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x265/doc/reST/presets.rst Normal file
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Preset Options
--------------
.. _presets:
Presets
=======
x265 has a number of predefined :option:`--preset` options that make
trade-offs between encode 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 enormous CPU cycles looking for the
absolute most efficient way to achieve that quality. As you go higher
than medium, the encoder takes shortcuts to improve performance at the
expense of quality and compression efficiency. As you go lower than
medium, the encoder tries harder and harder to achieve the best quailty
per bit compression ratio.
The presets adjust encoder parameters to affect these trade-offs.
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
| | ultrafast | superfast | veryfast | faster | fast | medium | slow | slower | veryslow | placebo |
+==============+===========+===========+==========+========+======+========+======+========+==========+=========+
| ctu | 32 | 32 | 32 | 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 |
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
| scenecut | 0 | 40 | 40 | 40 | 40 | 40 | 40 | 40 | 40 | 40 |
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
| refs | 1 | 1 | 1 | 1 | 2 | 3 | 3 | 3 | 5 | 5 |
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
| 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 |
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
| 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 | 0 | 0 | 0 | 0 | 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 |
+--------------+-----------+-----------+----------+--------+------+--------+------+--------+----------+---------+
Placebo mode enables transform-skip prediction evaluation.
.. _tunings:
Tuning
======
There are a few :option:`--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
~~~~~~~~~~~~~~~~~~~~
:option:`--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.
* :option:`--psy-rd` 0.5
* :option:`--rdoq-level` 0
* :option:`--psy-rdoq` 0
It lowers the strength of adaptive quantization, so residual energy can
be more evenly distributed across the (noisy) picture:
* :option:`--aq-strength` 0.3
And it similarly tunes rate control to prevent the slice QP from
swinging too wildly from frame to frame:
* :option:`--ipratio` 1.1
* :option:`--pbratio` 1.1
* :option:`--qcomp` 0.8
And lastly it reduces the strength of deblocking to prevent grain being
blurred on block boundaries:
* :option:`--deblock` -2
Fast Decode
~~~~~~~~~~~
:option:`--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:
* :option:`--no-deblock`
* :option:`--no-sao`
It disables weighted prediction, which tend to be bandwidth bottlenecks:
* :option:`--no-weightp`
* :option:`--no-weightb`
And it disables intra blocks in B frames with :option:`--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. :option:`--tune` *zerolatency*
removes latency from both sides. The decoder latency is removed by:
* :option:`--bframes` 0
Encoder latency is removed by:
* :option:`--b-adapt` 0
* :option:`--rc-lookahead` 0
* :option:`--no-scenecut`
* :option:`--no-cutree`
* :option:`--frame-threads` 1
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.