Highpass filters can enhance the edges and sharpen details in a grayscale image.
AutoCAD Raster Design toolset uses the following highpass filters:
- Highpass Filter #1 picks up differences between the target pixel and its neighbor's side to side and top and bottom. Diagonally connected pixels are ignored.
- Highpass Filter #2 evenly weighs all the surrounding pixels in the kernel but places an especially high weight is on the target pixel. Thus, if the target pixel differs significantly from its neighbors, the dimming effect of the neighboring pixels negligible.
- Highpass Filter #3 weighs side to side and top and bottom neighbors more heavily than diagonal neighbors.