linux-kernel - Re: camera application for testing (was Re: v4l subdevs without big device)

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Message-ID: <20160501140831.GH26360@valkosipuli.retiisi.org.uk>
Date:	Sun, 1 May 2016 17:08:31 +0300
From:	Sakari Ailus <sakari.ailus@....fi>
To:	Pavel Machek <pavel@....cz>
Cc:	Hans Verkuil <hverkuil@...all.nl>, pali.rohar@...il.com,
	sre@...nel.org, kernel list <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux-omap@...r.kernel.org, tony@...mide.com, khilman@...nel.org,
	aaro.koskinen@....fi, ivo.g.dimitrov.75@...il.com,
	patrikbachan@...il.com, serge@...lyn.com, tuukkat76@...il.com,
	mchehab@....samsung.com, linux-media@...r.kernel.org,
	laurent.pinchart@...asonboard.com
Subject: Re: camera application for testing (was Re: v4l subdevs without big
 device)

Hi Pavel,

On Sat, Apr 30, 2016 at 12:13:59AM +0200, Pavel Machek wrote:
> Hi!
> 
> What is reasonable camera application for testing?
> 
> N900 looks like a low-end digital camera. I have now have the hardware
> working (can set focus to X cm using command line), but that's not
> going to be useful for taking photos.

I guess you already knew about omap3camd; it's proprietary but from purely
practical point of view it'd be an option to support taking photos on the
N900. That would not be extensible any way, the best possible functionality
is limited what the daemon implements.

I'm just mentioning the option of implementing wrapper for the omap3camd so
that it can work with upsteam APIs, I don't propose that however.

> 
> In particular, who is going to do computation neccessary for
> autofocus, whitebalance and exposure/gain?

I think libv4l itself has algorithms to control at least some of these. It
relies on the image data so the CPU time consumption will be high.

AFAIR Laurent has also worked on implementing some algorithms that use the
histogram and some of the statistics. Add him to cc list.

> 
> There's http://fcam.garage.maemo.org/gettingStarted.html that should
> work on maemo, but a) it is not in Debian, b) it has non-trivial
> dependencies and c) will be a lot of fun to get working...
> 
> (and d), will not be too useful, anyway, due to 1sec shutter lag:

I believe this will be shorter nowadays. I don't remember the exact
technical solution which the text below refers to but I'm pretty sure it'll
be better with the current upstream. API-wise, there's work to be done there
(to port FCAM to upsteram APIs) but it's a possibility.

> 
> Fast resolution switching (less shutter lag)
> FCam is built on top of V4L2, which doesn't handle rapidly varying
> resolutions. When a Shot with a different resolution to the previous
> one comes down the pipeline, FCam currently flushes the entire V4L2
> pipeline, shuts down and restarts the whole camera subsystem, then
> starts streaming at the new resolution. This takes a long time (nearly
> a second), and is the cause of the horrible shutter lag on the N900. A
> brave kernel hacker may be able to reduce this time by fiddling with
> the FCam ISP kernel modules and the guts of the FCam library source
> (primarily Daemon.cpp).
> Anyone who solves this one will have our undying gratitude. An ideal
> solution would be able to insert a 5MP capture into a stream of
> 640x480 frames running at 30fps, without skipping more than the frame
> time of the 5MP capture. That is, the viewfinder would effectively
> stay live while taking a photograph.
> 
> )
> 
> Any other application I should look at? Thanks,

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk