Operations can include addition and subtraction, as well as logical operations either between images or between an image and a constant value. Image arithmetic is a fundamental building block in many vision systems. In particular, image subtraction is often used to highlight differences bet ween images. The examples show how image subtraction can be used to compare an input image with a known good template.
These are the components being checked. On the left is the good component, and on the right is the defective part. To make the image easier to process, the part is illuminated with a backlight to remove any unwanted image information. The defective part can then be easily recog nised by the software.
Another common implementation of image arithmetic is flat-field correction. This technique compensates for the effect of uneven lighting, or it can be used to remove lens vignetting effects. A number of reference images are captured with a blank background and any subsequent image is adjusted to compensate for the underlying illumination error by applying flat-field correction.
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