ffmpeg 编码h264码率控制

FFmpeg and H.264 Encoding Guide

The goal of this guide is to inform new users how to create a high-quality H.264 video using the encoder x264.

There are two rate control modes that are usually suggested for general use: Constant Rate Factor (CRF) or Two-Pass ABR. The rate control is a method that will decide how many bits will be used for each frame. This will determine the file size and also how quality is distributed. To know more about what the different rate control modes do, see this post.

If you need help compiling and installing libx264 see one of our FFmpeg and x264 compiling guides.

Constant Rate Factor (CRF)

This method allows the encoder to attempt to achieve a certain output quality for the whole file when output file size is of less importance. This provides maximum compression efficiency with a single pass. Each frame gets the bitrate it needs to keep the requested quality level. The downside is that you can't tell it to get a specific filesize or not go over a specific size or bitrate.

1. Choose a CRF value

The range of the quantizer scale is 0-51: where 0 is lossless, 23 is default, and 51 is worst possible. A lower value is a higher quality and a subjectively sane range is 18-28. Consider 18 to be visually lossless or nearly so: it should look the same or nearly the same as the input but it isn't technically lossless.

The range is exponential, so increasing the CRF value +6 is roughly half the bitrate while -6 is roughly twice the bitrate. General usage is to choose the highest CRF value that still provides an acceptable quality. If the output looks good, then try a higher value and if it looks bad then choose a lower value.

Note: The CRF quantizer scale mentioned on this page only applies to 8-bit x264 (10-bit x264 quantizer scale is 0-63). You can see what you are using by referring to the ffmpeg console output during encoding (yuv420p or similar for 8-bit, and yuv420p10le or similar for 10-bit). 8-bit is more common among distributors.

2. Choose a preset

A preset is a collection of options that will provide a certain encoding speed to compression ratio. A slower preset will provide better compression (compression is quality per filesize). This means that, for example, if you target a certain file size or constant bit rate, you will achieve better quality with a slower preset. Similarly, for constant quality encoding, you will simply save bitrate by choosing a slower preset.

The general guideline is to use the slowest preset that you have patience for. Current presets in descending order of speed are: ultrafast,superfastveryfastfasterfastmediumslowslowerveryslowplacebo. The default preset is medium. Ignore placebo as it is not useful (see FAQ). You can see a list of current presets with -preset help (see example below), and what settings they apply with x264 --fullhelp.

You can optionally use -tune to change settings based upon the specifics of your input. Current tunings include: filmanimationgrainstillimagepsnrssimfastdecodezerolatency. For example, if your input is animation then use the animation tuning, or if you want to preserve grain then use the grain tuning. If you are unsure of what to use or your input does not match any of tunings then omit the -tune option. You can see a list of current tunings with -tune help, and what settings they apply with x264 --fullhelp.

Another optional setting is -profile:v which will limit the output to a specific H.264 profile. This can generally be omitted unless the target device only supports a certain profile (see Compatibility). Current profiles include: baselinemainhighhigh10high422high444. Note that usage of -profile:v is incompatible with lossless encoding.

To list all possible internal preset and tunes:

ffmpeg -f lavfi -i nullsrc -c:v libx264 -preset help -f mp4 -

Note: Windows users may need to use NUL instead of - as the output.

3. Use your settings

Once you've chosen your settings apply them for the rest of your videos if you are encoding more. This will ensure that they will all have similar quality.

CRF Example

ffmpeg -i input -c:v libx264 -preset slow -crf 22 -c:a copy output.mkv

Note that in this example the audio stream of the input file is simply stream copied over to the output and not re-encoded.


This method is generally used if you are targeting a specific output file size and output quality from frame to frame is of less importance. This is best explained with an example. Your video is 10 minutes (600 seconds) long and an output of 200 MiB is desired. Since bitrate = file size / duration:

(200 MiB * 8192 [converts MiB to kBit]) / 600 seconds = ~2730 kBit/s total bitrate
2730 - 128 kBit/s (desired audio bitrate) = 2602 kBit/s video bitrate

You can also forgo the bitrate calculation if you already know what final (average) bitrate you need.

Two-Pass Example

ffmpeg -y -i input -c:v libx264 -preset medium -b:v 2600k -pass 1 -c:a aac -b:a 128k -f mp4 /dev/null && \
ffmpeg -i input -c:v libx264 -preset medium -b:v 2600k -pass 2 -c:a aac -b:a 128k output.mp4

Note: Windows users should use NUL instead of /dev/null.

As with CRF, choose the slowest preset you can tolerate.

In pass 1 specify a output format with -f that matches the output format in pass 2. Also in pass 1, specify the audio codec used in pass 2; in many cases -an in pass 1 will not work.

See Making a high quality MPEG-4 ("DivX") rip of a DVD movie. It is an MEncoder guide, but it will give you an insight about how important it is to use two-pass when you want to efficiently use every bit when you're constrained with storage space.

Lossless H.264

You can use -crf 0 to encode a lossless output. Two useful presets for this are ultrafast or veryslow since either a fast encoding speed or best compression are usually the most important factors.

Lossless Example (fastest encoding)

ffmpeg -i input -c:v libx264 -preset ultrafast -crf 0 output.mkv

Lossless Example (best compression)

ffmpeg -i input -c:v libx264 -preset veryslow -crf 0 output.mkv

Note that lossless output files will likely be huge, and most non-FFmpeg based players will not be able to decode lossless, so if compatibility or file size issues you should not use lossless. If you're looking for an output that is roughly "visually lossless" but not technically lossless use a -crf value of around 17 or 18 (you'll have to experiment to see which value is acceptable for you). It will likely be indistinguishable from the source and not result in a huge, possibly incompatible file like true lossless mode.

Overwriting default preset settings

You can overwrite default preset settings with the x264-params option, or by using the libx264 private options (see ffmpeg -h encoder=libx264). This is not recommended unless you know what you are doing. The presets were created by the x264 developers and tweaking values to get a better output is usually a waste of time.


ffmpeg -i input -c:v libx264 -preset slow -crf 22 -x264-params keyint=123:min-keyint=20 -c:a copy output.mkv

Note: The old option x264opts will be removed. Use x264-params instead.

Additional Information & Tips

CBR (Constant Bit Rate)

There is no native CBR mode, but you can "simulate" a constant bit rate setting by tuning the parameters of ABR:

ffmpeg -i input.mp4 -c:v libx264 -x264-params "nal-hrd=cbr" -b:v 1M -minrate 1M -maxrate 1M -bufsize 2M output.ts

In the above example, -bufsize is the "rate control buffer" so it will enforce your requested "average" (1 MBit/s in this case) across each 2 MBit worth of video. So basically it is assumed that the receiver/end player will buffer that much data so it's ok to fluctuate within that much.

Of course, if it's all just empty/black frames then it will still serve less than that many bits/s but it will raise the quality level as much as it can.

Constained encoding (VBV / maximum bit rate)

You can also use -crf or -b:v with a maximum bit rate by specifying both -maxrate and -bufsize:

ffmpeg -i input -c:v libx264 -crf 23 -maxrate 1M -bufsize 2M output.mp4

This will effectively "target" -crf 23, but if the output were to exceed 1 MBit/s, the encoder would increase the CRF to prevent bitrate spikes. However, be aware that libx264 does not strictly control the maximum bit rate as you specified (the maximum bit rate may be well over 1M for the above file). To reach a perfect maximum bit rate, use two-pass.

In another example, instead of using constant quality (CRF) as a target, the average bitrate is set. A two-pass approach is preferred here:

ffmpeg -i input -c:v libx264 -b:v 1M -maxrate 1M -bufsize 2M -pass 1 -f mp4 /dev/null
ffmpeg -i input -c:v libx264 -b:v 1M -maxrate 1M -bufsize 2M -pass 2 output.mp4

Low Latency

libx264 offers a -tune zerolatency option. See the StreamingGuide.


All devices

If you want your videos to have highest compatibility with older devices:

-profile:v baseline -level 3.0

This disables some advanced features but provides for better compatibility. Typically you may not need this setting (and therefore avoid using -profile:v and -level), but if you do use this setting it may increase the bit rate compared to what is needed to achieve the same quality in higher profiles.


iOS Compatability (source)
Profile Level Devices Options
Baseline 3.0 All devices -profile:v baseline -level 3.0
Baseline 3.1 iPhone 3G and later, iPod touch 2nd generation and later -profile:v baseline -level 3.1
Main 3.1 iPad (all versions), Apple TV 2 and later, iPhone 4 and later -profile:v main -level 3.1
Main 4.0 Apple TV 3 and later, iPad 2 and later, iPhone 4s and later -profile:v main -level 4.0
High 4.0 Apple TV 3 and later, iPad 2 and later, iPhone 4s and later -profile:v high -level 4.0
High 4.1 iPad 2 and later, iPhone 4s and later, iPhone 5c and later -profile:v high -level 4.1
High 4.2 iPad Air and later, iPhone 5s and later -profile:v high -level 4.2

Note: This table does not include any additional restrictions which may be required by your device.


QuickTime only supports YUV planar color space with 4:2:0 chroma subsampling (use -vf format=yuv420p or -pix_fmt yuv420p) for H.264 video.


See Authoring a professional Blu-ray Disc with x264.

Pre-testing your settings

Encode a random section instead of the whole video with the -ss and -t/-to options to quickly get a general idea of what the output will look like.

  • -ss: Offset time from beginning. Value can be in seconds or HH:MM:SS format.
  • -t: Duration. Value can be in seconds or HH:MM:SS format.
  • -to: Stop writing the output at specified position. Value can be in seconds or HH:MM:SS format.

faststart for web video

You can add -movflags +faststart as an output option if your videos are going to be viewed in a browser. This will move some information to the beginning of your file and allow the video to begin playing before it is completely downloaded by the viewer. It is not required if you are going to use a video service such as YouTube.

Custom preset file

Refer to the -vpre output option in the documentation.


Will two-pass provide a better quality than CRF?

No, though it does allow you to target a file size more accurately.

Why is placebo a waste of time?

It helps at most ~1% compared to the veryslow preset at the cost of a much higher encoding time. It's diminishing returns: veryslow helps about 3% compared to the slower preset, slower helps about 5% compared to the slow preset, and slow helps about 5-10% compared to the medium preset.

Why doesn't my lossless output look lossless?

Blame the RGB to YUV color space conversion. If you convert to yuv444 it should still be lossless (which is the default now).

Will a graphics card make x264 encode faster?

For x264 specifically, probably not. x264 supports OpenCL for some lookahead operations.

See HWAccelIntro for information on supported hardware based H.264 encoders and decoders.

Encoding for dumb players

You may need to use -vf format=yuv420p (or the alias -pix_fmt yuv420p) for your output to work in QuickTime and most other players. These players only supports the YUV planar color space with 4:2:0 chroma subsampling for H.264 video. Otherwise, depending on your source, ffmpeg may output to a pixel format that may be incompatible with these players.

Additional Resources

来自 http://trac.ffmpeg.org/wiki/Encode/H.264

ffmpeg 编码h264码率控制