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Version 41 (modified by Antoine Martin, 8 years ago) (diff)


Colourspace conversion step - CSC

Before passing the pixels to the video encoder, we may or may not include a colourspace conversion step. (ie: newer versions of x264 support BGRA pixels as input directly)

Information and Diagnostics

The pixel format used as input as well as the CSC module chosen to do this conversion work can be found via "xpra info".
It will be chosen automatically based on the desired speed/quality settings. Note that the CSC step may degrade the quality of the picture (YUV444P mode does not, but YUV422P and YUV420P do).

CSC Modules

On the server side we currently (as of v0.11) support 4 modules for doing this step:

Note: some of those modules may require you to add some libraries to your library path (ie: CUDA often does, Nvidia's OpenCL libraries do too)

On the client side, if a CSC step is needed, it will use swscale. The OpenGL mode does not need a CSC step as it can display YUV pixels directly on screen.

Choosing a CSC module

The best way to choose a CSC module is to test them all and compare them.

Specify a module or mode

To choose a specific CSC module, use:

XPRA_CSC_TYPE=name xpra ...

Where name is one of the csc modules above.

One can also force the use a specific CSC mode:

XPRA_FORCE_CSC_MODE=cscmode xpra ...

Where cscmode is one of: YUV420P, YUV422P, YUV444P.

Some modules can be configured further, see below:




  • select which device type is used as backend using:
    XPRA_OPENCL_DEVICE_TYPE=devicetype xpra start ...

Where devicetype is either GPU or CPU.

  • choose the OpenCL implementation to use when more than one are installed:
    XPRA_OPENCL_PLATFORM="Intel" xpra ...
  • select a specific OpenCL device when more than one is available:
    XPRA_OPENCL_DEVICE_NAME="GeForce GTX 760" xpra ...

To get more information use:



You may need to build xpra with the flag "--with-csc_nvcuda". You must use a sufficiently recent versionof the nvidia video driver, preferably obtained directly from nvidia, and not the nouveau drivers (which you should probably remove).



  • choose the device used with:
    XPRA_CUDA_DEVICE=N xpra ...

Where N is the device ID. Use:

XPRA_CUDA_DEBUG=1 xpra ...

to get more information.


As part of the colourspace conversion step, we can also downscale the frame if the XPRA_SCALING environment variable is unset or set to "1", and either:

  • the application requests scaling using the _XPRA_SCALING X11 window property (specified as a 32-bit fraction, 16-bits for each)
  • the XPRA_SCALING_HARDCODED environment variable is set on the server, using one of those 2 forms (the second one is preferred):
    • XPRA_SCALING_HARDCODED=N to downscale by N
    • XPRA_SCALING_HARDCODED=M:N to downscale by the fraction M/N
  • automatically, using some heuristics (the bigger the picture size and the lower the quality + the higher the speed - the more likely it is that it will be enabled)
  • manually using xpra control at runtime (see ticket:461#comment:9):
    xpra control DISPLAY scaling "*" 2:3
    will scale all windows ("*") by 2/3rd on DISPLAY (replace with your actual display number or URI)

Note: when downscaling is enabled, this may introduce a colourspace conversion step where none was necessary before (when the encoder was previously handling RGB pixel data directly).

Only some encoders can handle the scaling natively (and therefore at almost no cost), nvenc does.