Re: [PATCH 10/15] V4L: Add camera 3A lock control

[Sylwester Nawrocki <snjw23@gmail.com>]

Moikka Sakari,

On 04/23/2012 07:47 AM, Sakari Ailus wrote:
> Sylwester Nawrocki wrote:
>> On 04/17/2012 06:09 PM, Sakari Ailus wrote:
>>> On Tue, Apr 17, 2012 at 12:09:51PM +0200, Sylwester Nawrocki wrote:
>>>> The V4L2_CID_3A_LOCK bitmask control allows applications to pause
>>>> or resume the automatic exposure, focus and wite balance adjustments.
>>>> It can be used, for example, to lock the 3A adjustments right before
>>>> a still image is captured, for pre-focus, etc.
>>>> The applications can control each of the algorithms independently,
>>>> through a corresponding control bit, if driver allows that.
>>>
>>> How is disabling e.g. focus algorithm different from locking focus?
>>
>> The difference looks quite obvious to me. When some AUTO control is
>> switched from auto to manual mode there is no guarantee about the
>> related parameters the device will end up. E.g. lens may be positioned
>> into default position, rather than kept at current one, exposure might
>> be set to manual value from before AE was enabled, etc.
>>
>> I've seen separate registers at the sensor interfaces for AE, AWB
>> locking/unlocking and for disabling/enabling those algorithms.
>> With the proposed control applications can be sure that, for example,
>> exposure is retained when the V4L2_CID_3A_LOCK is set.
>>
>> Does it answer your question ?
> 
> Yes, it does.
> 
> I was thinking how does the situation really differ from disabling the
> corresponding automatic algorithm. There may be subtle differences in
> practice albeit in principle the two are no different. And if some of the
> sensors implement it as lock, then I guess it gives us few options for the
> user space interface.

Can you anticipate any any possible issues such diversity might bring to
applications ? I imagine such control can be quite useful for snapshot,
and with current control API design and the drivers' behaviour applications 
cannot be sure what settings a device ends up with after switching from
"auto" to "manual" - last auto settings or the manual values. Usually its
just the previous manual values.

Such a bitmask control looks quite useful to me. Moreover, at the moment 
there is no control that would provide similar functionality for auto focus.
The bitmask control allows to easily control auto exposure, wb and focus
atomically. However that's not a big deal, since this could be well achieved
with the extended control API.

Although V4L2_CID_3A_LOCK might be hard to implement in driver which use
the control framework, since it would depend on multiple other controls.
But this could be worked around by updating proper control current values
and sending a control event from driver manually, unless you want to cluster
almost all controls.

--

Regards,
Sylwester