Muhammad - please post the full error message (so all of the red text) so we can narrow down which line of the callback is generating the error.
Is it that the A is not defined? (Instead of using global variables, save the image (once loaded) to the handles structure which this callback then use to access the image.)
More likely, the error has to do with f1 variable which is being used as the step size in the two for loops....but it hasn't yet been defined. Please define this variable (again consider using the handles structure if this is set elsewhere in the code).
