Convolution of an image using convmtx2

Hello.

I've to apply a set of operators (for example: Prewitt,Sobel,Canny,Robinson) to an image.

I have the following function:

function name_fun( operador , imagen )
 % the image is loaded from file to a matrix
 img1 = imread(imagen,jpeg);

   % the original image is shown
   imshow(img1);

   % image is converted to gray scale
   img2 = rgb2gray(img1);

   % the new image is shown
   imshow(img2);

   % convolution (using the operator given as parameter) is applied (?) using 3x3 pixel window
    img3 = convmtx2(H,m,n)

   % the convultionated image is shown
   imshow(img3);

I just wonder what are m and n (like: img3 = ....). Is height and weight of the image? And Do I need to do something more or this way I get the result I want?

Thank you