completely fills the overall GIF's bounds.
   451  	if pm.Rect.Min != (image.Point{}) {
   452  		dup := *pm
   453  		dup.Rect = dup.Rect.Sub(dup.Rect.Min)
   454  		pm = &dup
   455  	}
   456  
   457  	return EncodeAll(w, &GIF{
   458  		Image: []*image.Paletted{pm},
   459  		Delay: []int{0},
   460  		Config: image.Config{
   461  			ColorModel: pm.Palette,
   462  			Width:      b.Dx(),
   463  			Height:     b.Dy(),
   464  		},
   465  	})
   466  }
   467
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