H265 currently implements three algorithms for intra-prediction mode selection:
- brute-force: test all 35 modes and compute exact bitrate. This should practically be the optimum possible.
- min-residual: compute a simple distortion measure on the residual image and select the mode with minimum distortion. See below for supported distortion measures.
- fast-brute: use a simple distortion measure to select the n-best modes with minimum distortion. Additionally, put the 3 candidate modes into the candidate list. For each candidate mode, compute the exact bitrate and select the best mode.
The following distortion measures are implemented in Libde265:
- SAD: sum of absolute differences
- SSD: sum of squared differences
- SATD-H: sum of absolute transformed differences (with Hadamard transform)
- SATD-D: sum of absolute transformed differences (with DCT/DST transform)
SSIM quality Considering SSIM quality, it can be observed that the HM quality is even below Min-Residual. It should be evaluated in the future if this is the case for other sequences, too. X265 has very good SSIM at high bitrates, but again drops off sharply at low bitrates. For Fast-Brute the results are again very close to optimum over the whole bitrate range.
Computation times Computation times (x265 is not included here, but it is much faster because of its optimized routines). Brute-force is by far the most computational expensive and computation time increases with bitrate because the exact bitrate computation takes more time at higher rates. The Fast-Brute algorithm is about a factor 4 faster and computation time does not increase as much. Computation time of Min-Residual is almost constant, because there is no bitrate dependent computation, only the final encoding takes a bit more time.