Unexpectedly expensive conversion from BGRA to BGR

[Nicolas Dufresne]

Le lundi 19 octobre 2020 à 09:52 -0700, Tyler Compton a écrit :
> Interesting, thank you Nicolas. Unfortunately, the majority of
> machine learning models I'm using require BGR or RGB as their input
> format, so there isn't a lot I can do about it right now. I guess
> I'll have to look for optimizations elsewhere.

Note that videoconvert element have few "fast_path". Even thous it's
unaligned, you know that RGB24 will re-aligne every 4 pixels. Perhaps
there a ways to add some 4 pixels (128bits -> 96bit compression) SIMD
optimization (using orc or not).

Nicolas

> 
> On Sat, Oct 17, 2020 at 7:38 AM Nicolas Dufresne <
> nicolas at ndufresne.ca> wrote:
> > Le vendredi 16 octobre 2020 à 16:10 -0700, Tyler Compton a écrit :
> > > Hi everyone,
> > > 
> > > I have a pipeline that, at some point, needs to convert video
> > frames from BGRA to BGR. I was surprised to see about a ~17%
> > increase in CPU usage just from doing this conversion. I wouldn't
> > expect it to be that expensive, considering it's just a matter of
> > stripping off some data for each pixel.
> > > 
> > > In my application, I have BGRA frames, but never any actual alpha
> > information. Is there perhaps an optimization opportunity because
> > of that?
> > > 
> > > On average I see about 17% CPU usage when running this pipeline:
> > > 
> > > videotestsrc is-live=true ! "video/x-
> > raw,format=BGRA,width=1920,height=1080,framerate=30/1" ! fakesink
> > > 
> > > However, I see around 38% CPU usage just by adding a videoconvert
> > to go from BGRA to BGR:
> > > 
> > > videotestsrc is-live=true ! "video/x-
> > raw,format=BGRA,width=1920,height=1080,framerate=30/1" !
> > videoconvert ! "video/x-raw,format=BGR" ! fakesink
> > 
> > This conversion will move 32bit aligned pixels to 24bits
> > (unaligned)
> > pixels. Unaligned writes are 3 time slower. Unless you need the
> > bandwidth gain (even though yuv formats may give a better
> > compression),
> > I would not recommending using 24bit packed formats in recent
> > applications. At least not when doing CPU access.
> > 
> > > 
> > > I'm running GStreamer 1.18.0 on Ubuntu 20.04.
> > > 
> > > Thanks,
> > > Tyler
> > > 
> > > 
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