Thread: Ray tracer and collision detection

score 1 for perspective

I was thinking about telling him that in class today.... but... I decided it was best he find out his silly mistake for himself

No, I'm just kidding, I was just as oblivious as he was .

Originally Posted by Perspective

>double dist = distRayPlane(Pos, velocity, pl.Normal, Dplane);
>if(dist < distRayPlane(Pos, velocity, pl.Normal, Dplane))

dist is never going to be less than itself

I kinda figured out. So all i have to do is determine the distance from the Original position till the plane's normal. Also, i have to check to see if the Origin position is inside the triangle or not (it is easier checking a rectangle than a triangle). Do you think you can help me figure out what the problem is?

Yes, finally i've made it work. This is how it goes:

Code:

bool checkPointTriangle(vect3 Point, vect3 Pa, vect3 Pb, vect3 Pc)
{
	double totalAngle = 0.0f;
	vect3 v1 = Point - Pa;
	vect3 v2 = Point - Pb;
	vect3 v3 = Point - Pc;

	NormalizeVect(v1);
	NormalizeVect(v2);
	NormalizeVect(v3);

	totalAngle += acos(dot(v1, v2));
	totalAngle += acos(dot(v2, v3));
	totalAngle += acos(dot(v3, v1));

	if(fabs(totalAngle - 2*PI) < 0.005)
		return true;
	else 
		return false;
}

So, the Position of the object (a vector) will be check if it is going to be inside a triangle or not, then with a distance calculation from the position to the triangle's normal, collision will occur.

Is there anything else i should add into my collision code to make it faster?

EDIT:
If i have a model with multiple triangles, what vector on the model should i check for the collide triangles? A point in a model's triangle or a point on its face?

If i have a model with multiple triangles, what vector on the model should i check for the collide triangles? A point in a model's triangle or a point on its face?

Could you please explain what you mean by this question?

I wrote this for determining if a point resides in a triangle...but this is also pretty nasty, and might actually be more expensive, but it works.

Code:

#include	"CollisionDetectionPrimitives.h"

extern	std::ofstream	trace;

#define	CCWPITDEBUG(x)	x

//Fixme: investigate to see how vector length plays into uncertainty here
//Normal x DiffVector = InwardVector
//InwardVector Dot Point >= 0 or >= -.0001
BOOL	CCWPointInTri(float	P1X,float	P1Y,float	P1Z,
					  float	P2X,float	P2Y,float	P2Z,
					  float	P3X,float	P3Y,float	P3Z,
					  float	NormalX,float	NormalY,float	NormalZ,
					  float	PointX,float PointY,float	PointZ)
{
	Vector3	Point1(P1X,P1Y,P1Z);	Vector3	Point2(P2X,P2Y,P2Z);	Vector3	Point3(P3X,P3Y,P3Z);
	Vector3	Normal(NormalX,NormalY,NormalZ);
	Vector3 Point(PointX,PointY,PointZ);

	Vector3	LocalPoint = Point - Point1; //The point in local coordintes with respect to a vertex 

	Vector3	DiffVector	=	Point2 - Point1;
	Vector3	InwardVector(0,0,0);
	InwardVector	=	CrossProduct(&Normal,&DiffVector);

	if(DotProduct(&InwardVector,&LocalPoint) < -.0001f)
	{
		CCWPITDEBUG(	trace << "Returning on first point" << "\n";	)
		return	FALSE;
	}

	LocalPoint	=	Point  - Point2;
	DiffVector	=	Point3 - Point2;
	InwardVector	=	CrossProduct(&Normal,&DiffVector);

	if(DotProduct(&InwardVector,&LocalPoint) < -.0001f)
	{
		CCWPITDEBUG(	trace << "Returning on seconed point" << "\n";	)
		return	FALSE;
	}

	LocalPoint	=	Point  - Point3;
	DiffVector	=	Point1 - Point3;
	InwardVector	=	CrossProduct(&Normal,&DiffVector);
	if(DotProduct(&InwardVector,&LocalPoint)<-.0001f)
	{
		CCWPITDEBUG(	trace << "Returning on third point" << "\n";	)
		return	FALSE;
	}

	return	TRUE;
}

while we're all in the mood for posting ray-triangle intersection algo's, here's mine. (I was forced to use this prototype, thats why i set the u, v, t params by reference). Also, this scence always had one object, that's why i just grab objects[0]. A better implimentation would pass the triangle as a paramater.

edit: oh btw: u and v are the "UV Coordinates" of the intersected triangle and t is the magnitude along the direction of the ray that the intersection takes place. So this algo doesn't just tell you if there is an intersection, it tells you the point on the triangle that the intersection takes place by setting the u,v, and t params (handy for phong shading or other types of lighting). Returns 0 if there is no intersection and 1 if there is.

Code:

// calculate ray - triangle intersection with backface culling
int RayTracer::calcTriangleIntersection(const Ray& ray, const VertexList* face,
					float *t, float *u, float *v) {

  Point3f vert1 = scene->objects[0].vertices[(face->vertex)-1];
  Point3f vert0 = scene->objects[0].vertices[(face->next->vertex)-1];
  Point3f vert2 = scene->objects[0].vertices[(face->next->next->vertex)-1];

  Vector3f edge1, edge2, tvec, pvec, qvec;
  float det, inv_det;

  // find vectors for two edges sharing vert0 
  edge1 = vert1 - vert0;
  edge2 = vert2 - vert0;

  // begin calculating determinant - also used to calculate U parameter
  pvec = ray.dir.cross(edge2);
  
  // if determinant is near zero, ray lies in plane of triangle
  det = edge1.dot(pvec);

  if (det < EPSILON)
    return 0;

  // calculate distance from vert0 to ray origin 
  tvec = ray.start - vert0;

  // calculate U parameter and test bounds 
  *u = tvec.dot(pvec);
  if (*u < -EPSILON || *u > det)
    return 0;

  // prepare to test V parameter 
  qvec = tvec.cross(edge1);

  // calculate V parameter and test bounds 
  *v = ray.dir.dot(qvec);
  if (*v < -EPSILON || (*u + *v) > det)
    return 0;
  
  // calculate t, scale parameters, ray intersects triangle 
  *t = -edge2.dot(qvec);

  inv_det = 1.0f / det;
  *t *= inv_det;
  *u *= inv_det;
  *v *= inv_det;
  
  return 1;
}

Originally Posted by Bubba

That acos() looks quite nasty. Anything you can do to get rid of it?

Well, i can put it like this:

Code:

totalAngle += (acos(dot(v1, v2)) + acos(dot(v2, v3)) + acos(dot(v3, v1)));

Anyway, i have to convert the decimal to degree to make a full 360* circle. Then minus 2 PI. I don't know how can you clean it up if you need the function.

Originally Posted by BobMcGee123

Could you please explain what you mean by this question?

I meant like this, I use a vector Pos, which i put in the glTranslatef() for testing. So all it does is check the Pos coordinate (x, y, z). Then i draw a triangle moved by the Pos coordinates. So, when i have more than one triangle, what point should i check so that all the triangle of the model can determine if it is inside/hit another objects.

That was suppose to be Bob, i messed up.

Anyway, if i ray trace every triangle of a model, it would be slow right?

About the bounding volume, if my model is really complex (curve, rectangles, all kind of stuff that make it look complex), do i have to find the center of the model? If the model is an aircraft with 2 wings on the side, would the bounding volume will make it look realistic.

The partitioning algorithm is something i have no idea, could you talk about it more.

I think he means something along the lines of checking whether or not two objects are even close to each other whatsoever...

If not they don't need to check collision between each other, saving valuable computation time .

How to do it? You could just as well ask a monkey for the answer :d.

Originally Posted by Shamino

I think he means something along the lines of checking whether or not two objects are even close to each other whatsoever...

If not they don't need to check collision between each other, saving valuable computation time .

How to do it? You could just as well ask a monkey for the answer :d.

So by that, we can use the ray to determine if the object parallel to the others. Find the ray direction of the first object, and calculate. If it perpendicular to the other objects' normal, we have a parallel line and we don't have to run the calculation.