Geometric Common Line Removal

Is this "invisible line removal"? I always though this problem is artificial and should not be solved in most cases. The problem can be difficult, not always solvable (for example, due to the problems of the shared line segment and its affect of pixel-size vicinity of the shared area); at the same time, the benefits of the solution can be miserable when compared with the effort.

(Also, remember, there are no lines in real life, they are all imaginary. In photo-realistic quality of rendering there no lines, too. So, if you show lines in your simplified picture, they are symbolic reflection of ideal mathematical object, not the realistic rendering model, so you may want not too take likes too seriously. You merely need a clear recognizable picture.)

Here is the idea. If the line is invisible, this is because it is masked by the object which is closer to the spectator. In 2D case, you need to keep Z-order to draw objects closer to the spectator on top of the others. In 3D case it's a bit more complex, but essentially, this is the same is Z-order; you need to order objects according to the position of the "camera". I did it in 3D and had no problems with performance, by the way. In other words: you have to draw all or some of the "invisible" lines; the objects placed on top of those line will cover them.

The only problem is performance, but it is not significant in most cases; you may loose more CPU time by trying to find a more complex solution. Anyway, even if you want to prevent excessive rendering (yes, my solution is based on excessive rendering), you should consider the problem of "invisible lines" as optimization one, not rendering.

To prevent flicker you need to use double-buffering; to improve performance, you need partial invalidation (with Windows GDI, InvalidateRect or InvalidateRgn).

—SA