How to compute image global gradient?

I know that imgradient computes image local gradient. Is there a way to compute the global gradient?

For example, for this image:

Here is the global gradient:

Answer 1

[Image Analyst]

Talya, I have no idea what that means. Gradients by definition are local - they're the slope of the function at some location. And you don't have a single number for the whole image - you have a 2D image, so presumably you have different gradient values for each pixel location. However your gradient does not look like the expected, which would be like an edge detection image. You're going to have to give some formula for turning your color image into this grayscale image, otherwise I have no idea.