Subversion Repositories openrisc

// 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 gif implements a GIF image decoder.
//
// The GIF specification is at http://www.w3.org/Graphics/GIF/spec-gif89a.txt.
package gif

import (
        "bufio"
        "compress/lzw"
        "errors"
        "fmt"
        "image"
        "image/color"
        "io"
)

// If the io.Reader does not also have ReadByte, then decode will introduce its own buffering.
type reader interface {
        io.Reader
        io.ByteReader
}

// Masks etc.
const (
        // Fields.
        fColorMapFollows = 1 << 7

        // Image fields.
        ifLocalColorTable = 1 << 7
        ifInterlace       = 1 << 6
        ifPixelSizeMask   = 7

        // Graphic control flags.
        gcTransparentColorSet = 1 << 0
)

// Section indicators.
const (
        sExtension       = 0x21
        sImageDescriptor = 0x2C
        sTrailer         = 0x3B
)

// Extensions.
const (
        eText           = 0x01 // Plain Text
        eGraphicControl = 0xF9 // Graphic Control
        eComment        = 0xFE // Comment
        eApplication    = 0xFF // Application
)

// decoder is the type used to decode a GIF file.
type decoder struct {
        r reader

        // From header.
        vers            string
        width           int
        height          int
        flags           byte
        headerFields    byte
        backgroundIndex byte
        loopCount       int
        delayTime       int

        // Unused from header.
        aspect byte

        // From image descriptor.
        imageFields byte

        // From graphics control.
        transparentIndex byte

        // Computed.
        pixelSize      uint
        globalColorMap color.Palette

        // Used when decoding.
        delay []int
        image []*image.Paletted
        tmp   [1024]byte // must be at least 768 so we can read color map
}

// blockReader parses the block structure of GIF image data, which
// comprises (n, (n bytes)) blocks, with 1 <= n <= 255.  It is the
// reader given to the LZW decoder, which is thus immune to the
// blocking.  After the LZW decoder completes, there will be a 0-byte
// block remaining (0, ()), but under normal execution blockReader
// doesn't consume it, so it is handled in decode.
type blockReader struct {
        r     reader
        slice []byte
        tmp   [256]byte
}

func (b *blockReader) Read(p []byte) (int, error) {
        if len(p) == 0 {
                return 0, nil
        }
        if len(b.slice) == 0 {
                blockLen, err := b.r.ReadByte()
                if err != nil {
                        return 0, err
                }
                if blockLen == 0 {
                        return 0, io.EOF
                }
                b.slice = b.tmp[0:blockLen]
                if _, err = io.ReadFull(b.r, b.slice); err != nil {
                        return 0, err
                }
        }
        n := copy(p, b.slice)
        b.slice = b.slice[n:]
        return n, nil
}

// decode reads a GIF image from r and stores the result in d.
func (d *decoder) decode(r io.Reader, configOnly bool) error {
        // Add buffering if r does not provide ReadByte.
        if rr, ok := r.(reader); ok {
                d.r = rr
        } else {
                d.r = bufio.NewReader(r)
        }

        err := d.readHeaderAndScreenDescriptor()
        if err != nil {
                return err
        }
        if configOnly {
                return nil
        }

        if d.headerFields&fColorMapFollows != 0 {
                if d.globalColorMap, err = d.readColorMap(); err != nil {
                        return err
                }
        }

Loop:
        for err == nil {
                var c byte
                c, err = d.r.ReadByte()
                if err == io.EOF {
                        break
                }
                switch c {
                case sExtension:
                        err = d.readExtension()

                case sImageDescriptor:
                        var m *image.Paletted
                        m, err = d.newImageFromDescriptor()
                        if err != nil {
                                break
                        }
                        if d.imageFields&fColorMapFollows != 0 {
                                m.Palette, err = d.readColorMap()
                                if err != nil {
                                        break
                                }
                                // TODO: do we set transparency in this map too? That would be
                                // d.setTransparency(m.Palette)
                        } else {
                                m.Palette = d.globalColorMap
                        }
                        var litWidth uint8
                        litWidth, err = d.r.ReadByte()
                        if err != nil {
                                return err
                        }
                        if litWidth < 2 || litWidth > 8 {
                                return fmt.Errorf("gif: pixel size in decode out of range: %d", litWidth)
                        }
                        // A wonderfully Go-like piece of magic.
                        lzwr := lzw.NewReader(&blockReader{r: d.r}, lzw.LSB, int(litWidth))
                        if _, err = io.ReadFull(lzwr, m.Pix); err != nil {
                                break
                        }

                        // There should be a "0" block remaining; drain that.
                        c, err = d.r.ReadByte()
                        if err != nil {
                                return err
                        }
                        if c != 0 {
                                return errors.New("gif: extra data after image")
                        }

                        // Undo the interlacing if necessary.
                        if d.imageFields&ifInterlace != 0 {
                                uninterlace(m)
                        }

                        d.image = append(d.image, m)
                        d.delay = append(d.delay, d.delayTime)
                        d.delayTime = 0 // TODO: is this correct, or should we hold on to the value?

                case sTrailer:
                        break Loop

                default:
                        err = fmt.Errorf("gif: unknown block type: 0x%.2x", c)
                }
        }
        if err != nil {
                return err
        }
        if len(d.image) == 0 {
                return io.ErrUnexpectedEOF
        }
        return nil
}

func (d *decoder) readHeaderAndScreenDescriptor() error {
        _, err := io.ReadFull(d.r, d.tmp[0:13])
        if err != nil {
                return err
        }
        d.vers = string(d.tmp[0:6])
        if d.vers != "GIF87a" && d.vers != "GIF89a" {
                return fmt.Errorf("gif: can't recognize format %s", d.vers)
        }
        d.width = int(d.tmp[6]) + int(d.tmp[7])<<8
        d.height = int(d.tmp[8]) + int(d.tmp[9])<<8
        d.headerFields = d.tmp[10]
        d.backgroundIndex = d.tmp[11]
        d.aspect = d.tmp[12]
        d.loopCount = -1
        d.pixelSize = uint(d.headerFields&7) + 1
        return nil
}

func (d *decoder) readColorMap() (color.Palette, error) {
        if d.pixelSize > 8 {
                return nil, fmt.Errorf("gif: can't handle %d bits per pixel", d.pixelSize)
        }
        numColors := 1 << d.pixelSize
        if d.imageFields&ifLocalColorTable != 0 {
                numColors = 1 << ((d.imageFields & ifPixelSizeMask) + 1)
        }
        numValues := 3 * numColors
        _, err := io.ReadFull(d.r, d.tmp[0:numValues])
        if err != nil {
                return nil, fmt.Errorf("gif: short read on color map: %s", err)
        }
        colorMap := make(color.Palette, numColors)
        j := 0
        for i := range colorMap {
                colorMap[i] = color.RGBA{d.tmp[j+0], d.tmp[j+1], d.tmp[j+2], 0xFF}
                j += 3
        }
        return colorMap, nil
}

func (d *decoder) readExtension() error {
        extension, err := d.r.ReadByte()
        if err != nil {
                return err
        }
        size := 0
        switch extension {
        case eText:
                size = 13
        case eGraphicControl:
                return d.readGraphicControl()
        case eComment:
                // nothing to do but read the data.
        case eApplication:
                b, err := d.r.ReadByte()
                if err != nil {
                        return err
                }
                // The spec requires size be 11, but Adobe sometimes uses 10.
                size = int(b)
        default:
                return fmt.Errorf("gif: unknown extension 0x%.2x", extension)
        }
        if size > 0 {
                if _, err := io.ReadFull(d.r, d.tmp[0:size]); err != nil {
                        return err
                }
        }

        // Application Extension with "NETSCAPE2.0" as string and 1 in data means
        // this extension defines a loop count.
        if extension == eApplication && string(d.tmp[:size]) == "NETSCAPE2.0" {
                n, err := d.readBlock()
                if n == 0 || err != nil {
                        return err
                }
                if n == 3 && d.tmp[0] == 1 {
                        d.loopCount = int(d.tmp[1]) | int(d.tmp[2])<<8
                }
        }
        for {
                n, err := d.readBlock()
                if n == 0 || err != nil {
                        return err
                }
        }
        panic("unreachable")
}

func (d *decoder) readGraphicControl() error {
        if _, err := io.ReadFull(d.r, d.tmp[0:6]); err != nil {
                return fmt.Errorf("gif: can't read graphic control: %s", err)
        }
        d.flags = d.tmp[1]
        d.delayTime = int(d.tmp[2]) | int(d.tmp[3])<<8
        if d.flags&gcTransparentColorSet != 0 {
                d.transparentIndex = d.tmp[4]
                d.setTransparency(d.globalColorMap)
        }
        return nil
}

func (d *decoder) setTransparency(colorMap color.Palette) {
        if int(d.transparentIndex) < len(colorMap) {
                colorMap[d.transparentIndex] = color.RGBA{}
        }
}

func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
        if _, err := io.ReadFull(d.r, d.tmp[0:9]); err != nil {
                return nil, fmt.Errorf("gif: can't read image descriptor: %s", err)
        }
        left := int(d.tmp[0]) + int(d.tmp[1])<<8
        top := int(d.tmp[2]) + int(d.tmp[3])<<8
        width := int(d.tmp[4]) + int(d.tmp[5])<<8
        height := int(d.tmp[6]) + int(d.tmp[7])<<8
        d.imageFields = d.tmp[8]
        return image.NewPaletted(image.Rect(left, top, left+width, top+height), nil), nil
}

func (d *decoder) readBlock() (int, error) {
        n, err := d.r.ReadByte()
        if n == 0 || err != nil {
                return 0, err
        }
        return io.ReadFull(d.r, d.tmp[0:n])
}

// interlaceScan defines the ordering for a pass of the interlace algorithm.
type interlaceScan struct {
        skip, start int
}

// interlacing represents the set of scans in an interlaced GIF image.
var interlacing = []interlaceScan{
        {8, 0}, // Group 1 : Every 8th. row, starting with row 0.
        {8, 4}, // Group 2 : Every 8th. row, starting with row 4.
        {4, 2}, // Group 3 : Every 4th. row, starting with row 2.
        {2, 1}, // Group 4 : Every 2nd. row, starting with row 1.
}

// uninterlace rearranges the pixels in m to account for interlaced input.
func uninterlace(m *image.Paletted) {
        var nPix []uint8
        dx := m.Bounds().Dx()
        dy := m.Bounds().Dy()
        nPix = make([]uint8, dx*dy)
        offset := 0 // steps through the input by sequential scan lines.
        for _, pass := range interlacing {
                nOffset := pass.start * dx // steps through the output as defined by pass.
                for y := pass.start; y < dy; y += pass.skip {
                        copy(nPix[nOffset:nOffset+dx], m.Pix[offset:offset+dx])
                        offset += dx
                        nOffset += dx * pass.skip
                }
        }
        m.Pix = nPix
}

// Decode reads a GIF image from r and returns the first embedded
// image as an image.Image.
func Decode(r io.Reader) (image.Image, error) {
        var d decoder
        if err := d.decode(r, false); err != nil {
                return nil, err
        }
        return d.image[0], nil
}

// GIF represents the possibly multiple images stored in a GIF file.
type GIF struct {
        Image     []*image.Paletted // The successive images.
        Delay     []int             // The successive delay times, one per frame, in 100ths of a second.
        LoopCount int               // The loop count.
}

// DecodeAll reads a GIF image from r and returns the sequential frames
// and timing information.
func DecodeAll(r io.Reader) (*GIF, error) {
        var d decoder
        if err := d.decode(r, false); err != nil {
                return nil, err
        }
        gif := &GIF{
                Image:     d.image,
                LoopCount: d.loopCount,
                Delay:     d.delay,
        }
        return gif, nil
}

// DecodeConfig returns the global color model and dimensions of a GIF image
// without decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, error) {
        var d decoder
        if err := d.decode(r, true); err != nil {
                return image.Config{}, err
        }
        return image.Config{
                ColorModel: d.globalColorMap,
                Width:      d.width,
                Height:     d.height,
        }, nil
}

func init() {
        image.RegisterFormat("gif", "GIF8?a", Decode, DecodeConfig)
}