Probing points in Mask (mean color) Ambilight project

I am just having difficulties to progress with my project.
So I am trying to make Camera based Ambilight color detection on TV screen.
In my project, user will be able to draw Trapezoid shape, and point where the screen is, so then based on that I can create the mask that looks like:

Now the question is, Assuming I have X number of LEDs Vertically and Y Horizontally,
How do I approach getting mean or average colors, around that mask, that I assume should be divided by some smaller sections ?

I also had an idea but don’t know if it’s possible, Could it be possible, to draw the line between those 4 points, and while it’s drawing, probing in ever X pixel distance or something like that ?

It’s all guessing I am not even sure that is the correct approach, using masks.
I would appreciate any suggestions

related: OpenCV how to get mean color of mask fragments - Stack Overflow