Video4Linux: Accessing camera aperture and focus.
 

I am doing a feasibility study of an application on N900 that would set the value of aperture and leave the expousure and focuse on automatic settings, however, I need to retreive the focus distance (V4L2_CID_FOCUS_ABSOLUTE). I read through the video4Linux API and in section 1.9.6 Camera Control Reference found the following.


Exposure and aperture:

V4L2_CID_EXPOSURE_AUTO integer
Enables automatic adjustments of the exposure time and/or iris aperture. The effect of manual changes of the exposure time or iris aperture while these features are enabled is undefined, drivers should ignore such requests. Possible values are:

V4L2_EXPOSURE_AUTO Automatic exposure time, automatic iris aperture.
V4L2_EXPOSURE_MANUAL Manual exposure time, manual iris.
V4L2_EXPOSURE_SHUTTER_PRIORITY Manual exposure time, auto iris.
V4L2_EXPOSURE_APERTURE_PRIORITY Auto exposure time, manual iris.


Focus:

V4L2_CID_FOCUS_ABSOLUTE integer
This control sets the focal point of the camera to the specified position. The unit is undefined. Positive values set the focus closer to the camera, negative values towards infinity.

V4L2_CID_FOCUS_AUTO boolean
Enables automatic focus adjustments. The effect of manual focus adjustments while this feature is enabled is undefined, drivers should ignore such requests.

Using this values one could get the camera in the appropriate mode of operation and theoretically through the control class retrieve any of those values.

Will the V4L2_CID_FOCUS_ABSOLUTE value change as the camera tries to autofocus on an object? If not, is there any other value I missed that could be used for that. Where can I check if these controls are supported by the N900 hardware.

Thx for any help,

Klen

Re: Video4Linux: Accessing camera aperture and focus.
 

I don't think that camera of n900 has any aperture (except for the aperture of lens - f/2.8) - only way to control exposition is therefore changing shutter speed and iso... and i'm not completely sure ISO changing is hardware-supported either (i.e. whether it isn't simulated by simply multiplying pixel values)..

dunno about the focusing - I've thought the video app has to control this, but seeing the focus_auto option makes me wonder........ :)

there's a "mirror" app in repos - a simple python script showing how to handle v4l devices using python.. let's try hacking it to play with the back camera..

Re: Video4Linux: Accessing camera aperture and focus.
 

First of all, Thank you for your reply!

You are right about the aperture. I got confused by the way they wrote 2.8/5.2 on the camera lens.

The aperture is now irrelevant. I would only want to access the focal distance which I guess is stored in (V4L2_CID_FOCUS_ABSOLUTE).

Are there any examples of how to use Video4Linux API in maemo 5 application except for the camera_example? Is it possible to get the full source of a camera example?

Thx,
Klen

Re: Video4Linux: Accessing camera aperture and focus.
 

well, you could e.g. have a look on the source of the "mirror" app (in extras-devel).. or here: http://wiki.maemo.org/Documentation/...ging_subsystem

Re: Video4Linux: Accessing camera aperture and focus.
 

Thank you for your reply.

Unfortunately there are no source fails of the mirror app available in the extras-devel repo. One can only find files of the debian packaging.

Re: Video4Linux: Accessing camera aperture and focus.
 

You're not kidding. Furthermore, it's full of .svn files.

Ugh.

Re: Video4Linux: Accessing camera aperture and focus.
 

It's written in Python. So I guess one could say that the program itself is its source.

Re: Video4Linux: Accessing camera aperture and focus.
 

Ah that didn't occur to me. I just saw the /opt and /usr directories and assumed it was precompiled.

Still a nasty mess with the subversion protected files everywhere though.

Re: Video4Linux: Accessing camera aperture and focus.
 

Wha' ? Who ? Where ? There was a version in testing that accidentally got packaged with .svn stuff but it was fixed, like, months ago. The current version (2.1-1, that is in Extras) certainly does not have subversion files. Be sure to check your version.

Also, Mirror uses gstreamer for the video operations, nothing really low level so it's (from a hacking standpoint) almost the same as if you experimented with gst-launch from the command line (which is actually how Mirror came to life in the first place :) )

Re: Video4Linux: Accessing camera aperture and focus.
 

Look at the source of v4l2ucp http://sourceforge.net/projects/v4l2ucp/. Especially the file v4l2ctrl.c. That can at least access the focus and shutter speed. However, I wanted to use that for bracketing shutterspeed (for making HDR pictures) but for some reason I cant get it to work.


Edit: Added link.