Export png of a specific stroke/s (or several strokes) by GreasePencil

I'm putting together a program passes automatic assistance and I don't know how to export using python a swap or surface, I did it in a bit of a caveman way, but I'd like this to make it blender.

in the image I only get to capture the points and the subtraction is not so good.

I mean, I want to export a spicy area with only one of the selected elements