NASA's Image Compression (DCTune) Comes to the Web
by Rich Deem

Introduction to image compression

DCTune is NASA's patented technology technology for optimizing JPEG image compression. It calculates the best JPEG compression for a specified perceptual error, given a particular image and a particular set of viewing conditions. However, the target perceptual error of 1.0 means that the compressed JPEG image is perceptually lossless, that is, it will appear exactly the same as the original uncompressed image. DCTune has recently been licensed by NASA commerically, although no commercial product is yet available.

Since a Windows demo version is available on the Web, I downloaded it to see how well it performed. The program is a command-line based DOS program that requires image files to be in Portable Pixmap (PPM/PBM/PGM) format. Since even Adobe Photoshop does not support this format, one has to download a graphic converter program to convert your JPEG/GIF/TIFF image to PPM format first. The command line allows you to specify the perceptual error or even the image size of your JPEG output. Having downloaded several new high resolution Hubble images, I compared the image quality of DCTune-produced JPEGs compared to JPEGs produced for "medium" image quality in Photoshop. The results were quite amazing, and can be seen in the new images in the Tour the Universe Slideshow. It takes a whole lot longer to generate those images, so I'm not sure I will use this program extensively on the site. However, a future commercial version might be more user friendly and time efficient.