how to find edges?

To find edges in a photo you can use a sobel filter.

float [] data = {-1,-1,-1,-1,8,-1,-1,-1,-1};//Sobel filter
Kernel kernel = new Kernel(3,3,data);
ConvolveOp cop = new ConvolveOp(kernel);
BufferedImage filteredImg = cop.filter(bufImg,dst);

filteredImg is a black-white colored image (edges are white,others are black).To detect an object i will use edge and color informations.