Extracting images from a camera in real-time
Most of the of the Canon A series cameras are able to run the free and stunningly capable  CHDK - Canon Hack Development Kit and also here. 


CHDK allows you to communicate via USB, control the camera's every know feature and add extra features. eg you can set up motion sensing of a selected area with action of choice to follow even though this facility is probably available as std on none of the cameras concerned.


CHDK is installed on an SD card which is inderted in the camera and the camera loads and runs it along with any "program" files at power up. The camera is not altered. I understand that the latest Canon A series camera are not compatible, but the project is ongoing and capable.

Overview here  



They say:

Enhanced Image Capture: CHDK supports RAW format for pictures (in addition to JPEG), longer recording time and length for videos (up to 1 hour or 2GBs), and several new compression options.

Additional Information On-Screen: With CHDK, you can customize your on-screen display to your heart's content, including worthwhile information like remaining battery life, histogram, depth of focus, and more.

Additional Photographic Settings: CHDK takes many of the features already available on your camera and gives you way more options, including longer exposure times (up to 65 seconds), faster shutter speeds (1/25,000 sec or faster), automatic bracketing of your photos, and more.

Support for Small Programs/Scripts: CHDK can run small scripts that will allow your camera to perform a set of actions based on the conditions of the script. Tons of scripts are available, and they provide functionality like motion-sensing photography (which reportedly works for lightning strikes) and unlimited interval time-lapse photography (see video below).
Click to view

Even better, CHDK is being actively developed, which means that new features are always on the horizon. There are actually several versions of CHDK in development, but the version we're going to focus on is called the Allbest build—a build that incorporates many of the best features available in other builds (get it?).

What You'll Need

To use CHDK, all you need is a supported Canon digital camera and an SD memory card. If you've already got a supported camera then it only follows that you've got the memory card, so you're already well on your way.