JPEG (Joint Photographic Experts Group)

JPEG is a true-colour or greyscale format that uses a 'lossy' compression technique based on a Discrete Fourier Transform (DCT). The algorithm is called 'lossy' because it loses colour information in the compression process, but the loss is usually hardly noticable (if at all), while the compression ratio is much greater than can be achieved with lossless compression methods.

The amount of loss can be influenced by specifying a 'quality' factor, expressed as a percentage, which (theoretically) may lie between 1% (very high compression ratio, low quality) and 100% (lowest compression ratio, highest quality). In reality, factors between around 30 and 95 are usable (even 100% loses colour information, while yielding very low compression ratios, 95% is a good maximum value to use).

Quality Settings

The -quality switch lets you trade off compressed file size against quality of the reconstructed image: the higher the quality setting, the larger the JPEG file, and the closer the output image will be to the original input. Normally you want to use the lowest quality setting (smallest file) that decompresses into something visually indistinguishable from the original image. For this purpose the quality setting should be between 50 and 95; the default of 75 is often about right. If you see defects at -quality 75, then go up 5 or 10 counts at a time until you are happy with the output image. (The optimal setting will vary from one image to another.)