Programmatically Reducing JPEG file size

Question

Apologies for any ignorance, but I have never worked with jpeg images (let alone any types of images) in Java before.

Supposing I want to send a jpeg image from a we

Answer 1

Two of the possible solutions are downscaling the image, here's how you'd do it:

BufferedImage original = //your image here
scaled = original.getScaledInstance(finalWidth, finalHeight, Image.SCALE_SMOOTH); // scale the image to a smaller one

BufferedImage result = new BufferedImage(finalWidth, finalHeight, original.getType());
Graphics2D g = result.createGraphics();     

g.drawImage(scaled, 0, 0, null); //draw the smaller image
g.dispose();

Obviously, you have to calculate the scaled width and height so the image stays by the same aspect ratio.

Once you have drawn it smaller, you can now turn this image into a JPEG file:

BufferedImage image = // this is the final scaled down image

JPEGImageEncoder jpegEncoder = JPEGCodec.createJPEGEncoder(output);
JPEGEncodeParam jpegEncodeParam = jpegEncoder.getDefaultJPEGEncodeParam(image);

jpegEncodeParam.setDensityUnit(JPEGEncodeParam.DENSITY_UNIT_DOTS_INCH);
jpegEncodeParam.setXDensity(92);
jpegEncodeParam.setYDensity(92);

jpegEncodeParam.setQuality( 0.8F , false);
jpegEncoder.encode(image, jpegEncodeParam);

These classes are from the JAI package (more exactly com.sun.image.codec.jpeg) and the JVM might complain that they should not be used directly, but you can ignore that.

You can possibly download JAI from here, if it does not work I have github mirrors setup for the two libraries, JAI core and JAI ImageIO.

Answer 2

Have a look at the ImageIO class. As for reducing file size: since the image would already be a JPEG the only things you could do is reduce the quality or the image size.

Another thing to keep in mind: if the image is a CMYK jpeg it might be bigger. Unfortunately ImageIO can't handle those, but you can try JAI ImageIO to convert from CMYK to RGB (which should be much smaller).

Answer 3

Another way to reduce image size is to change compression level. You can do that using ImageWriter.

    ImageWriter writer = null;
    Iterator<ImageWriter> iwi = ImageIO.getImageWritersByFormatName("jpg");
    if (!iwi.hasNext())
        return;
    writer = (ImageWriter) iwi.next();
    ImageWriteParam iwp = writer.getDefaultWriteParam();
    iwp.setCompressionMode(ImageWriteParam.MODE_EXPLICIT) ;
    iwp.setCompressionQuality(compressionQuality);
    writer.setOutput(...);
    writer.write(null, image, iwp);

Answer 4

The easiest way to do this is to decompress the byte stream into a Java Image, optionally resize it (which makes it smaller) and then regenerate a JPEG image from this with the desired quality setting.

This new image is then what is sent to the client.