[winswitch] Reducing the colour depth (client side)

Antoine Martin antoine at nagafix.co.uk
Mon Mar 4 10:55:25 GMT 2013

On 03/04/2013 05:42 PM, Nick Burrett wrote:
> On 4 March 2013 09:00, Antoine Martin <antoine at nagafix.co.uk> wrote:
>> Hi Nick,
>> xpra is a compositing window manager, and AFAIK this requires 24bpp or
>> higher. I've tried launching Xvfb/Xdummy with 8bpp and Composite gets
>> turned off. The -pixdepths switch does not help here. FYI: xpra seems to
>> crash rather than exit gracefully, but that's another problem.
>> As for achieving bandwidth savings by reducing the colour depth, x264
>> and vpx encodings use colour subsampling when the quality is lowered
>> (switching from 4:4:4 to 4:2:2 and 4:2:0), this is in the chroma+luma
>> space rather than plain RGB, but it does save about 50% on image size.
>> Also note that newer encodings (x264, webp, etc) are vastly superior to
>> the encodings found in VNC, so the bandwidth usage should already be
>> much lower (even without colour subsampling).
> Alright, that makes sense.   My gnome-terminal data transfer showed as
> 420Mbytes for around 6 hours of operation (using x264).  I'll have a
> go at tuning the quality some more and extend the time between full
> window refreshes.
> I wonder if there's some efficiency to be gained by pre-processing the
> image before compression (at the expense of quality) -- perhaps by
> masking out the bottom 3 bits of R, G and B values so that the
> compressors don't have to try to handle subtle colour variations.
>> If you find that the quality/bandwidth/framerate/latency isn't good
>> enough for you use case, let me know - it is quite possible that recent
>> versions have focused a little bit too much on the LAN usage scenario..
> It's not great at the moment -- it can take 2-3 seconds to switch a
> tab in gnome-terminal, for example.  But I'd rather ensure that I've
> exhausted my client option settings first.
2 to 3 seconds is way too high, I doubt you can find enough options to
lower that enough to make it bearable.
There is a lot of information for debugging such problems here:
Let me know what you find and I can make sure this gets auto-tuned properly.


> Regards,
> Nick
>> On 03/03/2013 07:26 PM, Nick Burrett wrote:
>>> Hi,
>>> Is it possible to reduce the colour depth client side when performing
>>> xpra attach?
>>> I'm attempting to use xpra over a 3G modem link where the X server is
>>> 140ms away.  For a while, I've been using a VNC setup in exactly this
>>> way and found that reducing the colour from 24bits to 8bits had a
>>> considerable impact on usability.
>>> Xvfb has a -pixdepths option, which appears to be a list of supported
>>> colour depths.  I just would like to get the client to allow the
>>> choice to be determined at connection time.
>>> Regards,
>>> Nick.
>>> _______________________________________________
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>>> shifter-users at lists.devloop.org.uk
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