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[i];
                            }
                            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;
            }
        }
    }

   
    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
    }

   
    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];
        }
    }

   
    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();

    }

   
    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
    }

   
    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());
    }

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

   
    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;
    }

   
    protected void skip() {
        do {
            readBlock();
        } while ((blockSize > 0) && !err());
    }

}