Re: [RFC/PATCH] v4l: Add V4L2_CID_FLASH_HW_STROBE_MODE control

[Sylwester Nawrocki <snjw23@gmail.com>]

Hi Sakari,

On 12/05/2011 11:41 PM, Sakari Ailus wrote:
> On Mon, Dec 05, 2011 at 08:56:46PM +0100, Sylwester Nawrocki wrote:
>> The V4L2_CID_FLASH_HW_STROBE_MODE mode control is intended
>> for devices that are source of an external flash strobe for flash
>> devices. This part seems to be missing in current Flash control
>> class, i.e. a means for configuring devices that are not camera
>> flash drivers but involve a flash related functionality.
>>
>> The V4L2_CID_FLASH_HW_STROBE_MODE control enables the user
>> to determine the flash control behavior, for instance, at an image
>> sensor device.
>>
>> The control has effect only when V4L2_CID_FLASH_STROBE_SOURCE control
>> is set to V4L2_FLASH_STROBE_SOURCE_EXTERNAL at a flash subdev, if
>> a flash subdev is present in the system.
>>
>> Signed-off-by: Sylwester Nawrocki <snjw23@gmail.com>
>> ---
>>
>> Hi Sakari,
>>
>> My apologies for not bringing this earlier when you were designing
>> the Flash control API.
>> It seems like a use case were a sensor controller drives a strobe
>> signal for a Flash and the sensor side requires some set up doesn't
>> quite fit in the Flash Control API.
>>
>> Or is there already a control allowing to set Flash strobe mode at
>> the sensor to: OFF, ON (per each exposed frame), AUTO ?

Thank you for the in-depth opinion (and sorry for the delayed response).

> The flash API defines the API for the flash, not for the sensor which might
> be controlling the flash through the hardware strobe pin. I left that out
> deliberately before I could see what kind of controls would be needed for
> that.
> 
> If I understand you correctly, this control is intended to configure the
> flash strobe per-frame? That may be somewhat hardware-dependent.

Yes, per captured frame. Actually the controls I proposed were meant to select
specific flash strobe algorithm. What refinements could be relevant for those
algorithms may be a different question. Something like the proposed controls
is really almost all that is offered by many of hardware we use.

> Some hardware is able to strobe the flash for the "next possible frame" or
> for the first frame when the streaming is started. In either of the cases,
> the frames before and after the one exposed with the flash typically are
> ruined because the flash has exposed only a part of them. You typically want
> to discard such frames.

Is this the case for Xenon flash as well, or LED only ?

I think the fact that we're using video capture like interface for still capture
adds complexity in such cases.

> The timing control of the flash strobe fully depends on the type of the
> flash: LED flash typically remains on for the whole duration of the frame
> exposure, whereas on xenon flash the full frame must be being exposed when
> the flash is being fired.

Indeed, I should have separated the LED and Xenon case in the first place.

Do you think we could start with separate menu controls for LED and Xenon
flash strobe, e.g.

V4L2_FLASH_LED_STROBE_MODE,
V4L2_FLASH_XENON_STROBE_MODE

and then think of what controls would be needed for each particular mode
under these menus ?

> Also different use cases may require different flash timing handling. [1]

I think we need to be able to specify flash strobe delay relative to exposure
start in absolute time and relative to exposure time units.

> 
> Some sensors have a synchronised electrical shutter (or what was it called,
> something like that anyway); that causes the exposure of all the lines of

I guess you mean two-curtain type shutter, like the one described here:

http://camerapedia.wikia.com/wiki/Focal_plane_shutter
http://www.photozone.de/hi-speed-flash-sync

> the sensor to stop at the same time. This effectively eliminates the rolling
> shutter effect. The user should know whether (s)he is using synchronised
> shutter or rolling shutter since that affects the timing a lot.
> 
> How the control of the hardware strobe should look like to the user?
> 
> I don't think the flash handling can be fully expressed by a single control
> --- except for end user applications. They very likely don't want to know
> about all the flash timing related details.

Agreed.

> 
> Are you able tell more about your use case? How about the sensor providing
> the hardware strobe signal?

As a light source a high intensity white LED is used. The LED current control
circuit is directly controlled by a sensor, let's say for simplicity through
one pin.
Now all the magic happens in the sensor firmware and the user can only select
flash programs, e.g. always on/off or auto. I've seen the front curtain and
rear curtain modes used here and there. As you may know these are used in
"slow sync" flash case, where the sensor is fired at the beginning or at the
end of long exposure period.

For example S5K6AA sensor provides following options in REG_TC_FLS_Mode
register for LED flash strobe:

0: TC_FLASH_DISABLE,
1: TC_FLASH_CONT_ENABLE,  // Always on
2: TC_FLASH_PULSE_ENABLE, // Use burst pulse on every capture
3: TC_FLASH_PULSE_AUTO    // Sensor controls the Flash status (burst mode)

For option 3 there is also a register:

REG_TC_FLS_Threshold - Set flash activation threshold in normalized
                       brightness units

For Xenon flash

REG_TC_FLS_XenonMode (Set Xenon flash mode):

0: TC_XENON_DISABLE,
1: TC_XENON_ONE_STROBE, // Use one strobe
2: TC_XENON_PRE_FLASH   // Use n strobes for pre-flash and another one, full

REG_TC_FLS_XenonPreFlashCnt - Number of Xenon pre-flash strobes


And this sensor has also register to trigger still (single- or multi-frame)
capture (REG_TC_GP_EnableCapture), i.e. switch from low resolution/high frame
rate operation to higher resolution image capture.


> [1] http://www.spinics.net/lists/linux-media/msg31363.html


--
Regards,
Sylwester