Beautiful Data

Image Compression

Just as the data produced by a robotic mission is precious, so is the communications bandwidth needed to return that data. For smaller images, such as those reduced by subframe or decimation operations, it could be acceptable to just downlink the image without compression. Larger images, such as the full-size SSI images, would consume a lot of downlink bandwidth, so compression was always considered as an option in such cases.

The ICS provided two forms of compression and two forms of size reduction using pixel mapping and scaling. Which type of compression or reduction would be used for a particular image depended largely on the level of image fidelity deemed necessary for the object of interest. In some cases, 8 bits per pixel would suffice; in other cases, the loss of fidelity inherent with JPEG compression was acceptable; and for the cases where the image had to retain as much fidelity as possible, there was a lossless compression method available.

In the ICS, a JPEG compressor, using all integer math and in-place operations, provided so-called "lossy" compression. JPEG is considered lossy because it discards some of the image data as a result of the compression process. It could compress image data to varying degrees by command. The final code was loosely based on the JPEG compressor flown on the Mars '98 mission, although only a part of that original code survived in the ICS for Phoenix. The original JPEG compressor used floating-point math, multiple full-size ...