Preset Options

Presets

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 slow slower veryslow placebo
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
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

Tuning

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

Film Grain Retention

--tune grain tries to improve the retention of film grain in the reconstructed output. It disables rate distortion optimizations in quantization, and increases the default psy-rd.

It lowers the strength of adaptive quantization, so residual energy can be more evenly distributed across the (noisy) picture:

And it similarly tunes rate control to prevent the slice QP from swinging too wildly from frame to frame:

And lastly it reduces the strength of deblocking to prevent grain being blurred on block boundaries:

Fast Decode

--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.

Zero Latency

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.

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