how to compress a PNG image using Java

Question

Hi I would like to know if there is any way in Java to reduce the size of an image (use any kind of compression) that was loaded as a BufferedImage and is going to be saved

Answer 1

By calling ImageIO.write you use the default compression quality for pngs. You can choose an explicit compression mode which reduces file size a lot.

String inputPath = "a.png";
String outputPath = "b.png";

BufferedImage image;
IIOMetadata metadata;

try (ImageInputStream in = ImageIO.createImageInputStream(Files.newInputStream(Paths.get(inputPath)))) {
    ImageReader reader = ImageIO.getImageReadersByFormatName("png").next();
    reader.setInput(in, true, false);
    image = reader.read(0);
    metadata = reader.getImageMetadata(0);
    reader.dispose();
}

try (ImageOutputStream out = ImageIO.createImageOutputStream(Files.newOutputStream(Paths.get(outputPath)))) {
    ImageTypeSpecifier type = ImageTypeSpecifier.createFromRenderedImage(image);
    ImageWriter writer = ImageIO.getImageWriters(type, "png").next();

    ImageWriteParam param = writer.getDefaultWriteParam();
    if (param.canWriteCompressed()) {
        param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
        param.setCompressionQuality(0.0f);
    }

    writer.setOutput(out);
    writer.write(null, new IIOImage(image, null, metadata), param);
    writer.dispose();
}

Answer 2

You may want to try pngtastic. It's a pure java png image optimizer that can shrink may png images down to some degree.

Answer 3

If it's going to be saved as PNG, compression will be done at that stage. PNG has a lossless compression algorithm (basically prediction followed by lempel-ziv compression) with few adjustable parameters (types of "filters") and not much impact in compression amount - in general the default will be optimal.

Answer 4

Have a look at the ImageWriterParam class in the same package as the ImageIO class. It mentions compression.

https://docs.oracle.com/javase/7/docs/api/javax/imageio/ImageWriteParam.html

Also look at the example at http://exampledepot.com/egs/javax.imageio/JpegWrite.html and see if it translates well for PNG files.