A method of determining slope of pixel intensities in an image includes the following steps: (a) receiving, from an imaging device, input pixel samples of a region of interest; (b) forming an impulse response function (IPR) for use in resampling the input pixel samples into output pixel intensities; and (c) calculating a derivative of the IPR. Also included is the step of convolving the derivative of the IPR with the input pixel samples to determine the slope of the output pixel intensities in the region of interest. In addition, the input pixel samples are generated in a first coordinate system by the imaging device, and the output pixel intensities are generated in a second coordinate system.