x265 has ten predefined --preset options that optimize the trade-off between encoding speed (encoded frames per second) and compression efficiency (quality per bit in the bitstream). The default preset is medium. It does a reasonably good job of finding the best possible quality without spending excessive CPU cycles looking for the absolute most efficient way to achieve that quality. When you use faster presets, the encoder takes shortcuts to improve performance at the expense of quality and compression efficiency. When you use slower presets, x265 tests more encoding options, using more computations to achieve the best quality at your selected bit rate (or in the case of –crf rate control, the lowest bit rate at the selected quality).
The presets adjust encoder parameters as shown in the following table. Any parameters below that are specified in your command-line will be changed from the value specified by the preset.
- ultrafast
- superfast
- veryfast
- faster
- fast
- medium (default)
- slow
- slower
- veryslow
- placebo
| preset | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|---|
| ctu | 32 | 32 | 64 | 64 | 64 | 64 | 64 | 64 | 64 | 64 |
| min-cu-size | 16 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 |
| bframes | 3 | 3 | 4 | 4 | 4 | 4 | 4 | 8 | 8 | 8 |
| b-adapt | 0 | 0 | 0 | 0 | 0 | 2 | 2 | 2 | 2 | 2 |
| rc-lookahead | 5 | 10 | 15 | 15 | 15 | 20 | 25 | 30 | 40 | 60 |
| lookahead-slices | 8 | 8 | 8 | 8 | 8 | 8 | 4 | 4 | 1 | 1 |
| scenecut | 0 | 40 | 40 | 40 | 40 | 40 | 40 | 40 | 40 | 40 |
| ref | 1 | 1 | 2 | 2 | 3 | 3 | 4 | 4 | 5 | 5 |
| limit-refs | 0 | 0 | 3 | 3 | 3 | 3 | 3 | 2 | 1 | 0 |
| me | dia | hex | hex | hex | hex | hex | star | star | star | star |
| merange | 57 | 57 | 57 | 57 | 57 | 57 | 57 | 57 | 57 | 92 |
| subme | 0 | 1 | 1 | 2 | 2 | 2 | 3 | 3 | 4 | 5 |
| rect | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 |
| amp | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 |
| limit-modes | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 0 |
| max-merge | 2 | 2 | 2 | 2 | 2 | 2 | 3 | 3 | 4 | 5 |
| early-skip | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| recursion-skip | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 |
| fast-intra | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
| b-intra | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 |
| sao | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| signhide | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| weightp | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| weightb | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 |
| aq-mode | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| cuTree | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| rdLevel | 2 | 2 | 2 | 2 | 2 | 3 | 4 | 6 | 6 | 6 |
| rdoq-level | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 | 2 | 2 |
| tu-intra | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 3 | 4 |
| tu-inter | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 3 | 4 |
There are a few --tune options available, which are applied after the preset.
Note
The psnr and ssim tune options disable all optimizations that sacrafice metric scores for perceived visual quality (also known as psycho-visual optimizations). By default x265 always tunes for highest perceived visual quality but if one intends to measure an encode using PSNR or SSIM for the purpose of benchmarking, we highly recommend you configure x265 to tune for that particular metric.
| –tune | effect |
|---|---|
| psnr | disables adaptive quant, psy-rd, and cutree |
| ssim | enables adaptive quant auto-mode, disables psy-rd |
| grain | improves retention of film grain. more below |
| fastdecode | no loop filters, no weighted pred, no intra in B |
| zerolatency | no lookahead, no B frames, no cutree |
--tune grain aims to encode grainy content with the best visual quality. The purpose of this option is neither to retain nor eliminate grain, but prevent noticeable artifacts caused by uneven distribution of grain. --tune grain strongly restricts algorithms that vary the quantization parameter within and across frames. Tune grain also biases towards decisions that retain more high frequency components.
It also enables a specialised ratecontrol algorithm --rc-grain that strictly minimises QP fluctuations across frames, while still allowing the encoder to hit bitrate targets and VBV buffer limits (with a slightly higher margin of error than normal). It is highly recommended that this algorithm is used only through the --tune grain feature.
--tune fastdecode disables encoder features which tend to be bottlenecks for the decoder. It is intended for use with 4K content at high bitrates which can cause decoders to struggle. It disables both HEVC loop filters, which tend to be process bottlenecks:
It disables weighted prediction, which tend to be bandwidth bottlenecks:
And it disables intra blocks in B frames with --no-b-intra since intra predicted blocks cause serial dependencies in the decoder.
There are two halves to the latency problem. There is latency at the decoder and latency at the encoder. --tune zerolatency removes latency from both sides. The decoder latency is removed by:
Encoder latency is removed by:
With all of these settings x265_encoder_encode() will run synchronously, the picture passed as pic_in will be encoded and returned as NALs. These settings disable frame parallelism, which is an important component for x265 performance. If you can tolerate any latency on the encoder, you can increase performance by increasing the number of frame threads. Each additional frame thread adds one frame of latency.