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# calculating the D component and ...

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i want to add collision detection to my engine, so i need some info: 1. i need to calculate a polygons(convex) plane. i used the cross-product to calculate the normal, but how can i calculate the D component(distance) to complete my plane ? 2. if i got the intersection point on a plane, i need to know if this point is within the boundaries of the polygon. i decided to use the method where i calculate a plane for each edge of the polygon and test the intersection point against them. but how can i calculate a plane for an edge ? or has anyone an easier algo to check if the intersection point is within the boundaries of a polygon ? sorry for that bad english, im german

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1) From the plane equation Ax+By+Cz+D=0. A,B,C are the x,y,z components of your plane normal (normalized !), and x,y,z are the coordinates of an arbitrary point on the plane, eg. a point of your original poly:

-D=N.x*P.x+N.y*P.y+N.z*P.z

2) A plane for an edge is simple: just take the crossproduct of the polygons plane normal (as above) and the normalized edge vector. Here you got your new plane normal.

If you use triangles, then there are very fast specialized point in triangle test algorithms.

-AH

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A surface normal should be normalized - that is, the length of the vector should equal a single unit. That usually means 1.

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thx,
now i got a buggy, but actually working collision detection in my engine (buggy: because some polygons dont react on the collision; im gonna fix that by myself).
now i have to implement collision response. i think i use the sliding-effect. i read in a tutorial that i can push the viewer back in the direction of the polygons normal. but how can i move a 3d-point in the direction of a vector ?

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Just add the vector''s x y and z values to the point''s x y and z values, respectively, but multiplied by the amount you want the player to bounce back. I guess you''ll want the bounce-back speed to be proportional to the speed at which the player hit the wall.

point.x = vector.x * bounce_speed;
point.y = vector.y * bounce_speed;
point.z = vector.z * bounce_speed;

I guess bounce_speed could either be a constant or a constant * player_speed.

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yeah, thanks thats all i need at the moment !!

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