Applying Prewitt Filters to an image and then using them to calculate an image gradient

I have applied a prewitt filter to an image with the following kernel [-1 -1 -1;0 0 0;1 1 1].

I would like to use the output as a way of calculating the image gradient.

THis is done by calculating (z7+z8+z9)-(z1+z2+z3) for the entire image.

How do I compute this for the entire image??

Answer 1

[Image Analyst]

Why not simply use imgradient()?