Re: [RFC] V4L2 API for flash devices

[Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>]

Sung Hee Park wrote:
> Here are two more use-cases for flash that might help inform the API design.
> Sakari encouraged me to post these. The person writing this is Andrew Adams,
> but I'm sending this from Sung Hee's account, because I only just subscribed
> to linux-media and can't immediately figure out how to reply to messages
> from before I subscribed. Sung Hee and I are both grad students at Stanford
> who work on FCam (http://fcam.garage.maemo.org/)

Hi Andrew,

Many thanks for the comments.

> Second-curtain-sync:
> 
> Sometimes you want to fire the flash at the end of a long exposure time.
> It's usually a way to depict motion. Here's an example:
> http://www.flickr.com/photos/latitudes/133206615/.
> 
> This only really applies to xenon flashes because you can't get a crisp
> image out of a longer duration LED flash. Even then it's somewhat
> problematic on rolling-shutter sensors but still possible provided you don't
> mind a slight shear to the crisp part of the image. From an API perspective,
> it requires you to be able to specify a trigger time at some number of
> microseconds into the exposure. On the N900 we did this with a real-time
> thread.
>
> Triggering external hardware:
> 
> This use-case is a little weirder, but it has the same API requirement. Some
> photographic setups require you to synchronize some piece of hardware with
> the exposure (e.g. an oscilloscope, or an external slave flash). On embedded
> devices this is generally difficult. If you can at least fire the flash at a
> precise delay into the exposure, you can attach a photodiode to it, and use
> the flash+photodiode as an opto-isolator to trigger your external hardware.
> 
> So we're in favor of having user-settable flash duration, and also
> user-settable trigger times (with reference to the start of the exposure
> time). I'm guessing that in a SMIA++ driver the trigger time would actually
> be a control on the sensor device, but it seemed relevant to bring up while
> you guys were talking about the flash API.

>From this I reckon that in a general case the handling of the flash
timing cannot be left for the drivers only. There must be a way to
control it.

So I'd think that also the level/edge trigger for the flash should be
visible. On edge trigger, the only limiting factor for the flash
duration on hardware would be the relatively coarse watchdog timer, and
I'd think the user should be able to know that.

The flash timing controls should be exposed by the sensor driver since
the sensor is what actually performs the timing.

Laurent; what do you think?

Cheers,

-- 
Sakari Ailus
sakari.ailus@maxwell.research.nokia.com