Help with collision tutorial

Hello everyone: I have read telemachos's tutorials both the "collision response" and "improved collision and response" and I have the following questions: The following codes are from the from collision response here is the codes for that part: //////////////////////////////////////////// double intersectRayPlane(D3DVECTOR rOrigin, D3DVECTOR rVector, D3DVECTOR pOrigin, D3DVECTOR pNormal) { double d = - (dot(pNormal,pOrigin)); double numer = dot(pNormal,rOrigin) + d; double denom = dot(pNormal,rVector); if (denom == 0) // normal is orthogonal to vector, cant intersect return (-1.0f); return -(numer / denom); } ///////////////////////////////////////////// from the code above, I don't know what "d" exactly is and why the "numer = dot(pNormal,rOrigin) + d" and why the distance to okane in world units(return values) = -(numer / denom) besically, I don't understand codes for this particle function. I would really appreciate if anyone could help to understand this? Millions of thanks [Edited by - muimui1911 on September 15, 2005 6:56:08 AM]