The quality value given when compression with JPEG2000 is a target size relative to the size of the source image. For example, 80 is specifies that the resulting image be 80 percent of the source image in size.
However, a certain amount of compression can be achieved almost losslessly. With an input image that has relatively low information content, the compressor will not make the output file larger than needed. In other words, if the compressor can create a virtually lossless image smaller than the size requested, it will.
For example, suppose an image can be compressed to 25 percent of its original size with virtually no loss. If compressing the image by 50 percent is specified, the compressor will still produce a file that is 25 percent of the original size.
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