Re: [V4L] Image clipping with mmap

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Trent Piepho wrote:
> 
> On Mon, 27 Nov 2000, Justin Schoeman wrote:
> > > I need to clip images with a bttv chipset framegrabber card. Since the
> > > only way i can read images is with mmap'ing the buffer, i need to set
> > > clipping parameters with this method. However using the VIDIOCSWIN ioctl
> > > doesnt work in this case, and im thus unable to set the clipping
> > > rectangles.
> > > Is there any way to do this with mmap io, and is it possible for me to
> > > simply read images with bt848 cards without going through the mmap
> > > interface?
> > >
> > > Thanks
> > > Abheek
> >
> > There is no way to do this with the v4l or v4l2 API.  The question from
> > my side is: Why?  You have to copy the image out of the mmap()ed region
> > anyway so any clips would be lost.  If you could explain what you want
> > to do, perhaps we could be of more help.
> 
> Save PCI bandwidth?  If you only want a fraction of the image, it doesn't make
> sense to DMA the whole thing into memory.

OK - That makes sense.

> You could also do memory overlay.  Place a background image into memory, then
> overlay the video on top of it.  Without clipping, a second copy is required to
> put the background into memory.  For example, on-screen captions usually stay
> on screen for many frames, so they can be read.  If memory overlay can't clip,
> you would have to redraw captions every single video frame, which is a lot of
> extra work.  Of course, everyone but me has a 1 GHz or faster processor and
> doesn't care about a few lost frames, so efficiency issues don't seem to
> matter.

The mmap()ed memory area is definitely NOT guaranteed to be user
writable, so this would not be a good idea.

> Also V4L and V4L2 have no concept of adjusting the analog TV timing
> parameters.  A crude workaround is to scale and crop the image to trick the
> driver into giving the desired timing.  This won't work for non-overlay
> capture.

OK v4l2 does have "digital zoom" capabilities that can let you select a
rectangular subpicture.  I have not implemented it for bttv2, but if
anybody wants to, they are welcome!

-justin





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