Roberts Edge
The Roberts Edge filter is use to detect edges based applying a horizontal and verticle filter in sequence. Both filters are applied to the image and summed to form the final result. The two filters are basic convolution filters of the form:
Horizontal Filter Verticle Filter
The Roberts Edge detector is fast since the filter is small but it is also subject to interference by noise. If edges are not very sharp the filter will tend not to detect the edge. See Prewitt or Sobel filters instead which are larger and less sensative to noise.