How to save lossless jpg in java?

问题:

I have to save a jpeg image lossless. I am work on a steganography project but Java compressing and saving my result. I research every forums and try everything but it didn't work.

Here my example code for lossless save a jpeg image:

BufferedImage image = ImageIO.read(new File("sources/image.jpg"));

ImageWriter writer = ImageIO.getImageWritersByFormatName("JPEG").next();
JPEGImageWriteParam jpegParams = new JPEGImageWriteParam(null);

jpegParams.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpegParams.setCompressionQuality(1f);

writer.setOutput(ImageIO.createImageOutputStream(new File("example.jpg")));
writer.write(null, new IIOImage(image,null,null), jpegParams);
writer.dispose();

After this process I compute PSNR value is 28.53173 and "example.jpg"'s size is bigger than "image.jpg".

I try import JAI library but I am not sure Java 8 is support JAI.

回答1:

JPEG is lossy all the time. Even at 100% compression quality there will be some loss of information, but it will be minimised.

The reason why your example.jpg has a bigger size is because it was encoded with a 100% quality factor, while most jpeg encoders have a default value of 50%-75%, which is what was most likely used for example.jpg. You can try different quality factors to see when both files have the same size.

A lossless JPEG format does exist (available in JAI), but it should be thought of as a different format to the conventional JPEG. However, it's not widely used and you'll probably not be able to view it in most applications, which would practically defeat the point of sharing an innocent image.