Re: [GnomeMeeting-list] how to support another capture card? (WAS: experience with the V4L1 compatibility of V4L2)

[<derek indranet co nz>]

Hi,
 openh323 is constained on frame size by the video codec it uses: H261

openh323 video code must generate a frame that is: 352x288 or 176x144.

We have found in developing code that some grabbers will not provide the 
176x144 frame size requested. 

Consequently, the library tries to grab a different frame size, and then 
convert the frame size into something the codec can use.

=====

Providing the appropriate frame size should be done in the library.

Providing frame size converters etc in the application is not wise.
Imagine, 10 people write 10 applications for displaying video with pwlib.
If the application provides frame size conversion, those 10 people have to
write 10 frame size converters. 
  not good.

Do it once, do it right. Do it in the library is my conclusion.

============
However, take this to another level.

Suppose there are 10 libraries out there that grab video, and present it 
for applications to use.
Those 10 libraries all contain framesize/format converters. 
Thus, 10 framesize/format converters have been written.
   not good.

Do it once, do it right. Do  the framesize/format conversion in
the kernel. Is this what v4l2 does?

=============================================================================



On Thu, 18 Apr 2002, Guilhem Tardy wrote:

> I added grabber->SetFrameSize(width, height) in videograbber.cpp, and got to
> the bottom of this:
> 
> Since the Videum card doesn't support 352x288 (but still advertises 640x288 as
> maximum frame size), it fall backs on 352x240 (same behavious with 'vctrl'),
> and VideoInputDevice::VerifyHardwareFrameSize() doesn't like it at all.
> 
> I am a bit confused with the logic here, because
> VideoInputDevice::SetFrameSize() first sets the size internally, then verifies
> hardware that returns FALSE (at this point, the card is set with 352x240, and
> internally to PWLib with 352x288, no converter is configured).
> 
> So now, my question is: does the PWLib library behave so by design? Should we
> address this in PWLib or work around it in the application (e.g. GnomeMeeting)?
> 
> Guilhem.
> 
> --- Guilhem Tardy <guilhem_tardy yahoo com> wrote:
> > Hi,
> > 
> > As I found out yesterday, the palette is not an issue. The problem lies in
> > the
> > VideoInputDevice::Open() function, which gets the device capabilities, sets
> > the
> > height and width internal values to the maximum values  (i.e. 640x240), and
> > mutes the audio.
> > 
> > In effect, the WinNov video frame size is changed from whatever I set it to
> > before (e.g. 352x240x24, via the 'vctrl' utility) to 640x240x8. This is also
> > reflected by the 'vctrl' utility after the facts.
> > 
> > Guilhem.
> > 
> > --- Damien Sandras <dsandras seconix com> wrote:
> > > On Wed, Apr 17, 2002 at 03:45:25PM -0700, Guilhem Tardy wrote:
> > > > I found that the problem is actually not the palette, but a subversive
> > > change
> > > > of frame size (640x288) when you Open() a VideoInputDevice.
> > > > Then, the SetFrameColourConverter() calls SetFrameColour(), which calls
> > > > SetFrameSizeConverter(), and returns FALSE (640x288 is definitely
> > > > non-standard).
> > > > Do you know why VideoInputDevice::Open() resets the frame size?
> > > > 
> > > 
> > > I'm not sure I understand what you mean.
> > > PWLib only tries sizes CIF (352x288) and QCIF (176x144).
> > 
> > 
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-- 
Derek Smithies Ph.D.
IndraNet Technologies Ltd.
Email: derek indranet co nz
ph +64 3 365 6485
Web: http://www.indranet-technologies.com/