Coding isotropic images

Rate-distortion functions for 2-dimensional homogeneous isotropic images are compared with the performance of five source encoders designed for such images. Both unweighted and frequency weighted mean-square error distortion measures are considered. The coders considered are a) differential pulse code modulation (DPCM) using six previous samples or picture elements (pels) in the prediction--herein called 6-pel DPCM, b) simple DPCM using single-sample prediction, c) 6-pel DPCM followed by entropy coding, d)
8 \times 8
discrete cosine transform coding, and e)
4 \times 4
Hadamard transform coding. Other transform coders were studied and found to have about the same performance as the two transform coders above. With the mean-square error distortion measure, 6-pel DPCM with entropy coding performed best. Next best was the
8 \times 8
discrete cosine transform coder and the 6-pel DPCM--these two had approximately the same distortion. Next were the
4 \times 4
Hadamard and simple DPCM, in that order. The relative performance of the coders changed slightly when the distortion measure was frequency weighted mean-square error. From
R = 1
to 3 bits/pel, which was the range studied here, the performances of all the coders were separated by only about 4 dB.