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A top hat filter is a type of image processing algorithm intended to make small objects in an image more prominent, i.e., increase their contrast, or make them stand out. The name indicates a filter-algorithm that, when processing each point of the image, makes use of image data within a radius and with different rules, image data between that radius and a larger radius (i.e., within its neighboring circular area, and within a ring around it), and it uses a threshold regarding points in the surrounding ring, all these suggestive a function, that graphed, would have the shape of a top hat. The circle and surrounding ring are basically a stencil used in processing a point. For pixelated images, the circular area and ring define which other pixels are used for a pixel's processing. Various strategies are used, one strategy being to reduce a pixel's brightness to the minimum of pixels within the ring-shaped area but only if its brightness fails to exceed the defined threshold added to the brightest pixel within the ring-shaped area. Much of the image's brightness is reduced, but small areas which stood out are left as is, thus standing out more. The effect of the filter is controlled by adjusting 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 have use in 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.