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Is PVS or portals really necessary?

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The are several methods for detemining which sectors of a world should be drawn in a 3d engine, such as PVS or portals, and many arguments which is best. But how much difference does it actually make? Now I don''t know if these figures are realistic, but imagine a quake level contains 1000 polygons. Of these an average of 50 are visible at a time. Therfore in theory a decent potentially visiable surfaces (PVS) algorithm will speed up the rendering by a factor of 20. But what if the level contains monsters, each consisting of 500 polygons, and you can see 5 of them. Now without PVS your rendering 3500 polygons, and with it your rendering 2550, which is not such a significant speed increase. Espesially considering you have to spend time running the PVS algorithm. You see surely without the complication of PVS or portals it would be much easier to do things like deformable or dynamic scenery. Now I realise there are some faults in my argument, an map may be more complex to render than a model beacuse of lightmaps and the like, but I still think the simplified engine would be worth it. What do other people think?

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That seems ok for a 1000 poly seen but what if you want a 10,000,000 polygon level. The beuty of the PVS is that with it the size of the level is insignificant(except memory wise). In addition, your argument about spending time claculating the PVS is flawed because the PVS is precalculated.

Without some kind of hierarchy things quicky get out of hand for such things as picking and collision detection.

ECKILLER

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Guest Anonymous Poster
Your resoning is true only if every single monster is in the same room with you. (That would suck.) PVS determination allows you to draw only those creatures that are visible, as well; it isn''t just for terrain.

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PVS still makes sense, as long as you don''t start clipping against portals... I don''t think that makes sense at all anymore.. and on the GF 3 it seems like Overdraw is more expensive than Z-Buffering, cause of that "clever" z-Buffer technology, where it does z-buffering first and then applies the textures or so... ;o). What''s important is, to push as many triangles as you can, that means Display lists and Vertex buffers as far as possible ;o)

cya,
Phil


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quote:
Original post by masonium
If I''m using OpenGL for example, how would I implement a PVS algorithm?


Implementing portals, or any other PVS algorithm, is not based on the graphics API. You need to compute the PVS yourself, using whatever method you''ve chosen, and then hand off the potentially visible vertex lists to the API.

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