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//The idea behind this is to create a ray (line) and check what interupts it.HRESULT D3DXIntersect(   LPD3DXBASEMESH	  pMesh,         //Mesh to check intersection   CONST D3DXVECTOR3 *pRayPos,       //Ray Origin   CONST D3DXVECTOR3 *pRayDir,       //Direction to cast ray   BOOL				 *pHit,          //Flag if intersection occurs   SWORD			 *pFaceIndex,    //Which face ray intersects   FLOAT			 *pU,   FLOAT			 *pV,   FLOAT 			 *pDist,	     //Distance to intersection   LPD3DXBUFFER		 *ppAllHits,     //Set to NULL   DWORD			 *pCountofHits); //Set to NULL   BOOL CheckIntersection(ID3DXMesh *Mesh,   float XStart, float YStart, float ZStart,   float XEnd,   float YEnd,   float ZEnd,   float *Distance)   {   	BOOL Hit;   	float u, v, Dist;				  //Flag if intersection occurs   	float XDiff, YDiff, ZDiff, Size;  //Misc/Distance to Intersection   	DWORD FaceIndex; 				  //Face that was intersected   	D3DXVECTOR3 vecDir;				  //Direction vector for ray   		//Calculate differences from start to end point   	XDiff = XEnd - XStart;   	YDiff = YEnd - YStart;   	ZDiff = ZEnd - ZStart;   	   	//Calculate the direction vector   	D3DXIntersect(Mesh,   	   &D3DXVECTOR3(XStart,YStart,ZStart), &vecDir,   	   &Hit, &FaceIndex, &u, &v, &Dist, NULL, NULL);   	      	//If there was an intersection, see if its within the path pf the ray.   	//Distance to intersection is less than the length of the ray   	if(Hit == TRUE) {   		//Get length of ray   		Size = (float)sqrt(XDiff*YDiff+YDiff*YDiff+ZDiff*ZDiff);   		   		//Ray did not intersect with a polygon   		if(Dist > Size)   		Hit = FALSE;   		else {   			//Ray intersected, store distance to intersection   			if(Length != NULL)   			*Length = Dist;   		}   	}   		//Return TRUE if intersection occured	return Hit;}float GetClosestHeight(ID3DXMesh *Mesh,   float XPos, float YPos, float ZPos)   {   	float YAbove, YBelow;   	   	//Get Hieght above and below point we check   	YAbove = GetHeightAbove(Mesh, XPos, YPos, ZPos);   	YBelow = GetHeightBelow(Mesh, XPos, YPos, ZPos);   	   	//See wich height is closer to the point we check, then return that value   	if(fabs(YAbove-YPos) < fabs(YBelow-YPos))   	//Height below is closer, return it	return YAbove;		//Height below is closer, return it	return YBelow;   }   float GetHeightBelow(ID3DXMesh *Mesh,   float XPos, float YPos, float XPos)   {   	BOOL Hit;         //Flag if polygon is hit   	float u, v, Dist; //Misc/Distance to intersection   	DWORD FaceIndex;  //Which face ray intersects with   	   	//Perform intersection test aginst mesh   	D3DXIntersec(Mesh.   	&D3DXVECTOR3(XPos,YPos,ZPos),   	&D3DXVECTOR3(0.0f, -1.0f, 0.0f),   	&Hit, &FaceIndex, &u, &v, &Dist, NULL, NULL);   	   	//Return closest height below if there was in intersection   	if(Hit == TRUE)   	return YPos-Dist;   	   	//Return height passed as no intersection   	return YPos;   }