[pvrusb2] Misplaced Filter-Controls?

[Hans Verkuil]

> Hi Mike,
>
> Am Sun, 22 Feb 2009 19:13:07 -0600 (CST)
> schrieb Mike Isely <isely at isely.net>:
>
>> Sorry about the delay.  I have been out of town and otherwise
>> distracted on other stuff.  Regarding your question..
>>
>
> No problem. I'm currenty also having some trouble with my back. (Hope
> you guys out there have a proper chair for sitting in front of the
> computer and also some outdoor-activity. Otherwise you'll earn a lot of
> pain...)
>
>> The filter controls you are looking at *are* actually part of the
>> mpeg encoder.  The same module in v4l-dvb which deals with the
>> encoder deals with these controls.
>>
>> I do understand what you are saying, but it is this way because (a)
>> the only thing that implements these controls are devices with an
>> mpeg encoder, and (b) all of those filter controls weren't even
>> conceived of in v4l until drivers came along that supported the
>> encoder.  Thus they got added later and are not a part of the "core"
>> set of video capture controls.
>>
>> This is one case where the implementation is affecting (somewhat) the
>> interface.  I generally don't like that sort of situation in
>> principle. I am a big proponent of careful partitioning of
>> implementation apart from interface (probably, some would say, to a
>> fault).  But you are also seeing an effect of history within the V4L
>> spec itself, since these controls did not even come into anyone's
>> thought process until the mpeg encoder chip showed up.
>>
>> As for the mpeg contorls being independent of channels, that's not my
>> call.  As far as a driver like pvrusb2 is concerned it's ALL
>> independent of channel mappings.  The 'illusion' that certain things
>> are specific to a channel and certain things are not is something
>> completely up to the app (e.g. mythtv).  In cases like that, the app
>> is going to be storing the settings on your behalf in a per-channel
>> app-specific data structure and swapping them in/out as you change
>> channels.  Filter settings should definitely be a per-channel thing
>> since the optimal settings are going to depend on the S/N ratio of
>> the signal.  The fact that they might be associated with mpeg
>> settings might be unfortunate, but there's no reason why an app
>> couldn't just swap in/out just the filter settings. The driver after
>> all is not going to care how the app chooses when/if to adjust the
>> various controls.
>>
>
> I'm not asking you to change the driver just to match my app... :) The
> channel- vs. global-profile just shows up the problem here. Of course I
> can hard-code the specific filters to the channels-profile but this
> would somehow defeat the generic control interface of v4l2 and lead to a
> solution where additional features of other devices aren't supported
> (like the non-existent filter-support in mythtv)
>
> Another test-case is probably the forthcoming raw-image-data support.
> If the pvrusb2 is used like a budget-card w/o an encoder by any TV-app,
> it would typically not show any MPEG-controls but user-controls. Here
> again, filters are completely inaccessible.

These filters are all MPEG specific and do not apply to raw video. That's
also the reason why they are all in the MPEG class. Typically you would
want a separate tab page for the MPEG controls since it is usually for
advanced users who know a bit about MPEG.

Regarding the temporal control: if I remember correctly it is set by
default to 8 for full-size PAL or NTSC images and to 0 for scaled images.
The encoder doesn't do a very good job when combining the temporal filter
with scaled images for some reason. I think the only time one might need
to change this filter is when the signal quality is pretty bad.

Regards,

      Hans

> But I'm still unsure about this... When thinking about it I find
> reasons both to put it in the MPEG- and the user-class :/
>
> However, don't bother. If you have a nice idea, let me know. :)
>
> Cheers,
> Carsten
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-- 
Hans Verkuil - video4linux developer - sponsored by TANDBERG