Three modifications of the Constant Area Quantization (CAQ) image bandwidth compression technique have been developed and tested in order to broaden the range of compression ratios obtainable with acceptable image quality. The first modification involved the introduction of an adaptive area threshold, the second was a two-threshold algorithm and the third was a hybrid of the CAQ with a Hadamard transform technique. Using these three algorithms together with the basic CAQ, images spanning the range from 0. 2 to 2 bits per picture element were obtained from an 8 bit original.
J. J. Pearson,
R. M. Simonds,
"Adaptive, Hybrid, And Multi-Threshold CAQ Algorithms", Proc. SPIE 0087, Advances in Image Transmission Techniques, (13 December 1976); doi: 10.1117/12.954977; https://doi.org/10.1117/12.954977