Sobel Filter implementation for Harris Corner Detector

I have to implement a Harris detector and am not quite sure about the following detail regarding the Sobel filter to obtain the image derivative. When applying the Sobel filter to a Grayscale image, I might get negative intensity values. Do I need to convert the Image back to a Matrix of o开发者_如何学Pythonnly positive values before I calculate the Harris Matrix for each pixel or am I supposed to use the values as they are?

I don't think you need to restrict it to only positive values.

You can look at VLFeat's Harris corner detection implementation (Matlab/C source included). It's in the toolbox directory: vl_harris.m