Video monitoring with Perl and a webcam
FISHING FOR IMAGES
Webcams are useful for monitoring. Using CPAN modules and custom-made C code, Perl scripts can automatically adjust the exposure and fish the most interesting images out of a video camera’s data stream.
Most webcams are accompanied by some kind of Windows software, which is of no use at all to Linux users. Luckily, more recent Linux distributions include Video4Linux, which gives you simple controls for many hot-pluggable USB cameras. The Creative NX Ultra camera I use for my experiments normally provides video output and costs about US$ 70. In fact, it is a waste of the camera’s abilities to use this camera as a simple webcam, but then again, it was just lying around in one of the drawers down at the Perlmeister lab, so why not use it? The camera does not need an external power supply, and it was detected immediately by the hot-plugging subsystem when I connected it to my PC. The video data typically goes to /dev/video0. The Linux::Capture::V4l Perl module from CPAN locks into the device entry, grabs the frame data, and lets programmers change exposure parameters such as the camera sensitivity on the fly. Listing 1 shows a simple application that first sets the camera sensitivity to 40000 before grabbing an image from the video stream, and then storing the image on disk as a JPEG photo (Figure 2).
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