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.

Re: Video4Linux: Accessing camera aperture and focus.
 

Sorry Attila, i was just checking to see that the source wasn't in the source package as he had said, and typed "apt-get source mirror" in the SDK, which most likely grabbed the -devel version.

Re: Video4Linux: Accessing camera aperture and focus.
 

Silly mistake. I missed the part that I am looking for phython script and was searching for "src" dir with ".c" and ".h" files instead. The sources are there, but are not the way forward for me.

The V4l2ucp looks much more promising.

Thx

Re: Video4Linux: Accessing camera aperture and focus.
 

Yeah,
Atleast I can control the focus with the ioctls. Dont know the units though. Something in the range of 550 seams to be focusing very close, whereas something like 150 means infinite.

But when I try to set V4L2_CID_EXPOSURE nothing happens. I tried fiddling with the V4L2_CID_EXPOSURE_AUTO control as you suggested but the driver doesnt seem to support this.

EDIT: Wait, now it works. Strange

Re: Video4Linux: Accessing camera aperture and focus.
 

Davost, thank you very much for your post. I am finally getting somwhere.

Now I also manage to gain access to the V4L2_CID_FOCUS_ABSOLUTE value and can manually control the focus of the camera.

The aim of my application is to only access the value of V4L2_CID_FOCUS_ABSOLUTE that is set by the auto focusing feature. If I use native camera app to focus on different objects, V4L2_CID_FOCUS_ABSOLUTE parameter changes.

Now I would like to write my own application that auto focuses on an object like the native camera. I managed to open a GStreamer pipeline, but can not activate auto focusing option.

The Question:
Does the native camera app implement the auto focusing algorithms, or, there is a library or hardware option that enables auto focus functionality?

Bellow are all the values that are supported by the N900 main camera.

ID: Control Name: Value
9963776: Brightness:0
9963777: Contrast:16
9963793: Exposure time [us]:33000
9963795: Gain [0.1 EV]:28
9963807: Color Effects:0
10094858: Focus, Absolute:105
10094862: Flash timeout [us]:1000
10094863: Flash intensity:12
10094864: Torch intensity:0
10094865: Indicator intensity:0
10096766: Test pattern mode:0
10096815: Focus ramping time [us]:3200
10096816: Focus ramping mode:2
10096881: Short-circuit fault:0
10096882: Overtemperature fault:0
10096883: Timeout fault:0
10096884: Overvoltage fault:0
10160385: Frame width:1593
10160386: Frame height:504
10160387: Visible width:684
10160388: Visible height:492
10160389: Pixel clock [Hz]:96533333
10160390: Sensitivity:65536

Re: Video4Linux: Accessing camera aperture and focus.
 

That would be the camera daemon. As per the dev guide:

Unfortunately, that's a License: Nokia thing

Re: Video4Linux: Accessing camera aperture and focus.
 

I'm following a very similar path as you are. My purpose is to make HDR pics. When I was researching this I finally found out that the mbarcode does autofocus using the gstreamer photography api. I guess this is also the proper way to do exposure control.

Example on how to set up the pipe line and do autofocus in:
maemo-barcode/trunk/maemo-barcode/camera_n900.c

Re: Video4Linux: Accessing camera aperture and focus.
 

Yes, gstreamer is the proper way, but there are a few things that are supported on lower levels but are not implemented in gstreamer. Apparently, they take patches :)

Re: Video4Linux: Accessing camera aperture and focus.
 

Great stuff guys!

Now I have all pieces of the puzzle for my app :).

Many thanks to you and the rest of the community.

Cheers,
Klen

Re: Video4Linux: Accessing camera aperture and focus.
 

¿Do you know how to obtain the focal distance to the objetive? or in other case, how can be changed the zone wheres is the autofocus applied? For example, If I want to focus one object in the corner that isn't in the center or isn't the nearest object, how can we do that?

Thanks in advance!

Re: Video4Linux: Accessing camera aperture and focus.
 

I've searched on the web, but I can't seem to find any existing free software that could be used as a base for contrast-based autodetection.

Basically, if you can detect the total contrast of a scene, you play with the focus until you get the contrast to a maximum. That is your focused point. you could do selective focus by using a bounding box around the area you wish to focus...

Does anyone know of a library that supports getting the overall image contrast level?

Re: Video4Linux: Accessing camera aperture and focus.
 

Why do you want to do that? You can always point the camera to the object you want to have in focus and then move it back to the position where you where before to take the shot.

Alternative would be to do manual focusing, you can do that through v4l2 driver API.

I noticed that the N900 native camera app does much better auto-focusing compared to the Gstreamer "photography.h" auto-focusing functionality? Does anybody know how they do it?

Cheers,
Klen

Re: Video4Linux: Accessing camera aperture and focus.
 

I would also like to know that.

:) Klen

Re: Video4Linux: Accessing camera aperture and focus.
 

I have never implemented the contrast based autofocusing, but I don't think the overall image contrast level would help here, I would filter out the low frequencies and maybe also some amount of high, to avoid detection of sensor noise.

You would also like not to process the whole image but limit to the smaller area or number of areas.

So the simple version may look like:

total contrast of some area = sum( ( very slightly blurred or denoised image - more blurred image )^2 )

Re: Video4Linux: Accessing camera aperture and focus.
 

I was just reading through some documentation on the camera of the N900 and it said the I2C command is used to adjust the delay of data and strobe signal in CCP2 to compensate skew between data and strobe.

Can someone explain what this means please :p thanks