This fourth type of algorithm is a unique approach to hiding polygons.

The approch is this:

1> Take a given section of the screen (the entire screen, in the first pass)
2> Check to see if it is "simple enough"
3> If it is, display it
4> If it isn't, subdivide the screen into four sections and check each of the new sections (starting at step 1)

The approach is elegantly simple. let's take the example at the right.

The screen isn't "simple enough", so it is split into 4 parts (lines marked "1")

Now the upper right, lower right, and lower left are "simple enough" so they would be displayed, but the upper left is no, so it is further broken down (lines marked "2")

Now both upper quadrants of "2" are "simple enough" but the two lower ones are not, so they are further broken down

And so on and so on. (often going down to the pixel)