Re: Gain controls in v4l2-ctrl framework

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

Hi Chris,

On Wednesday 26 September 2012 07:42:41 Chris MacGregor wrote:
> On 09/26/2012 12:42 AM, Sakari Ailus wrote:
> > On Wed, Sep 26, 2012 at 12:14:36PM +0530, Prabhakar Lad wrote:
> >> On Sun, Sep 23, 2012 at 4:56 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
> >>> 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.

I think it should be fine for now. If we later find out that a user space 
application really needs to control the analog and digital gains individually 
and precisely we can always split the controls then. For now I think a single 
gain control (per channel) that groups analog and digital gains should be 
enough.

> 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...

-- 
Regards,

Laurent Pinchart