2011/8/30 Guennadi Liakhovetski <g.liakhovet...@gmx.de>:
> 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.
best regards,
Bastian
> Thanks
> Guennadi
> ---
> Guennadi Liakhovetski, Ph.D.
> Freelance Open-Source Software Developer
> http://www.open-technology.de/
>
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