These are more powerful than previous methods (back-face removal and depth sorting - painter's algorithm)
We are through and can scan convert the testing area if one of the following is true:
Test for 1® Test boundary rectangles (extents) of surfaces against boundary area.
Test for 2 ® Same as Test 1 for inside surfaces of surrounding surface. For overlapping surface, we need to check if actual surface inside testing area so use line intersection.
Test for 3 ® Do Z depth ordering , use extents to test for surrounding surface, if Zmax of surrounding surface is less than Z min for enclosed surfaces then test 3 is satisfied.
Note that test 3 is satisfied for surfaces 2 and 3 but not for 1, even though S surrounds 1. So we could do more elaborate testing for 1 but easier to just subdivide and continue with simple tests.