Mike Schilli
and 1 contributors


Video::Capture::V4l::Imager - Capture images from a video webcam


    use Video::Capture::V4l::Imager;

    my $vcap = Video::Capture::V4l::Imager->new(
        width  => 320,
        height => 240,

      # Adjust camera brightness if necessary
      # Capture an image, back comes an Imager object
    my $img = $vcap->capture();

      # Save it as JPEG
    $img->write(file => 'mycapture.jpg')
      or die "Can't write: $!";


Video::Capture::V4l::Imager captures still images from a USB video cam connected to your Linux box. The pictures come back as Imager objects (see the Imager module on CPAN for details) and can be easily manipulated and saved to disk.

Video::Capture::V4l::Imager is a convenience wrapper around Marc Lehmann's Video::Capture::V4l module. It uses the following procedure to obtain the still images from the video stream:


Capturing Images

To initialize V4l with your connected video camera, call the constructor with the width and height setting according to what's supported by the video camera:

    my $vcap = Video::Capture::V4l::Imager->new(
        width   => 320,
        height  => 240,

Note that this call will fail if your video camera doesn't support the specified width and height setting.

To capture a picture from the video stream, call


which will return an Imager object on success or undef otherwise.

Adjusting the Camera

To adjust the brightness setting of the camera, use


and pass the integer value of the desired brightness setting.

To calibrate the camera to obtain an image that has a given average brightness, use calibrate:

    my $vcap = Video::Capture::V4l::Imager->new(
        width             => 320,
        height            => 240,
        avg_optimal       => 128,
        avg_tolerance     => 20,
        brightness_min    => 0,
        brightness_max    => 65535,
        calibration_tries => 5,


This will start a simple binary search by taking a picture, checking its average brightness via img_avg() (see below) and adjusting the camera brightness to get to a desired optimal brightness avg_optimal. within a selectable tolerance avg_tolerance.

calibrate() returns the best camera brightness setting found.

The minimum and maximum settings for camera brightness (which depend on the camera used) are set in brightness_min and brightness_max.

The maximum number of test exposures is set in calibration_tries, after this number is reached, the camera brightness that worked best so far gets returned.

Utility functions

The following utility methods for Imager objects have been included in this module:


Calculates the average brightness of an image by adding up all channels of every pixel and then dividing it by 3 times the number of pixels. Handy to get a feel if an image is over- or underexposed, by a very crude measure.

$i->img_changed($other_img, $diff)>

Checks if two images are significantly different, used to tell if two consecutive still images taken from a video stream show that something's going on in the scene.

Calculates the difference in channel values of all pixels and compares it to the number passed in $diff. If the sum of all channel differences is greater than $diff, img_changed returns true and false otherwise.

Note that the counter will be increased by up to 3 points per pixel (red, green, blue channels), so factor that into $diff.


Note that this module contains inline C code and will take significantly longer to start up on the first run. It will speed up significantly on consecutive runs, because the object code will be preserved once it has been compiled.


Copyright 2007 by Mike Schilli, all rights reserved. This program is free software, you can redistribute it and/or modify it under the same terms as Perl itself.


2007, Mike Schilli <cpan@perlmeister.com>