Re: [RFC PATCH 1/4] v4l: Add V4L2_CID_PRESET_WHITE_BALANCE menu control

[Sylwester Nawrocki <snjw23@gmail.com>]

Hi Sakari,

On 12/30/2011 12:34 AM, Sakari Ailus wrote:
> On Wed, Dec 28, 2011 at 02:51:38PM +0100, Laurent Pinchart wrote:
>> On Wednesday 28 December 2011 14:35:00 Sylwester Nawrocki wrote:
>>> On 12/28/2011 07:23 AM, HeungJun, Kim wrote:
>>>> It adds the new CID for setting White Balance Preset. This CID is
>>>> provided as menu type using the following items:
>>>> 0 - V4L2_WHITE_BALANCE_INCANDESCENT,
>>>> 1 - V4L2_WHITE_BALANCE_FLUORESCENT,
>>>> 2 - V4L2_WHITE_BALANCE_DAYLIGHT,
>>>> 3 - V4L2_WHITE_BALANCE_CLOUDY,
>>>> 4 - V4L2_WHITE_BALANCE_SHADE,
>>>
>>> I have been also investigating those white balance presets recently and
>>> noticed they're also needed for the pwc driver. Looking at
>>> drivers/media/video/pwc/pwc-v4l2.c there is something like:
>>>
>>> const char * const pwc_auto_whitebal_qmenu[] = {
>>> 	"Indoor (Incandescant Lighting) Mode",
>>> 	"Outdoor (Sunlight) Mode",
>>> 	"Indoor (Fluorescent Lighting) Mode",
>>> 	"Manual Mode",
>>> 	"Auto Mode",
>>> 	NULL
>>> };
>>>
>>> static const struct v4l2_ctrl_config pwc_auto_white_balance_cfg = {
>>> 	.ops	= &pwc_ctrl_ops,
>>> 	.id	= V4L2_CID_AUTO_WHITE_BALANCE,
>>> 	.type	= V4L2_CTRL_TYPE_MENU,
>>> 	.max	= awb_auto,
>>> 	.qmenu	= pwc_auto_whitebal_qmenu,
>>> };
>>>
>>> ...
>>>
>>> 	cfg = pwc_auto_white_balance_cfg;
>>> 	cfg.name = v4l2_ctrl_get_name(cfg.id);
>>> 	cfg.def = def;
>>> 	pdev->auto_white_balance = v4l2_ctrl_new_custom(hdl, &cfg, NULL);
>>>
>>> So this driver re-defines V4L2_CID_AUTO_WHITE_BALANCE as a menu control
>>> with custom entries. That's interesting... However it works in practice
>>> and applications have access to what's provided by hardware.
>>> Perhaps V4L2_CID_AUTO_WHITE_BALANCE_TEMPERATURE would be a better fit for
>>> that :)
>>>
>>> Nevertheless, redefining standard controls in particular drivers sounds
>>> a little dubious. I wonder if this is a generally agreed approach ?
>>
>> No agreed with me at least :-)
>>
>>> Then, how does your V4L2_CID_PRESET_WHITE_BALANCE control interact with
>>> V4L2_CID_AUTO_WHITE_BALANCE control ? Does V4L2_CID_AUTO_WHITE_BALANCE need
>>> to be set to false for V4L2_CID_PRESET_WHITE_BALANCE to be effective ?
>>
>> Is the preset a fixed white balance setting, or is it an auto white balance 
>> with the algorithm tuned for a particular configuration ? In the first case, 
>> does it correspond to a fixed white balance temperature value ?
> 
> While I'm waiting for a final answer to this, I guess it's the second. There
> are three things involved here:
> 
> - V4L2_CID_WHITE_BALANCE_TEMPERATURE: relatively low level control telling
>   the colour temperature of the light source. Setting a value for this
>   essentially means using manual white balance.
> 
> - V4L2_CID_AUTO_WHITE_BALANCE: automatic white balance enabled or disabled.

Was the third thing the V4L2_CID_DO_WHITE_BALANCE control that you wanted to
say ? It's also quite essential functionality, to be able to fix white balance
after pointing camera to a white object. And I would expect
V4L2_CID_WHITE_BALANCE_PRESET control's documentation to state how an
interaction with V4L2_CID_DO_WHITE_BALANCE looks like.

> The new control proposed by HeungJun is input for the automatic white
> balance algorithm unless I'm mistaken. Whether or not the value is static,
> however, might be considered of secondary importance: it is a name instead
> of a number and clearly intended to be used as a high level control. I'd
> still expect it to be a hint for the algorithm.
> 
> The value of the new control would have an effect as long as automatic white
> balance is enabled.

The idea to treat the preset as a hint to the algorithm is interesting, however
as it turns out this are just static values (R/B balance) in manual WB mode.

I expect some parameters for adjusting auto WB algorithm (WB (R/G/B) gain bias
or something similar) to be present in sensor's ISP as well. If I remember well
I've seen something like this in one of sensor's documentations.

>>>> diff --git a/Documentation/DocBook/media/v4l/controls.xml
>>>> b/Documentation/DocBook/media/v4l/controls.xml index c0422c6..350c138
>>>> 100644
>>>> --- a/Documentation/DocBook/media/v4l/controls.xml
>>>> +++ b/Documentation/DocBook/media/v4l/controls.xml
>>>> @@ -2841,6 +2841,44 @@ it one step further. This is a write-only
>>>> control.</entry>
>>>>
>>>>  	  </row>
>>>>  	  <row><entry></entry></row>
>>>>
>>>> +	  <row id="v4l2-preset-white-balance">
>>>> +	    <entry
>>>> spanname="id"><constant>V4L2_CID_PRESET_WHITE_BALANCE</constant>&nbsp;</
>>>> entry>
>>>
>>> Wouldn't V4L2_CID_WHITE_BALANCE_PRESET be better ?
>>
>> That's what I was about to say.
> 
> And the menu items would contain the same prefix with CID_ removed. They're
> going to be long, but I don't see that as an issue for menu items.

Should we call it V4L2_CID_WB_PRESET then ?

Anyway V4L2_WHITE_BALANCE_PRESET_INCADESCENT for example is not that long,
we have control names that almost reach 80 characters :)

--

Regards,
Sylwester