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# Collision Detection and Plane Equation

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Hello, First off before anyone flames me, I read many articles and looked through prior posts on Math & Physics and in the DirectX forum. Anyways my first question is, The plane equation: Ax + By + Cz ( + | - ) d = 0; D is the plane''s distance to the origin .. what Origin? (0,0,0)? ( If it''s the dotproduct of the normalized vectors (ABC and xyz) I know how to get that value - otherwise I''m really confused on this aspect ) Next is, I know how to do collision detection but my issue is - say I''m walking around my world ( 3D ) How do I know which polygon face I should be testing my bounding sphere ( or box ) with? A world would have thousands of faces to test against, how will I know which one to target? I.e. walking forward then I hit a wall north of me instead of South East? Thanks for your time, David

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When you have the equation Ax + By + Cz - D = 0, you know that using the coordinates (x,y,z) of ANY point on the plane it must be equal to zero.

Then, to get the D value you may just use one point on plane:
D = Ax + By + Cz.

In other words, D is the dot product of the plane''s normal (A,B,C) and any plane''s point (x,y,z). And, yes, it''s the distance from (0,0,0).

When you are working with large worlds, you must use a way to organize all the polygons (BSP, Octrees, Portals). Not just for rendering, but for collision detection too.

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"When you are working with large worlds, you must use a way to organize all the polygons (BSP, Octrees, Portals). Not just for rendering, but for collision detection too. "

Ah very nice, just thought bsp/octrees/portal/and such were just for rendering much thanks to you!

Thanks!

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