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A top hat filter is a type of image processing algorithm intended to make small objects in a digital image more prominent, i.e., increase their contrast and make them stand out more. Top hat indicates a particular class of filter-algorithm that chooses whether a pixel value is to be used as is, versus whether it is to be zeroed. The criteria is based upon all the pixel-values within a circle surrounding the pixel-of-interest, i.e., within a certain distance, which treats the pixels within a smaller circle (within a shorter distance) differently than those surrounding it. The outer and inner circle are basically a stencil used in processing the pixel. Various criteria are used, a typical one being that the value pixel is kept if the average of the values of nearer pixels within the smaller circle must exceed the average of the rest of the larger circle's pixels by some given threshold. The effect is that much of the image's brightness is reduced, but small areas that already stood out a bit are left "as is", thus standing out more. The effect of the filter can be adjusted by modifying the radii and threshold: which offers a means of adjusting what size of objects within the image are made to stand out.
Top hat filters also carry out useful functions when applied to an image's Fourier space.
The term top hat filter is also used for some physical filters, e.g., to filter a fluid, that are physically in a shape similar to a top hat.