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Encodings.md

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Encoding Encodings

Xpra supports a wide variety of picture and video encodings for sending the window contents to the client fast and efficiently.
For some background information on picture encodings, see https://images.guide/.

Choosing which encoding to use for a given window is best left to the xpra engine.
It will make this decision using the window's characteristics (size, state, metadata, etc), network performance (latency, congestion, etc), user preference, client and server capabilities and performance, etc

Generally, if any tuning is needed, instead of trying to guess what should be used and overriding the encodings and encoding options, it is best to use the min-speed and min-quality options instead.

Encodings:

pseudo encodings

The following pseudo encodings just control which actual encodings can be selected by the engine:

  • auto which is the default, allows all options
  • grayscale does the same, but without sending colours - which saves some bandwidth (this saving is not always significant)
  • scroll will try harder to send the screen updates using a list of motion vectors, if possible

You can select the pseudo-encoding using the --encoding=ENC switch.

picture encodings
Codename Bit Depths Characteristics Details
mmap all fastest only available with local connections, selected automatically
rgb all very fast raw RGB pixels, potentially compressed with a stream compressor (ie: lz4)
webp 24 / 32 good fast, supports transparency, lossy and lossless modes
jpeg 24 fast easy to support
png 24 / 32 slow easy to support
png/P 8 slow only useful for 8-bit desktop mode
png/L 8 slow greyscale
video encodings

Using a video stream is often the most efficient way of sending large amounts of screen updates without consuming too much bandwidth. The xpra engine should automatically detect when it makes sense to switch to a video codec.

Codename Bit Depths Characteristics
vp8 24 fast but less efficient
vp9 24 / 30 more efficient but somewhat slower
h264 24 / 30 licensing issues
hevc 24 / 30 licensing issues - usually slower

Which ones of these video encodings are available depends on the video encoders enabled:

Video Encoders

Xpra ships the following encoder modules:

Codename Encodings supported Notes
vpx vp8, vp9
x264 h264 fast
x265 hevc slower
nvenc h264, hevc fastest (requires hardware)
ffmpeg all capabilities vary

Which encodings are actually supported by each encoder may vary, depending on the version used, the build options, hardware capabilities, etc.

You can choose which video encoders are loaded at runtime using the video-encoders option.

Some of these video encoders may require a colorspace conversion step:

colorspace Conversion

These modules are used for:

  • converting the pixel data received by the xpra server into a pixel format that can be consumed by the video encoders
  • converting the pixel data from the video decoders into a pixel format that can be used to paint the client's window (different windows may have different capabilities)
  • up / down scaling the pixel data when needed |Codename|Colorspaces supported|Notes| |--------|-------------------|-----| |cython|r210, BGR48, GBRP10, YUV444P10|slow but useful for some high bit depth modes| |swscale|RGB24, BGR24, 0RGB, BGR0, ARGB, BGRA, ABGR, YUV420P, YUV422P, YUV444P, GBRP, NV12|fast| |libyuv|BGRX, YUV420P, NV12|fastest|

You can choose which colorspace conversion modules are loaded at runtime using the csc-modules option.

video decoders

Xpra ships the following decoder modules:

Codename Encodings supported
avcodec2 all
vpx vp8, vp9

You can choose which video decoders are loaded at runtime using the video-decoders option.

Diagnostics

list all the encodings available with the current installation 
xpra encoding

(on MS Windows and MacOS, you can also use the Encodings_info wrapper)

list all the video codecs and colorspace conversion modules available 
xpra video
list encodings available to the client 
xpra attach --encoding=help
list encodings available to the server 
xpra start --encoding=help
debug logging switches 
xpra start -d damage,compress,encoding

Tuning

Warning: tuning is very often misused and ends up being counterproductive.

Preventing blurry screen updates

Rather than selecting a lossless picture encoding, which may use far too much bandwidth and cause performance issues:

  • make sure that the applications are correctly detected: either using the application's command content-type and content-categories mapping
  • raise the min-quality and / or lower the min-speed
  • maybe lower the auto-refresh delay - just be aware that the lossless auto-refresh can be costly (as all lossless frames are)
Quality

Acceptable values range from 1 (lowest) to 100 (lossless).
Rather than tuning the quality option, it is almost always preferable to set the min-quality instead.
Using lower values saves bandwidth and CPU, but the screen updates may become more blurry.

Speed

Acceptable values range from 1 (lowest) to 100 (lossless).
Rather than tuning the speed option, it is almost always preferable to set the min-speed instead.
Using lower values costs more CPU, which reduces bandwidth consumption but may also lower the framerate.

Best

The best possible setup is to use NVENC or another hardware encoder supported by libva: hardware encoders compress very well and do so incredibly fast.

Further reading!

When comparing performance, make sure that you use the right metrics...
The number of updates per second (aka fps) is not always a good one: if there are many small regions, this can be a good or a bad thing.