How to calculate polygon depth (z-buffer method)?

Question

Mathematics

Hello. I need to z-buffer not for every pixel on the screen, but for polygons. Could you please explain how to calculate the depth of a polygon?
For example: There are several polygons (rectangles) and each has 4 vertices with some x, y, z coordinates, how to calculate the depth, for example, for a blue and red rectangle?

Answer 1

This is no longer z-buffering, but an artist's algorithm. Take, for example, the z-coordinate of the center.