On Tue, 30 Aug 2011, Hans Verkuil wrote:
> On Tuesday, August 30, 2011 16:24:55 Guennadi Liakhovetski wrote:
> > Hi Hans
> >
> > On Tue, 30 Aug 2011, Hans Verkuil wrote:
[snip]
> > > The problem with S_FMT changing the crop rectangle (and I assume we are not
> > > talking about small pixel tweaks to make the hardware happy) is that the
> > > crop operation actually removes part of the frame. That's not something you
> > > would expect S_FMT to do, ever. Such an operation has to be explicitly
> > > requested by the user.
> > >
> > > It's also why properly written applications (e.g. capture-example.c) has
> > > code like this to reset the crop rectangle before starting streaming:
> > >
> > > if (0 == xioctl(fd, VIDIOC_CROPCAP, &cropcap)) {
> > > crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
> > > crop.c = cropcap.defrect; /* reset to default */
> > >
> > > if (-1 == xioctl(fd, VIDIOC_S_CROP, &crop)) {
> > > switch (errno) {
> > > case EINVAL:
> > > /* Cropping not supported. */
> > > break;
> > > default:
> > > /* Errors ignored. */
> > > break;
> > > }
> > > }
> > > }
> > >
> > > (Hmm, capture-example.c should also test for ENOTTY since we changed the
> > > error code).
> >
> > I agree, that preserving input rectangle == output rectangle in reply to
> > S_FMT is not nice, and should be avoided, wherever possible. Still, I
> > prefer this to sticking with just one fixed output geometry, especially
> > since (1) the spec doesn't prohibit this behaviour,
>
> Hmm, I think it should be prohibited. Few drivers actually implement crop,
> and fewer applications use it. So I'm not surprised the spec doesn't go into
> much detail.
>
> > (2) there are already
> > precedents in the mainline.
>
> Which precedents? My guess is that any driver that does this was either not
> (or poorly) reviewed, or everyone just missed it.
My first two sensor drivers mt9m001 and mt9v022 do this, but, I suspect, I
didn't invent it at that time, I think, I copied it from somewhere, cannot
say for sure though anymore.
> > Maybe, a bit of hardware background would help: the sensor is actually
> > supposed to be able to both crop and scale, and we did try to implement
> > scales other than 1:1, but the chip just refused to produce anything
> > meaningful.
>
> I still don't see any reason why S_FMT would suddenly crop on such a sensor.
> It's completely unexpected and the user does not get what he expects.
Good, let's make it simple for all (except Bastian) then: Bastian, sorry
for having misguided you, please, switch to .s_crop().
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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