Re: Gain controls in v4l2-ctrl framework

[Chris MacGregor <chris@cybermato.com>]

Hi, Hans.

On 09/24/2012 11:46 AM, Hans de Goede wrote:
> Hi,
>
> On 09/24/2012 07:17 PM, Chris MacGregor wrote:
>>
>> On 09/24/2012 07:42 AM, Prabhakar Lad wrote:
>>> Hi Hans,
>>>
>>> On Mon, Sep 24, 2012 at 4:25 PM, Hans de Goede <hdegoede@redhat.com> 
>>> wrote:
>>>> Hi,
>>>>
>>>>
>>>> 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.

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.

>
> Regards,
>
> Hans

Thanks,
     Chris