During development of HEVC and after finalization some practitioners criticized the standard for a ‘huge’ amount of diagonal prediction modes – 31 pure angular prediction modes (except vertical,horizontal, DC and plane) due to impact on complexity.
My conjecture on Abundance of Intra Prediction Modes in HEVC:
DCT is poor in approximation of diagonal edges (because DCT basis functions are ‘rectangular-formed’). Consequently inaccuracy in DCT approximation of oblique edges sometimes causes such visual impairment as staircases (visible around edges):
Therefore main efforts were applied to diminish residual errors in intra blocks (in case of abundance of oblique edges) by increasing amount and granularity of diagonal intra prediction modes.
By the way the emerging standard AV1 has 56 angular intra prediction modes!!!
In addition to enhancement of visual perception, richness of diagonal modes is tailored to reduce Intra-frame bitrate spikes (since spatial redundancy can be better exploited).
Moreover, error resilience is also enhanced since intra blocks tend to be selected more frequently in Inter frames and as a result temporal error propagation (due to packet loss or bit-flips) gets limited.
Appendix: Differential Pulse Code Modulation (DPCM) method to exploit Spatial Redundancy
The Spatial Redundancy in video data means a strong correlation of neighboring pixels, which is observed within frames.
Therefore, a predictive coding is beneficial if the correlation is strong enough among the spatially adjacent pixels.
The basis of the spatial predictive coding is to predict a pixel value from previous pixels which were already coded (hence they are available at the decoding side). The prediction error (i.e., the difference between the current pixel and its predicted value) is quantized and then it is entropy coded.
This is a very popular form of predictive coding known as differential pulse code modulation (DPCM).
The implicit assumption here – neighboring pixels are highly correlated, hence the prediction error tends to be small.
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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