Painter algorithm

The painter algorithm ( Engl. Painter's algorithm ) is a simple solution to the visibility problem in 3D computer graphics . When displaying a three-dimensional scene on a two-dimensional one, it is often necessary to decide which polygons are visible and which are hidden.

First the mountains are drawn, then the ground and finally the trees

The name painter algorithm is an allusion to a painter who first draws the distant objects in a scene and then paints over them with the closer ones. According to the algorithm, in the implementation of a computer graphics application are used: First, all polygons their depth sorted ( depth sorting , engl. Depth sort and then the order they are drawn by). The visibility problem is solved by overdrawing the parts of the image that are normally not visible.

Why the painter algorithm fails

This approach creates a number of problems. What happens if polygon A partially intersects polygon B, B partially C and C partially intersect A? It can no longer be decided which polygon is in front of which. A similar case occurs when two polygons intersect each other in three-dimensional space. In such cases, at least one of the affected polygons must be subdivided so that the sorting is possible and the painter algorithm delivers a correct result.

Another problem is that the painter's algorithm is inefficient because the computer has to calculate the intensities of all points on a polygon, even if the polygon is not visible in the final scene.

These and other problems with the painter algorithm led to the development of the Z-buffer , which can be viewed as a logical further development of the painter algorithm. By using a z-buffer, the objects no longer need to be rendered in the order of their depth.