painter's algorithm not working

If the sign of the dot product of the view vector and the polygons normal is negative then it faces away from you.

yes. i am aware of that. the idea was to not use backface culling yet untill i know that the painter''s algo is working!


thanks for trying anyway though.

That''s the problem with Z-sorting your triangles based on a single point. Whichever point you choose to sort by, you''re more than likely going to end up with ordering errors. Even if you could find a method that works for a cube, in the more general case you will stil get errors (eg if 2 triangles intersect, you''ll get errors whichever way you order them).

The only way to be really sure about the ordering is to use a z-buffer, but I take it you don''t want to do it that way or you wouldn''t be trying the painter''s algorithm?

Another option could be to split triangles that intersect along all axes to form mutually exclusive triangles. Or if you''re objects are always convex, you could pre-order the faces. I seem to remember a theory from a long time ago that said that convex objects could always be preordered so that rendering the faces in that order would always be correct. Can''t remember any algorithms for calcualting the order though (and I think it relies on backface culling as well).

Sorry I don''t have any more useful suggestions ...

Mike.

How about considering the highest or lowest z-value instead of the average one??

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Fleejay

As he mentioned in the original post, he''s already tried min and max z values and still gets the errors.

In the case when triangles are non-intersecting, I think sorting by min z will always work (correct me if I''m wrong). But in the case of intersecting triangles you can''t find any point to sort by which will always be correct. In the case of a cube, all triangles intersect other traingles at the edges (i.e will give the same z value). So depending on how the sort works (<= or <) you will end up rendering either one or the other first. If it looks correct from one side, then it will look incorrect from the other.

problem solved: i just wrote a z-buffer instead.