J2ME GIF 解码器（支持动画）

Displaying GIF Images on J2ME Mobile Phones 
J2SE GIF Source

第一篇文章讲解比较详细，曾被多次转载。而第二篇则是GIF编解码器的J2SE源代码。

我正是在参考了以上文章之后，才开始对该代码展开移植。包括测试也就用了半天时间。感兴趣的朋友最好自己动手试一试。有麻烦的话，可以联系我。

下面先把J2ME版本的GIF解码器源代码公布如下。稍后提供编码器的代码（工作比较忙，抱歉）。

 
import java.io.InputStream;
import java.util.Vector;

import javax.microedition.lcdui.Image;

/**
 * Class GifDecoder - Decodes a GIF file into one or more frames. <br>
 *
 * <pre>
 * Example:
 *    GifDecoder d = new GifDecoder();
 *    d.read(&quot;sample.gif&quot;);
 *    int n = d.getFrameCount();
 *    for (int i = 0; i &lt; n; i++) {
 *       BufferedImage frame = d.getFrame(i);  // frame i
 *       int t = d.getDelay(i);  // display duration of frame in milliseconds
 *       // do something with frame
 *    }
 * </pre>
 *
 * No copyright asserted on the source code of this class. May be used for any
 * purpose, however, refer to the Unisys LZW patent for any additional
 * restrictions. Please forward any corrections to kweiner@fmsware.com.
 *
 * @author Kevin Weiner, FM Software; LZW decoder adapted from John Cristy's
 *         ImageMagick.
 * @version 1.03 November 2003
 *
 */

public class GifDecoder {

    /**
     * File read status: No errors.
     */
    public static final int STATUS_OK = 0;

    /**
     * File read status: Error decoding file (may be partially decoded)
     */
    public static final int STATUS_FORMAT_ERROR = 1;

    /**
     * File read status: Unable to open source.
     */
    public static final int STATUS_OPEN_ERROR = 2;

    protected InputStream in;
    protected int status;

    protected int width; // full image width
    protected int height; // full image height
    protected boolean gctFlag; // global color table used
    protected int gctSize; // size of global color table
    protected int loopCount = 1; // iterations; 0 = repeat forever

    protected int[] gct; // global color table
    protected int[] lct; // local color table
    protected int[] act; // active color table

    protected int bgIndex; // background color index
    protected int bgColor; // background color
    protected int lastBgColor; // previous bg color
    protected int pixelAspect; // pixel aspect ratio

    protected boolean lctFlag; // local color table flag
    protected boolean interlace; // interlace flag
    protected int lctSize; // local color table size

    protected int ix, iy, iw, ih; // current image rectangle
    protected int lrx, lry, lrw, lrh;
    protected Image image; // current frame
    protected Image lastImage; // previous frame

    protected byte[] block = new byte[256]; // current data block
    protected int blockSize = 0; // block size

    // last graphic control extension info
    protected int dispose = 0;
    // 0=no action; 1=leave in place; 2=restore to bg; 3=restore to prev
    protected int lastDispose = 0;
    protected boolean transparency = false; // use transparent color
    protected int delay = 0; // delay in milliseconds
    protected int transIndex; // transparent color index

    protected static final int MaxStackSize = 4096;
    // max decoder pixel stack size

    // LZW decoder working arrays
    protected short[] prefix;
    protected byte[] suffix;
    protected byte[] pixelStack;
    protected byte[] pixels;

    protected Vector frames; // frames read from current file
    protected int frameCount;

    static class GifFrame {
        public GifFrame(Image im, int del) {
            image = im;
            delay = del;
        }

        public Image image;
        public int delay;
    }

    /**
     * Gets display duration for specified frame.
     *
     * @param n
     *            int index of frame
     * @return delay in milliseconds
     */
    public int getDelay(int n) {
        delay = -1;
        if ((n >= 0) && (n < frameCount)) {
            delay = ((GifFrame) frames.elementAt(n)).delay;
        }
        return delay;
    }

    /**
     * Gets the number of frames read from file.
     *
     * @return frame count
     */
    public int getFrameCount() {
        return frameCount;
    }

    /**
     * Gets the first (or only) image read.
     *
     * @return BufferedImage containing first frame, or null if none.
     */
    public Image getImage() {
        return getFrame(0);
    }

    /**
     * Gets the "Netscape" iteration count, if any. A count of 0 means repeat
     * indefinitiely.
     *
     * @return iteration count if one was specified, else 1.
     */
    public int getLoopCount() {
        return loopCount;
    }

    /**
     * Creates new frame image from current data (and previous frames as
     * specified by their disposition codes).
     */
    protected void setPixels() {
        // expose destination image's pixels as int array
        int[] dest = new int[width * height];

        // fill in starting image contents based on last image's dispose code
        if (lastDispose > 0) {
            if (lastDispose == 3) {
                // use image before last
                int n = frameCount - 2;
                if (n > 0) {
                    lastImage = getFrame(n - 1);
                } else {
                    lastImage = null;
                }
            }

            if (lastImage != null) {
                lastImage.getRGB(dest, 0, width, 0, 0, width, height);
                // copy pixels

                if (lastDispose == 2) {
                    // fill last image rect area with background color
                    int c = 0;
                    if (!transparency) {
                        c = lastBgColor;
                    }
                    for (int i = 0; i < lrh; i++) {
                        int n1 = (lry + i) * width + lrx;
                        int n2 = n1 + lrw;
                        for (int k = n1; k < n2; k++) {
                            dest[k] = c;
                        }
                    }
                }
            }
        }

        // copy each source line to the appropriate place in the destination
        int pass = 1;
        int inc = 8;
        int iline = 0;
        for (int i = 0; i < ih; i++) {
            int line = i;
            if (interlace) {
                if (iline >= ih) {
                    pass++;
                    switch (pass) {
                    case 2:
                        iline = 4;
                        break;
                    case 3:
                        iline = 2;
                        inc = 4;
                        break;
                    case 4:
                        iline = 1;
                        inc = 2;
                    }
                }
                line = iline;
                iline += inc;
            }
            line += iy;
            if (line < height) {
                int k = line * width;
                int dx = k + ix; // start of line in dest
                int dlim = dx + iw; // end of dest line
                if ((k + width) < dlim) {
                    dlim = k + width; // past dest edge
                }
                int sx = i * iw; // start of line in source
                while (dx < dlim) {
                    // map color and insert in destination
                    int index = ((int) pixels[sx++]) & 0xff;
                    int c = act[index];
                    if (c != 0) {
                        dest[dx] = c;
                    }
                    dx++;
                }
            }
        }
        image = Image.createRGBImage(dest, width, height, false);
    }

    /**
     * Gets the image contents of frame n.
     *
     * @return BufferedImage representation of frame, or null if n is invalid.
     */
    public Image getFrame(int n) {
        Image im = null;
        if ((n >= 0) && (n < frameCount)) {
            im = ((GifFrame) frames.elementAt(n)).image;
        }
        return im;
    }

    /**
     * Reads GIF image from stream
     *
     * @param BufferedInputStream
     *            containing GIF file.
     * @return read status code (0 = no errors)
     */
    public int read(InputStream is) {
        init();
        if (is != null) {
            in = is;
            readHeader();
            if (!err()) {
                readContents();
                if (frameCount < 0) {
                    status = STATUS_FORMAT_ERROR;
                }
            }
        } else {
            status = STATUS_OPEN_ERROR;
        }
        try {
            is.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return status;
    }

    /**
     * Decodes LZW image data into pixel array. Adapted from John Cristy's
     * ImageMagick.
     */
    protected void decodeImageData() {
        int NullCode = -1;
        int npix = iw * ih;
        int available, clear, code_mask, code_size, end_of_information, in_code, old_code, bits, code, count, i, datum, data_size, first, top, bi, pi;

        if ((pixels == null) || (pixels.length < npix)) {
            pixels = new byte[npix]; // allocate new pixel array
        }
        if (prefix == null)
            prefix = new short[MaxStackSize];
        if (suffix == null)
            suffix = new byte[MaxStackSize];
        if (pixelStack == null)
            pixelStack = new byte[MaxStackSize + 1];

        // Initialize GIF data stream decoder.

        data_size = read();
        clear = 1 << data_size;
        end_of_information = clear + 1;
        available = clear + 2;
        old_code = NullCode;
        code_size = data_size + 1;
        code_mask = (1 << code_size) - 1;
        for (code = 0; code < clear; code++) {
            prefix[code] = 0;
            suffix[code] = (byte) code;
        }

        // Decode GIF pixel stream.

        datum = bits = count = first = top = pi = bi = 0;

        for (i = 0; i < npix;) {
            if (top == 0) {
                if (bits < code_size) {
                    // Load bytes until there are enough bits for a code.
                    if (count == 0) {
                        // Read a new data block.
                        count = readBlock();
                        if (count <= 0)
                            break;
                        bi = 0;
                    }
                    datum += (((int) block[bi]) & 0xff) << bits;
                    bits += 8;
                    bi++;
                    count--;
                    continue;
                }

                // Get the next code.

                code = datum & code_mask;
                datum >>= code_size;
                bits -= code_size;

                // Interpret the code

                if ((code > available) || (code == end_of_information))
                    break;
                if (code == clear) {
                    // Reset decoder.
                    code_size = data_size + 1;
                    code_mask = (1 << code_size) - 1;
                    available = clear + 2;
                    old_code = NullCode;
                    continue;
                }
                if (old_code == NullCode) {
                    pixelStack[top++] = suffix[code];
                    old_code = code;
                    first = code;
                    continue;
                }
                in_code = code;
                if (code == available) {
                    pixelStack[top++] = (byte) first;
                    code = old_code;
                }
                while (code > clear) {
                    pixelStack[top++] = suffix[code];
                    code = prefix[code];
                }
                first = ((int) suffix[code]) & 0xff;

                // Add a new string to the string table,

                if (available >= MaxStackSize)
                    break;
                pixelStack[top++] = (byte) first;
                prefix[available] = (short) old_code;
                suffix[available] = (byte) first;
                available++;
                if (((available & code_mask) == 0)
                        && (available < MaxStackSize)) {
                    code_size++;
                    code_mask += available;
                }
                old_code = in_code;
            }

            // Pop a pixel off the pixel stack.

            top--;
            pixels[pi++] = pixelStack[top];
            i++;
        }

        for (i = pi; i < npix; i++) {
            pixels = 0; // clear missing pixels
        }

    }

    /**
     * Returns true if an error was encountered during reading/decoding
     */
    protected boolean err() {
        return status != STATUS_OK;
    }

    /**
     * Initializes or re-initializes reader
     */
    protected void init() {
        status = STATUS_OK;
        frameCount = 0;
        frames = new Vector();
        gct = null;
        lct = null;
    }

    /**
     * Reads a single byte from the input stream.
     */
    protected int read() {
        int curByte = 0;
        try {
            curByte = in.read();
        } catch (Exception e) {
            status = STATUS_FORMAT_ERROR;
        }
        return curByte;
    }

    /**
     * Reads next variable length block from input.
     *
     * @return number of bytes stored in "buffer"
     */
    protected int readBlock() {
        blockSize = read();
        int n = 0;
        if (blockSize > 0) {
            try {
                int count = 0;
                while (n < blockSize) {
                    count = in.read(block, n, blockSize - n);
                    if (count == -1)
                        break;
                    n += count;
                }
            } catch (Exception e) {
                e.printStackTrace();
            }

            if (n < blockSize) {
                status = STATUS_FORMAT_ERROR;
            }
        }
        return n;
    }

    /**
     * Reads color table as 256 RGB integer values
     *
     * @param ncolors
     *            int number of colors to read
     * @return int array containing 256 colors (packed ARGB with full alpha)
     */
    protected int[] readColorTable(int ncolors) {
        int nbytes = 3 * ncolors;
        int[] tab = null;
        byte[] c = new byte[nbytes];
        int n = 0;
        try {
            n = in.read(c);
        } catch (Exception e) {
            e.printStackTrace();
        }
        if (n < nbytes) {
            status = STATUS_FORMAT_ERROR;
        } else {
            tab = new int[256]; // max size to avoid bounds checks
            int i = 0;
            int j = 0;
            while (i < ncolors) {
                int r = ((int) c[j++]) & 0xff;
                int g = ((int) c[j++]) & 0xff;
                int b = ((int) c[j++]) & 0xff;
                tab[i++] = 0xff000000 | (r << 16) | (g << 8) | b;
            }
        }
        return tab;
    }

    /**
     * Main file parser. Reads GIF content blocks.
     */
    protected void readContents() {
        // read GIF file content blocks
        boolean done = false;
        while (!(done || err())) {
            int code = read();
            switch (code) {

            case 0x2C: // image separator
                readImage();
                break;

            case 0x21: // extension
                code = read();
                switch (code) {
                case 0xf9: // graphics control extension
                    readGraphicControlExt();
                    break;

                case 0xff: // application extension
                    readBlock();
                    String app = "";
                    for (int i = 0; i < 11; i++) {
                        app += (char) block;
                    }
                    if (app.equals("NETSCAPE2.0")) {
                        readNetscapeExt();
                    } else
                        skip(); // don't care
                    break;

                default: // uninteresting extension
                    skip();
                }
                break;

            case 0x3b: // terminator
                done = true;
                break;

            case 0x00: // bad byte, but keep going and see what happens
                break;

            default:
                status = STATUS_FORMAT_ERROR;
            }
        }
    }

    /**
     * Reads Graphics Control Extension values
     */
    protected void readGraphicControlExt() {
        read(); // block size
        int packed = read(); // packed fields
        dispose = (packed & 0x1c) >> 2; // disposal method
        if (dispose == 0) {
            dispose = 1; // elect to keep old image if discretionary
        }
        transparency = (packed & 1) != 0;
        delay = readShort() * 10; // delay in milliseconds
        transIndex = read(); // transparent color index
        read(); // block terminator
    }

    /**
     * Reads GIF file header information.
     */
    protected void readHeader() {
        String id = "";
        for (int i = 0; i < 6; i++) {
            id += (char) read();
        }
        if (!id.startsWith("GIF")) {
            status = STATUS_FORMAT_ERROR;
            return;
        }

        readLSD();
        if (gctFlag && !err()) {
            gct = readColorTable(gctSize);
            bgColor = gct[bgIndex];
        }
    }

    /**
     * Reads next frame image
     */
    protected void readImage() {
        ix = readShort(); // (sub)image position & size
        iy = readShort();
        iw = readShort();
        ih = readShort();

        int packed = read();
        lctFlag = (packed & 0x80) != 0; // 1 - local color table flag
        interlace = (packed & 0x40) != 0; // 2 - interlace flag
        // 3 - sort flag
        // 4-5 - reserved
        lctSize = 2 << (packed & 7); // 6-8 - local color table size

        if (lctFlag) {
            lct = readColorTable(lctSize); // read table
            act = lct; // make local table active
        } else {
            act = gct; // make global table active
            if (bgIndex == transIndex)
                bgColor = 0;
        }
        int save = 0;
        if (transparency) {
            save = act[transIndex];
            act[transIndex] = 0; // set transparent color if specified
        }

        if (act == null) {
            status = STATUS_FORMAT_ERROR; // no color table defined
        }

        if (err())
            return;

        decodeImageData(); // decode pixel data
        skip();

        if (err())
            return;

        frameCount++;

        // create new image to receive frame data
        image = Image.createImage(width, height);

        setPixels(); // transfer pixel data to image

        frames.addElement(new GifFrame(image, delay)); // add image to frame
        // list

        if (transparency) {
            act[transIndex] = save;
        }
        resetFrame();

    }

    /**
     * Reads Logical Screen Descriptor
     */
    protected void readLSD() {

        // logical screen size
        width = readShort();
        height = readShort();

        // packed fields
        int packed = read();
        gctFlag = (packed & 0x80) != 0; // 1 : global color table flag
        // 2-4 : color resolution
        // 5 : gct sort flag
        gctSize = 2 << (packed & 7); // 6-8 : gct size

        bgIndex = read(); // background color index
        pixelAspect = read(); // pixel aspect ratio
    }

    /**
     * Reads Netscape extenstion to obtain iteration count
     */
    protected void readNetscapeExt() {
        do {
            readBlock();
            if (block[0] == 1) {
                // loop count sub-block
                int b1 = ((int) block[1]) & 0xff;
                int b2 = ((int) block[2]) & 0xff;
                loopCount = (b2 << 8) | b1;
            }
        } while ((blockSize > 0) && !err());
    }

    /**
     * Reads next 16-bit value, LSB first
     */
    protected int readShort() {
        // read 16-bit value, LSB first
        return read() | (read() << 8);
    }

    /**
     * Resets frame state for reading next image.
     */
    protected void resetFrame() {
        lastDispose = dispose;
        lrx = ix;
        lry = iy;
        lrw = iw;
        lrh = ih;
        lastImage = image;
        lastBgColor = bgColor;
        dispose = 0;
        transparency = false;
        delay = 0;
        lct = null;
    }

    /**
     * Skips variable length blocks up to and including next zero length block.
     */
    protected void skip() {
        do {
            readBlock();
        } while ((blockSize > 0) && !err());
    }
}

使用起来比较简单，可以参考如下代码

 
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;

import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;
import javax.microedition.midlet.MIDlet;

public class gifme extends MIDlet {

    private gifCanvas canvas;

    public gifme() {
        canvas = new gifCanvas();
    }

    protected void destroyApp(boolean arg0) {

    }

    protected void pauseApp() {

    }

    protected void startApp() {
        Display.getDisplay(this).setCurrent(canvas);
        new Thread(canvas).start();
    }

    public class gifCanvas extends Canvas implements Runnable {

        private GifDecoder d;
        private int ind;
        private int count;
        private long delay;
        private Image frame;

        private long start;

        public gifCanvas() {
            long start = System.currentTimeMillis();
            d = new GifDecoder();
            d.read(getClass().getResourceAsStream("/rr.gif"));

            System.out.println(System.currentTimeMillis() - start);
            ind = 0;
            count = d.getFrameCount();
        }

        public void run() {
            while (true) {
                long now = System.currentTimeMillis();
                if (now - start >= delay) {
                    repaint();
                    try {
                        Thread.sleep(delay);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }

        public void paint(Graphics g) {
            start = System.currentTimeMillis();
            frame = d.getFrame(ind);
            delay = d.getDelay(ind);
            System.out.println("frame = " + ind + "  delay = " + delay);
            ind++;
            if (ind >= count) {
                ind = 0;
            }
            g.drawImage(frame, 0, 0, 0);
        }
    }
}