Hi
A little bit of context for the situation which prompted me to send in
this mail. I need to perform 'digital zooms' on segments of the image, and
since the app is a real time thing would like it to be as efficient as
possible. So the policy i thought of using, since V4L doesnt support this,
was to scale and crop, which is why i needed to do clip the image.
I would welcome any ideas on a way around this, or otherwise i guess
working on the bttv2 driver for this feature might not be a bad place for
me to start with. :-)
Abheek
On Mon, 27 Nov 2000, Justin Schoeman wrote:
> Trent Piepho wrote:
> >
> > On Mon, 27 Nov 2000, Justin Schoeman wrote:
> >
> > 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!
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