On Tue, 30 Aug 2011, Bastian Hecht wrote:
> 2011/8/30 Guennadi Liakhovetski <g.liakhovetski@xxxxxx>:
> > 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().
>
> Sure, no problem. So s_fmt() shall be not available at all then,
> right? Instead the cropping rectangle can be changed and the output
> rectangle is adjusted accordingly.
I would keep .s_fmt() and just return the current configuration from it.
Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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