Re: Gain controls in v4l2-ctrl framework

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

Hi Hans,

On Monday 24 September 2012 12:55:53 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
> 
> 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 ?
> 
> 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 ...

I've never had to set different gains for the two green components either, 
although I haven't done much with them.

> > 4: V4L2_CID_GAIN_BLUE
> > 5: V4L2_CID_GAIN_OFFSET
> 
> GAIN_OFFSET that sounds a bit weird... GAIN_OFFSET sounds like it is
> a number which gets added to the 3/4 gain settings before the gain gets
> applied, but I assume that you just mean a number which gets added to the
> value from the pixel, either before or after the gain is applied and I must
> admit I cannot come up with a better name.
> 
> I believe (not sure) that some sensors have these per color ...

Some might at least.

> The question is if it makes sense to actually control this per color though,
> I don't think it does as it is meant to compensate for any fixed measuring
> errors, which are the same for all 3/4 colors.

The offset is usually applied after the gain, so you might need different 
offsets to compensate for a fixed error that is multiplied by different gains.

> Note that all the sensor cells are exactly the same, later on a color grid
> gets added on top of the sensors to turn them into r/g/b cells, but
> physically they are the same cells, so with the same process and temperature
> caused measuring errors...

-- 
Regards,

Laurent Pinchart