:: > � �These are all driver-specific properties and not something that would
:: > be in a general API description. �For the most part these just shadow
:: > the V4L attributes.
::
:: Right - but AFAIK there is no such thing as setting the grabbing frequency in
:: v4l drivers.
::
:: > � � Video is not in RGB. �Video is always in YUV. �I don't think it's
:: > possible for an RGB format to have better quality if the video source
:: > is in YUV.
::
:: Right - but if you have a dvb card at hands you can see that the overlay
:: quality in RGB565 mode is much better than YUV2 although both formats are
:: 16bits and although the input is MPEG2, especially yellow and brown colors
:: have little resolution.
::
:: That means, eventually Xvideo is not was i need - i'll will try GL instead,
:: because Xv uses to much CPU (X-server, sys) anyway...
I'm coming (butting?) into the middle of this so my comments
may not be germane (in which case I won't be offended if you tell
me to buzz off).
I'm not quite sure what YUV2 is but ordinary MPEG2 YUV (more properly,
Y, Cb, Cr) is such that U and V are subsampled x2 horizontally w.r.t. Y and that
is how you get "16 bits".
In fact looking horizontally at MPEG2 (studio profile) you see {Y Cb Y
Cr Y Cb Y Cr ...}. Where Y, Cb, and Cr, are each 8-bits. Hence each
grouping ((Y Cx): (x\in {b,r}) is 16bits. The more usual MPEG2 (main profile)
also subsamples the U and V vertically x2.
As a result the chrominance signal can support only
half (or less) the resolution of the luminance signal.
For RGB-565 you are chopping bits from each subpixel, a non-linear
operation not (easily) amenable to "linear systems" type analysis.
Bit chopping does not, technically speaking, result in loss of resolution.
Instead you get various "contouring" and other artifacts due to the
limited bit-depth. Note however that there is an R, G, and B sub-pixel
at every location in the sampling lattice so the chrominance signal
can support just as high frequencies as the luminance.
Caveat to the Cognoscenti: Those who know this subject will detect
that I've fudged a tiny bit in my explanation. But what I've said gets
across is layman's terms why you would see low chroma
resolution in the YUV case.
dean S. Messing
Center for Displayed Appearance
Information Systems Technologies Dept.
Sharp Laboratories of America
E-Mail: [EMAIL PROTECTED]
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