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Frei and Chen operators can be combined to make kernels that detect specific morphological patterns in small neighborhoods. The following procedure creates a new () image that highlights edges in an old () image:

[compute FC01, FC02, and FC09 here] result = SQRT(FC01 * FC01 + FC02 * FC02) new(x,y) = (result / (FC09+1) NEXT x

NEXT y where result is a variable that holds the output of combining the horizontal and vertical edges detectors and SQRT is the square root function. Dividing by the average value means that this operator finds edges with little regard to the image brightness.

• Tip: AIP4Win includes the Frei and Chen operators for detecting edges, lines, ripples, and points. In astronomy, the edge detector and the line detector are by far the most useful of the Frei and Chen operators.

The new image created by the Frei and Chen line operator is a grayscale image, and the boundary lines tend to be two to three pixels wide. To produce lines that are one pixel wide, first threshold the image and then run the skeletonize operator described in the following section.

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