Edge detection on a texture (shader)

I need to do edge detection (enhancement) on a texture. A movie texture specifically. This is not about 3D edge or normals, but just old-fashion image processing contrast-enhancing edge detection. I could write this in C# but clearly this is a job for a shader, right? Anyone know how to go about setting this up, or is there anything like this already (Note: not looking for a full-screen effect). Thanks.