[PD] Re: [GEM-dev] Re: pix_video and dv1394 capture on Edgy

Roman Haefeli reduzierer at yahoo.de
Thu Dec 28 20:44:39 CET 2006


--- Mathieu Bouchard <matju at artengine.ca> schrieb:

> On Thu, 28 Dec 2006, Ivica Ico Bukvic wrote:
> 
> > Thanks for the advice but I am hoping to avoid
> webcam due to inherent
> > quality/fps decrease.
> 
> USB2 is 40 times faster than USB1: at 400 mbps, in
> 720x576, full color 24 
> bits per pixel, cameras could do 40 fps... but
> typically they don't, they 
> use half the data rate (decimation of chroma
> columns, as in most broadcast 
> video) and have a cap at 30 fps because anyway the
> motion is already 
> blurry enough at 30 fps.

anyway, though it theoretically should be possible to
capture a webcam with 720x576 at 30fps, i couldn't find a
cam yet, that supports these values. also, it is so
confusing, that often they are advertized as usbII
capable cams, but actually they don't make use of the
bandwidth. further you find often a maximum framerate
in the specs, but not which dimensions these maximum
rate is related to. webcam sellers are really pissing
me off. i bought a spc900nc, which works with the
opensource pwc linux webcam driver. though this cam
should be capable of capturing at 90fps according to
the specs, i couldn't reach that rate neither with the
proprietary windows driver nor with the pwc driver.
the maximum with the pwc i could get is 320x240 at 30fps,
which is less than 1/4 of what you say should be
possible with usbII.
more moanings about webcams:
it's hard to tell in advance, if it will be possible
to switch automatic gain control/automatic white
balance/automatic shutter time on and off, which could
be important when using a cam with gem/pdp/gridflow.
with the cam i mentioned above, i have the following
colospace problems:
-pdp is capturing the cam as expected  
-gridflow captures it only in yuv420p colorspace and
one needs to convert it to RGB using gridflow objects,
which is heavily cpu expensive.
-gem shows the captured image as bgr instead of rgb,
so colors red and blue are swapped. it is possible to
convert it to rgb with [pix_colormatrix], which is
also very cpu expensive. 

fazit: webcams are kind of an annoying topic and i can
really understand ivico's efforts to get a dv-cam
working with gem.  

roman 




		
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