how bad is jpeg?

There has been much criticism of JPEG as a "lossy" format. Lossy compression means that the image is allowed to change (degrade) in order to achieve smaller file sizes. How much does JPEG compression degrade image quality? Well, it depends.

Much of the criticism of JPEG compression is in fact a criticism of implementation choices by software authors. JPEG is capable of remarkable compression with very little degradation in quality. First let's examine the kind of problem that produced disappointment in some JPEG users.

The original is rendered as a PNG, a lossless compression algorithm.

The original PNG (24,195 bytes):

The JPEG (28,238 bytes):

The good news: the images are hard to distinguish without very close inspection, and the file size of the JPEG is only 29% the size of the uncompressed PSP original and less than half the size of the PNG. Most portions of the image look identical even under extreme maginfication. However, if you know where to look, you can find a few spots with significant differences.

This kind of problem is often cited by critics of JPEG. However, the above is really a criticism of only one JPEG approach to reducing file size. It is a manifestation of chroma subsampling, where color information is stored at only half the resolution of the intensity information. A few programs, most notably Paint Shop Pro, make chroma subsampling optional.

So how bad is JPEG? Not bad at all! It is a lot better than most people think, especially at low compression settings, and especially with chroma subsampling turned off.

However, it is important to remember that JPEG is designed for photographic images. Computer generated images with sharp transitions between areas of uniform color are particularly difficult for JPEG to handle. (See JPEG vs GIF.) Also, colored stripes that are only one pixel wide, as in the above image, cause problems for the great majority of graphics programs that force subsampling of chroma information in 2x2 blocks. For best JPEG quality, it is absolutely essential to use a program such as Paint Shop Pro that allows the user to disable chroma subsampling.