One of main properties of DCT transform is de-correlation (i.e. minimizing or even removing the inter-dependencies between coefficients). Actually coefficients within a DCT block get statistically independent. Hence, there is no sense to code a couple (or generally n-tuple) of DCT coefficients as one entity. It’s more efficient and simpler to code each DCT coefficient separately with taking into consideration inter-block dependencies only, since intra (or inter-coefficient-within-same-block) dependencies are removed by DCT transform.
For example, in JPEG (baseline) each DC coefficient is coded as the difference between the current DC and the previous one (to utilize the inter-block dependency).
DCT de-correlation is not full, therefore in HEVC/H.265 standard in coding of DCT coefficients spatial dependencies between surrounding DCT coefficients are still taken into consideration.
The human visual system has different sensitivities to different DCT basis functions – more sensitive to low frequencies and less sensitive to high frequencies.
The assumption “sensitivity to a particular coefficient’s quantization error is independent of the magnitudes of all the other coefficients (except the DC)” is reported to be incorrect, in the paper “Visibility of DCT basis functions: Effects of contrast masking“, by Joshua A. Solomon et al. is revealed:
sensitivity to a particular coefficient’s quantization error is affected by the magnitudes of other coefficients.
The authors of the paper coined this phenomenon as “between-coefficient contrast masking“.
23+ years’ programming and theoretical experience in the computer science fields such as video compression, media streaming and artificial intelligence (co-author of several papers and patents).
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