

OK, never heard of Cross filters, but Roberts, Prewitt and Sobel filters
are standard spatial masks for edge detection.  Highpass filtering does a
good job of detecting edges, too.  A good reference for all sorts of general
digital image processing is the book _Digital Image Processing_ by
Rafael C. Gonzalez and Richard E. Woods.  Addison-Wesley Publishing Co. 1992.
ISBN 0-201-50803-6

I've used this source to do the above filters and many other things as a
grad project.  Not too dificult to do.  The worst part is loading in the
images from TIFF/GIF/IFF-ILBM/whatever!  :)