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# A beginner BSP question...

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On page 1104 of Michael Abrash''s "Graphics Programming Black Book" their is a top down picture of a bunch of walls, and next to them, is the BSP tree created by these walls... If you have the book, Why doesn''t wall D, which splits wall C, actually show up in the BSP? If you don''t have the book, my question is basically why 1 of the walls splits another, which creates sub-spaces, but another of the walls which also splits another wall in the picture does not... Does this make any sense? Tony

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Are you sure the wall you''re talking about is actually used as a dividing plane? If not, even if it spans both sides of some other wall it won''t have to be split.

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I happen to have that book right here, and...

D does show up in the BSP, as the front child of C. But maybe I''m misinterpreting what you''re saying.

But, to answer your question. In this case, C is picked as the first splitting plane. This gives us two half-spaces, and two new sets of polys, B and AE on one side and D on the other.

A poly can only split another poly in its own half-space. D is the only poly in its half-space, so it doesn''t split any other polys. On the other side we have the polys AE and B. If AE had been chosen (rather than B) as the splitting plane, there would be no further splits. Since B is chosen, it splits AE into A and E. Note, however, that B bisects, but does not split, C. This is because B can only split polys in its own half-space, and C is not in its half-space.

Hmm. This stuff is a little hard to explain. Did that help at all?

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I think I get it... Basically, if you have a wall A, and it''s left subspace contains B, C, and D... Regardless of whether B,C, or D bisect A, they cannot split it since it is not really in their space... They are in ITS half space...

Tony

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"I think I get it... Basically, if you have a wall A, and it''s left subspace contains B, C, and D... Regardless of whether B,C, or D bisect A, they cannot split it since it is not really in their space... They are in ITS half space..."

Correct.

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