How to encode an animated GIF in Java, using ImageWriter and ImageIO?

Question

I've looked all over the place, but can't seem to find any easy to understand explanation. (I've found classes and methods written by other Java users that can do this, but I'm hoping to write my own.)

Answer 1

Here is the createImage() method of GIFanim. Perhaps that will give you a start.

public byte[] createImage() throws Exception {

    ImageWriter iw = ImageIO.getImageWritersByFormatName("gif").next();

    ByteArrayOutputStream os = new ByteArrayOutputStream();
    ImageOutputStream ios = ImageIO.createImageOutputStream(os);
    iw.setOutput(ios);
    iw.prepareWriteSequence(null);
    int i = 0;

    for (AnimationFrame animationFrame : frameCollection) {

        BufferedImage src = animationFrame.getImage();
        ImageWriteParam iwp = iw.getDefaultWriteParam();
        IIOMetadata metadata = iw.getDefaultImageMetadata(
            new ImageTypeSpecifier(src), iwp);

        configure(metadata, "" + animationFrame.getDelay(), i);

        IIOImage ii = new IIOImage(src, null, metadata);
        iw.writeToSequence(ii, null);
        i++;
    }

    iw.endWriteSequence();
    ios.close();
    return os.toByteArray();
}

Note that this is a very naïve implementation, that produces images that are significantly larger than can be made with a library that compresses the color palette and performs other optimizations. Implementing a library like that would be a significant task.