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# D3DXComputeBoundingSphere

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Hey, I''m trying to do collision detection (simple bounding sphere right now) but am having some weird results. I use the D3DXComputeBoundingSphere to compute the radius of the bounding sphere around a mesh, but i get really weird collisions when I do a simple interect test. I basically do this: CheckCollision(*obj1, *obj2) { //get radius of each rad1 = *obj1->getRadius(); rad2 = *obj2->getRadius(); //compute difference in distances in each axis float diffx = fabs(obj1->x - obj2->x); float diffy = fabs(obj1->y - obj2->y); float diffz = fabs(obj1->z - obj2->z); if((rad1+rad2) >= diffx) { if((rad1+rad2) >= diffy) { if((rad1+rad2) >= diffz) { return true; } } } return false; } Anyway, I draw a wireframe sphere around my meshes in debug mode so I can see that the radius computed is correct, but I get collisions even when two meshes are quite far apart. But it doesn''t make sense. For example, I get that the radius of each mesh in the collision is 5.14, but the distance between them is computed as only 2.4 even though the objects are not interecting at all. Could it be some sort of conversion I''m missing from object space to world coordinate space? Thanks, # andrew.c

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You're not actually calculating the distance between the two.

Try this:

    bool CheckCollision (CObject *obj1, CObject *obj2){    //get radius of each    float rad1 = *obj1->getRadius();    float rad2 = *obj2->getRadius();    //Calculate threshold and square it    float threshold = rad1 + rad2;    threshold *= threshold;    //compute difference in distances in each axis    float diffx = obj1->x - obj2->x;    float diffy = obj1->y - obj2->y;    float diffz = obj1->z - obj2->z;    //Calculate distance - leave it squared to prevent sqrt    float distance = diffx * diffx + diffy * diffy + diffz * diffz;    return distance < threshold;}

[edited by - Kippesoep on March 7, 2003 5:35:14 PM]

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Distance between two points in space:
d = sqrt( delta_x^2 + delta_y^2 + delta_z^2 );

delta is v2 - v1 where v is a variable

so if d is less than radius_of_object1 + radius_of_object2 you have a collision.

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[edited by - Enselic on March 7, 2003 2:39:04 PM]

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Skip the sqrt if you are only checking for collisions. The squared distance compared to the squared radius is fine.

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