Source file src/pkg/image/png/writer.go

     1	// Copyright 2009 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 png
     6	
     7	import (
     8		"bufio"
     9		"compress/zlib"
    10		"hash/crc32"
    11		"image"
    12		"image/color"
    13		"io"
    14		"strconv"
    15	)
    16	
    17	type encoder struct {
    18		w      io.Writer
    19		m      image.Image
    20		cb     int
    21		err    error
    22		header [8]byte
    23		footer [4]byte
    24		tmp    [3 * 256]byte
    25	}
    26	
    27	// Big-endian.
    28	func writeUint32(b []uint8, u uint32) {
    29		b[0] = uint8(u >> 24)
    30		b[1] = uint8(u >> 16)
    31		b[2] = uint8(u >> 8)
    32		b[3] = uint8(u >> 0)
    33	}
    34	
    35	type opaquer interface {
    36		Opaque() bool
    37	}
    38	
    39	// Returns whether or not the image is fully opaque.
    40	func opaque(m image.Image) bool {
    41		if o, ok := m.(opaquer); ok {
    42			return o.Opaque()
    43		}
    44		b := m.Bounds()
    45		for y := b.Min.Y; y < b.Max.Y; y++ {
    46			for x := b.Min.X; x < b.Max.X; x++ {
    47				_, _, _, a := m.At(x, y).RGBA()
    48				if a != 0xffff {
    49					return false
    50				}
    51			}
    52		}
    53		return true
    54	}
    55	
    56	// The absolute value of a byte interpreted as a signed int8.
    57	func abs8(d uint8) int {
    58		if d < 128 {
    59			return int(d)
    60		}
    61		return 256 - int(d)
    62	}
    63	
    64	func (e *encoder) writeChunk(b []byte, name string) {
    65		if e.err != nil {
    66			return
    67		}
    68		n := uint32(len(b))
    69		if int(n) != len(b) {
    70			e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))
    71			return
    72		}
    73		writeUint32(e.header[0:4], n)
    74		e.header[4] = name[0]
    75		e.header[5] = name[1]
    76		e.header[6] = name[2]
    77		e.header[7] = name[3]
    78		crc := crc32.NewIEEE()
    79		crc.Write(e.header[4:8])
    80		crc.Write(b)
    81		writeUint32(e.footer[0:4], crc.Sum32())
    82	
    83		_, e.err = e.w.Write(e.header[0:8])
    84		if e.err != nil {
    85			return
    86		}
    87		_, e.err = e.w.Write(b)
    88		if e.err != nil {
    89			return
    90		}
    91		_, e.err = e.w.Write(e.footer[0:4])
    92	}
    93	
    94	func (e *encoder) writeIHDR() {
    95		b := e.m.Bounds()
    96		writeUint32(e.tmp[0:4], uint32(b.Dx()))
    97		writeUint32(e.tmp[4:8], uint32(b.Dy()))
    98		// Set bit depth and color type.
    99		switch e.cb {
   100		case cbG8:
   101			e.tmp[8] = 8
   102			e.tmp[9] = ctGrayscale
   103		case cbTC8:
   104			e.tmp[8] = 8
   105			e.tmp[9] = ctTrueColor
   106		case cbP8:
   107			e.tmp[8] = 8
   108			e.tmp[9] = ctPaletted
   109		case cbTCA8:
   110			e.tmp[8] = 8
   111			e.tmp[9] = ctTrueColorAlpha
   112		case cbG16:
   113			e.tmp[8] = 16
   114			e.tmp[9] = ctGrayscale
   115		case cbTC16:
   116			e.tmp[8] = 16
   117			e.tmp[9] = ctTrueColor
   118		case cbTCA16:
   119			e.tmp[8] = 16
   120			e.tmp[9] = ctTrueColorAlpha
   121		}
   122		e.tmp[10] = 0 // default compression method
   123		e.tmp[11] = 0 // default filter method
   124		e.tmp[12] = 0 // non-interlaced
   125		e.writeChunk(e.tmp[0:13], "IHDR")
   126	}
   127	
   128	func (e *encoder) writePLTE(p color.Palette) {
   129		if len(p) < 1 || len(p) > 256 {
   130			e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
   131			return
   132		}
   133		for i, c := range p {
   134			r, g, b, _ := c.RGBA()
   135			e.tmp[3*i+0] = uint8(r >> 8)
   136			e.tmp[3*i+1] = uint8(g >> 8)
   137			e.tmp[3*i+2] = uint8(b >> 8)
   138		}
   139		e.writeChunk(e.tmp[0:3*len(p)], "PLTE")
   140	}
   141	
   142	func (e *encoder) maybeWritetRNS(p color.Palette) {
   143		last := -1
   144		for i, c := range p {
   145			_, _, _, a := c.RGBA()
   146			if a != 0xffff {
   147				last = i
   148			}
   149			e.tmp[i] = uint8(a >> 8)
   150		}
   151		if last == -1 {
   152			return
   153		}
   154		e.writeChunk(e.tmp[:last+1], "tRNS")
   155	}
   156	
   157	// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
   158	// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
   159	// should be relatively infrequent, since writeIDATs uses a bufio.Writer.
   160	//
   161	// This method should only be called from writeIDATs (via writeImage).
   162	// No other code should treat an encoder as an io.Writer.
   163	func (e *encoder) Write(b []byte) (int, error) {
   164		e.writeChunk(b, "IDAT")
   165		if e.err != nil {
   166			return 0, e.err
   167		}
   168		return len(b), nil
   169	}
   170	
   171	// Chooses the filter to use for encoding the current row, and applies it.
   172	// The return value is the index of the filter and also of the row in cr that has had it applied.
   173	func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
   174		// We try all five filter types, and pick the one that minimizes the sum of absolute differences.
   175		// This is the same heuristic that libpng uses, although the filters are attempted in order of
   176		// estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than
   177		// in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).
   178		cdat0 := cr[0][1:]
   179		cdat1 := cr[1][1:]
   180		cdat2 := cr[2][1:]
   181		cdat3 := cr[3][1:]
   182		cdat4 := cr[4][1:]
   183		pdat := pr[1:]
   184		n := len(cdat0)
   185	
   186		// The up filter.
   187		sum := 0
   188		for i := 0; i < n; i++ {
   189			cdat2[i] = cdat0[i] - pdat[i]
   190			sum += abs8(cdat2[i])
   191		}
   192		best := sum
   193		filter := ftUp
   194	
   195		// The Paeth filter.
   196		sum = 0
   197		for i := 0; i < bpp; i++ {
   198			cdat4[i] = cdat0[i] - paeth(0, pdat[i], 0)
   199			sum += abs8(cdat4[i])
   200		}
   201		for i := bpp; i < n; i++ {
   202			cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])
   203			sum += abs8(cdat4[i])
   204			if sum >= best {
   205				break
   206			}
   207		}
   208		if sum < best {
   209			best = sum
   210			filter = ftPaeth
   211		}
   212	
   213		// The none filter.
   214		sum = 0
   215		for i := 0; i < n; i++ {
   216			sum += abs8(cdat0[i])
   217			if sum >= best {
   218				break
   219			}
   220		}
   221		if sum < best {
   222			best = sum
   223			filter = ftNone
   224		}
   225	
   226		// The sub filter.
   227		sum = 0
   228		for i := 0; i < bpp; i++ {
   229			cdat1[i] = cdat0[i]
   230			sum += abs8(cdat1[i])
   231		}
   232		for i := bpp; i < n; i++ {
   233			cdat1[i] = cdat0[i] - cdat0[i-bpp]
   234			sum += abs8(cdat1[i])
   235			if sum >= best {
   236				break
   237			}
   238		}
   239		if sum < best {
   240			best = sum
   241			filter = ftSub
   242		}
   243	
   244		// The average filter.
   245		sum = 0
   246		for i := 0; i < bpp; i++ {
   247			cdat3[i] = cdat0[i] - pdat[i]/2
   248			sum += abs8(cdat3[i])
   249		}
   250		for i := bpp; i < n; i++ {
   251			cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)
   252			sum += abs8(cdat3[i])
   253			if sum >= best {
   254				break
   255			}
   256		}
   257		if sum < best {
   258			best = sum
   259			filter = ftAverage
   260		}
   261	
   262		return filter
   263	}
   264	
   265	func writeImage(w io.Writer, m image.Image, cb int) error {
   266		zw := zlib.NewWriter(w)
   267		defer zw.Close()
   268	
   269		bpp := 0 // Bytes per pixel.
   270	
   271		switch cb {
   272		case cbG8:
   273			bpp = 1
   274		case cbTC8:
   275			bpp = 3
   276		case cbP8:
   277			bpp = 1
   278		case cbTCA8:
   279			bpp = 4
   280		case cbTC16:
   281			bpp = 6
   282		case cbTCA16:
   283			bpp = 8
   284		case cbG16:
   285			bpp = 2
   286		}
   287		// cr[*] and pr are the bytes for the current and previous row.
   288		// cr[0] is unfiltered (or equivalently, filtered with the ftNone filter).
   289		// cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the
   290		// other PNG filter types. These buffers are allocated once and re-used for each row.
   291		// The +1 is for the per-row filter type, which is at cr[*][0].
   292		b := m.Bounds()
   293		var cr [nFilter][]uint8
   294		for i := range cr {
   295			cr[i] = make([]uint8, 1+bpp*b.Dx())
   296			cr[i][0] = uint8(i)
   297		}
   298		pr := make([]uint8, 1+bpp*b.Dx())
   299	
   300		for y := b.Min.Y; y < b.Max.Y; y++ {
   301			// Convert from colors to bytes.
   302			i := 1
   303			switch cb {
   304			case cbG8:
   305				for x := b.Min.X; x < b.Max.X; x++ {
   306					c := color.GrayModel.Convert(m.At(x, y)).(color.Gray)
   307					cr[0][i] = c.Y
   308					i++
   309				}
   310			case cbTC8:
   311				// We have previously verified that the alpha value is fully opaque.
   312				cr0 := cr[0]
   313				if rgba, _ := m.(*image.RGBA); rgba != nil {
   314					j0 := (y - b.Min.Y) * rgba.Stride
   315					j1 := j0 + b.Dx()*4
   316					for j := j0; j < j1; j += 4 {
   317						cr0[i+0] = rgba.Pix[j+0]
   318						cr0[i+1] = rgba.Pix[j+1]
   319						cr0[i+2] = rgba.Pix[j+2]
   320						i += 3
   321					}
   322				} else {
   323					for x := b.Min.X; x < b.Max.X; x++ {
   324						r, g, b, _ := m.At(x, y).RGBA()
   325						cr0[i+0] = uint8(r >> 8)
   326						cr0[i+1] = uint8(g >> 8)
   327						cr0[i+2] = uint8(b >> 8)
   328						i += 3
   329					}
   330				}
   331			case cbP8:
   332				if p, _ := m.(*image.Paletted); p != nil {
   333					offset := (y - b.Min.Y) * p.Stride
   334					copy(cr[0][1:], p.Pix[offset:offset+b.Dx()])
   335				} else {
   336					pi := m.(image.PalettedImage)
   337					for x := b.Min.X; x < b.Max.X; x++ {
   338						cr[0][i] = pi.ColorIndexAt(x, y)
   339						i += 1
   340					}
   341				}
   342			case cbTCA8:
   343				// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
   344				for x := b.Min.X; x < b.Max.X; x++ {
   345					c := color.NRGBAModel.Convert(m.At(x, y)).(color.NRGBA)
   346					cr[0][i+0] = c.R
   347					cr[0][i+1] = c.G
   348					cr[0][i+2] = c.B
   349					cr[0][i+3] = c.A
   350					i += 4
   351				}
   352			case cbG16:
   353				for x := b.Min.X; x < b.Max.X; x++ {
   354					c := color.Gray16Model.Convert(m.At(x, y)).(color.Gray16)
   355					cr[0][i+0] = uint8(c.Y >> 8)
   356					cr[0][i+1] = uint8(c.Y)
   357					i += 2
   358				}
   359			case cbTC16:
   360				// We have previously verified that the alpha value is fully opaque.
   361				for x := b.Min.X; x < b.Max.X; x++ {
   362					r, g, b, _ := m.At(x, y).RGBA()
   363					cr[0][i+0] = uint8(r >> 8)
   364					cr[0][i+1] = uint8(r)
   365					cr[0][i+2] = uint8(g >> 8)
   366					cr[0][i+3] = uint8(g)
   367					cr[0][i+4] = uint8(b >> 8)
   368					cr[0][i+5] = uint8(b)
   369					i += 6
   370				}
   371			case cbTCA16:
   372				// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
   373				for x := b.Min.X; x < b.Max.X; x++ {
   374					c := color.NRGBA64Model.Convert(m.At(x, y)).(color.NRGBA64)
   375					cr[0][i+0] = uint8(c.R >> 8)
   376					cr[0][i+1] = uint8(c.R)
   377					cr[0][i+2] = uint8(c.G >> 8)
   378					cr[0][i+3] = uint8(c.G)
   379					cr[0][i+4] = uint8(c.B >> 8)
   380					cr[0][i+5] = uint8(c.B)
   381					cr[0][i+6] = uint8(c.A >> 8)
   382					cr[0][i+7] = uint8(c.A)
   383					i += 8
   384				}
   385			}
   386	
   387			// Apply the filter.
   388			f := filter(&cr, pr, bpp)
   389	
   390			// Write the compressed bytes.
   391			if _, err := zw.Write(cr[f]); err != nil {
   392				return err
   393			}
   394	
   395			// The current row for y is the previous row for y+1.
   396			pr, cr[0] = cr[0], pr
   397		}
   398		return nil
   399	}
   400	
   401	// Write the actual image data to one or more IDAT chunks.
   402	func (e *encoder) writeIDATs() {
   403		if e.err != nil {
   404			return
   405		}
   406		var bw *bufio.Writer
   407		bw = bufio.NewWriterSize(e, 1<<15)
   408		e.err = writeImage(bw, e.m, e.cb)
   409		if e.err != nil {
   410			return
   411		}
   412		e.err = bw.Flush()
   413	}
   414	
   415	func (e *encoder) writeIEND() { e.writeChunk(e.tmp[0:0], "IEND") }
   416	
   417	// Encode writes the Image m to w in PNG format. Any Image may be encoded, but
   418	// images that are not image.NRGBA might be encoded lossily.
   419	func Encode(w io.Writer, m image.Image) error {
   420		// Obviously, negative widths and heights are invalid. Furthermore, the PNG
   421		// spec section 11.2.2 says that zero is invalid. Excessively large images are
   422		// also rejected.
   423		mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())
   424		if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 {
   425			return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mw, 10))
   426		}
   427	
   428		var e encoder
   429		e.w = w
   430		e.m = m
   431	
   432		var pal color.Palette
   433		// cbP8 encoding needs PalettedImage's ColorIndexAt method.
   434		if _, ok := m.(image.PalettedImage); ok {
   435			pal, _ = m.ColorModel().(color.Palette)
   436		}
   437		if pal != nil {
   438			e.cb = cbP8
   439		} else {
   440			switch m.ColorModel() {
   441			case color.GrayModel:
   442				e.cb = cbG8
   443			case color.Gray16Model:
   444				e.cb = cbG16
   445			case color.RGBAModel, color.NRGBAModel, color.AlphaModel:
   446				if opaque(m) {
   447					e.cb = cbTC8
   448				} else {
   449					e.cb = cbTCA8
   450				}
   451			default:
   452				if opaque(m) {
   453					e.cb = cbTC16
   454				} else {
   455					e.cb = cbTCA16
   456				}
   457			}
   458		}
   459	
   460		_, e.err = io.WriteString(w, pngHeader)
   461		e.writeIHDR()
   462		if pal != nil {
   463			e.writePLTE(pal)
   464			e.maybeWritetRNS(pal)
   465		}
   466		e.writeIDATs()
   467		e.writeIEND()
   468		return e.err
   469	}