Our description of the typical machine vision system in the preceding section identified three functions: sensing and digitizing, image processing and analysis, and application.

This and the following sections in the chapter will elaborate on these functions. The present section is concerned with the sensing and digitizing aspects of machine vision.

Image sensing requires some type of image formation device such as a camera and a digitizer which stores a video frame in the computer memory.

We divide the sensing and digitizing functions into several steps. The initial step involves capturing the image of the scene with the vision camera.

The image consists of relative light intensities corresponding to the various portions of the scene. These light intensities are continuous analog values which must be sampled and converted into a digital form.

The second step, digitizing, is achieved by an analog-to-digital (A/D) converter. The A/D converter is either a part of a digital video camera or the front end of a frame grabber.

The choice is dependent on the type of hardware in the system. The frame grabber, representing the third step, is an image storage and computation device which stores a given pixel array.

The frame grabber can vary in capability from one which simply stores an image to significant computation capability.

In the more powerful frame grabbers, thresholding, windowing, and histogram modification calculations can be carried out under computer control.

The stored image is then subsequently processed and analyzed by the combination of the frame grabber and the vision controller.