Re: Gain controls in v4l2-ctrl framework

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 09/24/2012 01:00 PM, Laurent Pinchart wrote:
> 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.

Hmm, so some have per color, some don't, so then we need:

V4L2_CID_GAIN_OFFSET
V4L2_CID_BLUE_OFFSET
V4L2_CID_RED_OFFSET
V4L2_CID_GREEN_OFFSET

Where GAIN_OFFSET is for the ones with just 1 offset register. Anyone have
a better name for that ?

Regards,

Hans