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# Sphere to sphere collision

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Hi all; I''m trying to get the first time ( not the closest point) that two spheres touch each other, where dist = 2*r. I tried to do that modifying the code from Oleg Dopertchouk, at gamedev, but i failed. Does anybody knows how to do that? Here is the code i was trying to change (Oleg Dopertchouk) BOOL bSphereTest(CObject3D* obj1, CObject3D* obj2 ) { // Relative velocity D3DVECTOR dv = obj2->prVelocity - obj1->prVelocity; // Relative position D3DVECTOR dp = obj2->prPosition - obj1->prPosition; //Minimal distance squared float r = obj1->fRadius + obj2->fRadius; //dP^2-r^2 float pp = dp.x * dp.x + dp.y * dp.y + dp.z * dp.z - r*r; //(1)Check if the spheres are already intersecting if ( pp < 0 ) return true; //dP*dV float pv = dp.x * dv.x + dp.y * dv.y + dp.z * dv.z; //(2)Check if the spheres are moving away from each other if ( pv >= 0 ) return false; //dV^2 float vv = dv.x * dv.x + dv.y * dv.y + dv.z * dv.z; //(3)Check if the spheres can intersect within 1 frame if ( (pv + vv) <= 0 && (vv + 2 * pv + pp) >= 0 ) return false; //Discriminant/4 float D = pv * pv - pp * vv; return ( D > 0 ); }

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Try something like that; it works for me

...{        float dt = 1.0f / UPDATE_FPS; // UPDATE_FPS == 60        Vector Vp = obj1->GetLinVelocity();        Vector Vq = obj2->GetLinVelocity();#define sign(n) (n >= 0.0f ? 1.0f : -1.0f)	Vector Vab = Vp	- Vq;	Vector l   = m_Position - Sphere.m_Position;	float aa   = Vab * Vab;	float bb   = 2.0f * (l * Vab*dt) * dt;	float cc   = l * l - (m_Radius + Sphere.m_Radius) *		     (m_Radius + Sphere.m_Radius);	float q	   = -0.5f *                    bb + sign(bb) * sqrt(bb * bb - 4 * aa * cc));	float t0   = q / aa;	if (t0 < 0.0f || t0 > 1.0f)	    return 0;	if (t0 > dt)	    t0 = dt;}

good luck,
David

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ok...i use the algorithm above but i have a problem...may i normalize position before use it???
i''m working with Coluna in the game...so i''m using the same post.

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no need; it''s just the absolute sphere position.

David

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