= Colourspace conversion step - CSC =

Before passing the pixels to the [/wiki/Encodings video encoder], we may or may not include a colourspace conversion step. (ie: newer versions of x264 support {{{BGRA}}} pixels as input directly)
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== 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}}}".
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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).
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== CSC Modules ==

On the server side we currently (as of v0.11) support 4 modules for doing this step:
* {{{csc_swscale}}} which uses [http://www.ffmpeg.org/ FFmpeg]'s swscale library (CPU based), [/browser/xpra/trunk/src/xpra/codecs/csc_swscale source here]
* [/wiki/CSC/OpenCL csc_opencl] (GPU or CPU based via [http://www.khronos.org/opencl/ OpenCL])
* [/wiki/CSC/NVCUDA csc_nvcuda] (GPU based via [http://www.nvidia.com/object/cuda_home_new.html CUDA])
* {{{csc_cython}}} (a slower fallback module in Cython - without scaling support) [/browser/xpra/trunk/src/xpra/codecs/csc_cython source here]

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)
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On the client side, if a CSC step is needed, it will use {{{swscale}}} only (for now). The {{{OpenGL}}} mode does not need a CSC step as it can display YUV pixels directly on screen, and do the scaling itself too.
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== Choosing a CSC module ==

The best way to choose a CSC module is to test them all and [/wiki/CSC/Performance compare them].
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== Specify a module or mode ==

As of version 0.12 (r5376), one can choose which csc modules get loaded into the server using the {{{--csc-modules=}}} command line option. ie:
{{{
xpra start :10 --csc-modules=nvcuda,swscale
}}}

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In version 0.11, 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 [/wiki/CSC/OpenCL] and [/wiki/CSC/NVCUDA]
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== Scaling ==

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)

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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), [/wiki/Encodings/nvenc nvenc] does.
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