Reduced DCT approximations for low bit rate coding

Transform approximations are explored for speeding up the software compression of images and video. Faster approximations are used to replace the regular DCT whenever only a few DCT coefficients are actually encoded. In one instance, we employ a data analyzer to drive a bank of coders by switching to the fastest coder depending on the image contents or on the output demand. The compression speed is adapted to the image contents in order to homogenize the data production rate, i.e. smoother areas produce fewer bits than detailed ones, but faster. The approximations are applicable to high compression (or constant output rate) environments and where lower complexity is required.