Re: Gain controls in v4l2-ctrl framework

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Monday 24 September 2012 12:16:01 Chris MacGregor wrote:
> On 09/24/2012 11:46 AM, Hans de Goede wrote:
> > On 09/24/2012 07:17 PM, Chris MacGregor wrote:
> >> On 09/24/2012 07:42 AM, Prabhakar Lad wrote:
> >>> On Mon, Sep 24, 2012 at 4:25 PM, Hans de Goede wrote:
> >>>> On 09/23/2012 01:26 PM, Prabhakar Lad wrote:
> >>>>> Hi All,
> >>>>> 
> >>>>> The CCD/Sensors have the capability to adjust the R/ye, Gr/Cy, Gb/G,
> >>>>> B/Mg gain values.
> >>>>> Since these control can be re-usable I am planning to add the
> >>>>> following gain controls as part of the framework:
> >>>>> 
> >>>>> 1: V4L2_CID_GAIN_RED
> >>>>> 2: V4L2_CID_GAIN_GREEN_RED
> >>>>> 3: V4L2_CID_GAIN_GREEN_BLUE
> >>>> 
> >>>> Not all sensors have separate V4L2_CID_GAIN_GREEN_RED /
> >>>> V4L2_CID_GAIN_GREEN_BLUE, so we will need a separate control for
> >>>> sensors which have one combined gain called simply V4L2_CID_GAIN_GREEN
> >>> 
> >>> Agreed
> >>> 
> >>>> Also do we really need separate V4L2_CID_GAIN_GREEN_RED /
> >>>> V4L2_CID_GAIN_GREEN_BLUE controls? I know hardware has them, but in my
> >>>> experience that is only done as it is simpler to make the hardware this
> >>>> way (fully symmetric sensor grid), have you ever tried actually using
> >>>> different gain settings for the 2 different green rows?
> >>> 
> >>> Never tried it.
> >>> 
> >>>> I've and that always results in an ugly checker board pattern. So I
> >>>> think we can and should only have a V4L2_CID_GAIN_GREEN, and for
> >>>> sensors with 2 green gains have that control both, forcing both to
> >>>> always have the same setting, which is really what you want anyways ...
> >>> 
> >>> Agreed.
> >> 
> >> Please don't do this. I am working with the MT9P031, which has separate
> >> gains, and as we are using the color version of the sensor (which we can
> >> get much more cheaply) with infrared illumination, we correct for the
> >> slightly different response levels of the different color channels by
> >> adjusting the individual gain controls.
> > 
> > Ok, sofar I'm following you, but are you saying that the correction
> > you need to apply for the green pixels on the same row as red pixels,
> > is different then the one for the green pixels on the same row as blue
> > pixels ?
> 
> IIRC, when we were calibrating, the two greens were at some times
> different.  The gain settings we're using at the moment are in fact the
> same for both greens - we had to compromise to avoid getting into higher
> values that increase the noise more than we like - but I don't know that
> it would be that way in the future.
> 
> > I can understand that the green "lenses" let through a different
> > amount of infrared light then sat the red lenses, but is there any
> > (significant) differences between the green lenses on 2 different rows?
> 
> I don't have time right now to dig out the datasheet and re-read it, but
> IIRC the auto-BLC (for instance) is row-wise, and consequently the
> greens could actually need slightly different values.  I think the
> datasheet made it fairly clear that the motivation for including
> separate green controls was because you might need them in practice, not
> because the hardware guys were punting the problem over to the software
> side.
> 
> >   (I have patches to add the controls, but I haven't had time yet to
> > 
> > get them into good enough shape to submit - sorry!)
> > 
> >> It seems to me that for applications that want to set them to the
> >> same value (presumably the vast majority), it is not so hard to set
> >> both the green_red and green_blue.  If you implement a single
> >> control, what happens for the (admittedly rare) application that
> >> needs to control them separately?
> > 
> > Well if these are showing up in something like a user oriented
> > control-panel (which they may) then having one slider for both
> > certainly is more userfriendly.

Those gains are low-level controls, they will very likely not appear in a 
generic panel.

> Okay, that's a fair point.  But an application that wanted to could insulate
> the user from it fairly easily.
> 
> I'm not opposed to having a single control, *if* there is some way for apps
> to control the greens separately when they need to. I don't have a brilliant
> solution for this offhand, other than just exposing the separate controls.

As you have a use case for separate controls I'd vote for having separate 
controls.

-- 
Regards,

Laurent Pinchart