# Ray tracer and collision detection

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• 01-11-2006
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
• 01-11-2006
Shamino
score 1 for perspective :D

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

No, I'm just kidding, I was just as oblivious as he was :D.
• 01-11-2006
hdragon
Quote:

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?
• 01-11-2006
VirtualAce
There is a ray-to-triangle intersection algo somewhere on this board. Bob and I talked about it extensively in several posts. I would recommend searching for posts either started by Bubba or by Bob.
• 01-17-2006
hdragon
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?
• 01-18-2006
VirtualAce
That acos() looks quite nasty. Anything you can do to get rid of it?
• 01-18-2006
BobMcGee123
Quote:

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;
}

• 01-18-2006
VirtualAce
That doesn't look so nasty. It's only subtraction's and cross product's. Shouldn't hit too hard.
• 01-18-2006
Perspective
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;
}

• 01-18-2006
hdragon
Quote:

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.

Quote:

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.
• 01-18-2006
Perspective
>>
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.
<<

Well, I didn't post the question but anywho...

You can ray trace against every triangle in the model, you can ray trace against a bounding volume of the model, or you can use a space partitioning algorithm to determine which triangles are in the same volume of space as your ray and test the triangles in that volume.

Those 3 options range from easy to difficult while simultaniously ranging from innefficient to efficient :)
• 01-18-2006
hdragon
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.
• 01-18-2006
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.
• 01-19-2006
hdragon
Quote:

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.
• 01-19-2006
VirtualAce
Well you could probably use an octree. Each leaf in the tree holds the vertices for that volume and each leaf is responsible for drawing itself. I'm not sure how to do this in a raytracer except that you wouldn't ray-intersect test any of the tri's in a leaf unless it passed certain tests.

I have very little experience with pure ray-tracing but I'm very experienced with ray-casting and vertical ray coherence. However ray-tracing is a whole nuther ball game since you basically are tracing a ray of light through pixel at x,y to completion.

I hope you get it working because I'm looking forward to seeing the final result.

Also a curve is simply a ray-to-sphere intersection test and you would want to break down the geometry into vertices if the sphere passed the test. Again I'm pretty much in the dark here with little experience in ray-tracing.
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