``Top Hat'' Operator

``Top hat'' operator, introduced by MEYER, results from the theory of mathematical morphology and allows peaks of intensity to be extracted from an image.

This filter can be used by supposing that the required lineaments are brighter than the environment . The main advantage of this filter is to be able to detect an over brightness even if the environment is not uniform. Moreover it is possible to regulate the size or the width of the over brightnesses very easily.

The principle is based on the subtraction of an image from its ``opening''. The opening consists of an erosion followed by a dilation, the size of the structuring element being conditioned by the width of the lineament to be detected.

In such a case, a simple thresholding could not have extracted the two brightness peaks. The efficiency of the processing depends on the dynamics of the image and especially of the noise, (see [Destival, 1987]), since the points of over brightness are detected, so are the peaks of noise. In the case of dark lineaments on clear background, it is possible to process the reversed image.