How much data is really lost when “lossy” compression is used?
Lossy compression is to be used on imaging and plasma wave data. Let’s focus on imaging data for now. Each picture is made up of an array of up to 800×800 picture elements, or pixels. Each pixel has 256 possible values, depending on the brightness of that part of the image. The lossy compression takes a group of these pixels and approximates their brightness values with a formula. On the ground this formula is reconverted into a set of data which is supposed to be close to the original data set. The fidelity of the reconstructed image depends on the selected compression factor, which will range from 3:1 to 80:1. Tests of the compression routine on Galileo and Voyager images demonstrated that pixel values were correctly reconstructed to within about one percent, which is considered excellent for most investigations.
