This interactive tutorial illustrates a realistic application of cross correlation to locating approximate matches to a complex target in an image. The tutorial shows an image of bubbles, with illumination from one side that produces bright and dark regions that make automatic thresholding to detect the features difficult. Cross correlation with an image of a single bubble locates the features, even though they vary somewhat in size and texture.

The tutorial initializes with a specimen of air bubbles imaged in the microscope appearing in the left-hand window entitled Specimen Image. The Display Image buttons display in the right-hand window either the selected Target image, or the result of Cross-Correlation (in which the brightness of the spots measures the goodness of the match with the target), or the result of applying a Top Hat filter to the cross-correlation result, which locates the bright spots and thus marks the location of each bubble in the original image.

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