### Digital Image Gradient

Greetings .

I want to implement the Fast Radial Symmetry Transform (described here http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.97.4064&rep=rep1&type=pdf) to identify nuclei of cells in histological images of breast cancer for this I need to first calculate the gradient of the image. Until now he had not reviewed very afond what the gradient of a digital image is itself. Now I have several questions I would like to clarify:

(1) According understand the gradient of each point of a function is a vector formed by the partial derivatives of each variable, so for a digital image (which is a discretization of a 2D function) the gradient at each point in the image? which is exactly?. I guess it should be a two-channel digital image or am I wrong?

(2) How I can calculate or approximate the gradient of an image?

(3) Is there a function in OpenCV to compute or approximate the gradient of an image?

Thanks in advance for any help and/or suggestions and I apologize if my questions seem silly or too elementary .