In this lesson, you will write a function to compute the digital gradient of an image. The digital gradient approximates the two-dimensional gradient of an image and emphasizes the edges. This simple function consists of three lines, corresponding to the three required components of procedures and functions: 1) the procedure or function declaration, 2) the body of the procedure or function, and 3) the terminating END statement.

To write the function, do the following:

FUNCTION GRAD, IMAGE

; Write the following code, which evaluates and returns the

; result. Result = abs (dz/dx) + abs (dz/dy), which is the sum

; of the absolute values of the derivative in the x and y

; directions.

RETURN, ABS(IMAGE - SHIFT(IMAGE, 1, 0))+ $ 
ABS(IMAGE - SHIFT(IMAGE, 0, 1))

; Use END to end of function.

END

The function has one parameter called IMAGE. There are no local variables. (Local variables are variables within a module that are not parameters and are not contained in Common Blocks.)