Re: Gain controls in v4l2-ctrl framework

[Chris MacGregor <chris@cybermato.com>]

Hi.

On 09/26/2012 12:42 AM, Sakari Ailus wrote:
> On Wed, Sep 26, 2012 at 12:14:36PM +0530, Prabhakar Lad wrote:
>> Hi All,
>>
>> On Sun, Sep 23, 2012 at 4:56 PM, Prabhakar Lad
>> <prabhakar.csengg@gmail.com> 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
>>> 4: V4L2_CID_GAIN_BLUE
>>> 5: V4L2_CID_GAIN_OFFSET
>>>
>>> I need your opinion's to get moving to add them.
>>>
>> I am listing out the gain controls which is the outcome of above discussion:-
>>
>> 1: V4L2_CID_GAIN_RED
>> 2: V4L2_CID_GAIN_GREEN_RED
>> 3: V4L2_CID_GAIN_GREEN_BLUE
>> 4: V4L2_CID_GAIN_BLUE
>> 5: V4L2_CID_GAIN_OFFSET
>> 6: V4L2_CID_BLUE_OFFSET
>> 7: V4L2_CID_RED_OFFSET
>> 8: V4L2_CID_GREEN_OFFSET
> Hi Prabhakar,
>
> As these are low level controls, I wonder whether it would make sense to
> make a difference between digital and analogue gain. I admit I'm not quite
> as certain whether there's such a large difference as there is for global
> gains for the camera control algorithms.

Sorry to make this more complicated, but the Aptina MT9P031, for 
instance (datasheet at 
http://www.aptina.com/assets/downloadDocument.do?id=865 - see page 35), 
has Digital Gain, an Analog Multiplier, and Analog Gain (for each of R, 
Gr, Gb, and B). For each color channel, there is one register, with the 
bits divided up into the three gain types. Furthermore, the different 
gain types have different units (increments).

Currently (at least in the last version I've used), the driver hides all 
this and provides a single gain control, and prioritizes which gain 
types are adjusted at different user-level gain settings in accordance 
with the datasheet recommendations (e.g. keep the analog gain between 1 
and 4 for best noise performance, and use the multiplier for gains 
between 4 and 8). This seems very sensible. If we try to distinguish 
between analog and digital gains in the control definitions, what should 
this driver do? And what about the multiplier? I suppose it could be 
"hidden" by the driver as part of the analog gain, as the driver 
currently does for the entire gain...

Cheers,
Chris