koobas.hobune.stream
Line > triangle intersection
(I know unity has a physics component, this is for a school assignment)
I'm trying to implement these methods for mesh collision but Its failing.
http://fwheel.net/gdw/wiki/index.pl?LineSegmentTriangleIntersection http://fwheel.net/gdw/wiki/index.pl?ParametricPlaneEquation
I do think I misunderstood something from either of the two tutorials. And I can't figure out what exactly.
My code so far: (Obviously this isn't about speed or efficiency, it just needs to work =) )
int index = 0;
for (int i = 0; i < faces.Length; i += 3) {
{
Vector3 A = vertices[faces[i]];
Vector3 B = vertices[faces[i+1]];
Vector3 C = vertices[faces[i+2]];
Vector3 Adelta = B - A;
Vector3 Bdelta = C - A;
Vector3 n = normals[index];
Vector3 nNormalized = n;
nNormalized.Normalize ();
Vector3 negativeN = -(nNormalized);
for (int j = 0; j < ballArray.Length; j++) {
Ball b = ballArray[j].GetComponent<Ball> ();
Vector3 u = (b.transform.position + (nNormalized * b.radius)) - b.transform.position;
Vector3 v = A - b.transform.position;
float nu = Vector3.Dot(u,nNormalized);
float nv = Vector3.Dot(v,nNormalized);
if(nu == 0)
{
continue;
}
float r = (nv/nu);
if(r >= 0 && r <= 1)
{
Vector3 interSectionPoint = b.transform.position + r * u;
Vector3 w = interSectionPoint - A;
float sParamTop = (Vector3.Dot(Adelta,Bdelta) * Vector3.Dot(w,Bdelta)) - (Vector3.Dot(Bdelta,Bdelta) * Vector3.Dot(w,Adelta));
float tParamTop = (Vector3.Dot(Adelta,Bdelta) * Vector3.Dot(w,Adelta)) - (Vector3.Dot(Adelta,Adelta) * Vector3.Dot(w,Bdelta));
float dominator = (Vector3.Dot(Adelta,Bdelta) * Vector3.Dot(Adelta,Bdelta)) - (Vector3.Dot(Adelta,Adelta) * Vector3.Dot(Bdelta,Bdelta));
float s = sParamTop/dominator;
float t = sParamTop/dominator;
if( s>=0 && t>=0 && s+t<=1)
{
//print("true");
b.reflect(n);
}
}
continue;
}
index++;
}
}
Thank you for reading
Ok, I'm a bit confused about your line direction...
As far as i understand the normal array contains the face-normals (which could be calculated easily). Your testing line segment start at the balls position and goes along the facenormal of the face you're testing. That doesn't make much sense to me.
This line is funny:
Vector3 u = (b.transform.position + (nNormalized * b.radius)) - b.transform.position;
It's the same as
Vector3 u = nNormalized * b.radius;
I would recommend to rename your variable so they help to understand what they actually hold.
u is your line direction vector (the vector between start and end point)
Since your line direction is always the same as the face normal, this:
float nu = Vector3.Dot(u,nNormalized);
equals this:
float nu = b.radius;
That's a really strange case of intersection. Do you actually want a line-triangle or a sphere-triangle intersection?
edit
I've uploaded my test scene if you'r interested ;)
You can drag the sphere around during playmode when you switch to the scene-view. If there's an intersection point with the plane it will show as yellow 3D-cross
If the barycentric coordinates are within 0..1 (so it's inside the triangle) it will show the cross in magenta
I'm rather confused myself. I want to check if a line intersects with a triangle. Since we are talking about "spheres" representing the balls, I guess you could say I would want sphere-triangle intersection. But since I'm working with the radius I thought Ill just check if a vector from the center point of the ball to radius bounds intersect with a triangle. If so I need to reflect it on said face normal.
I need to find those parameters which are 0<= p <= 1.
For the direction vector, I was actually trying to create a vector wich points in the opposite direction ( not in this code i've shown) of the normal and see if that one intersects with the triangle...
The assignment is due tomorrow, and I have no clue how to fix this.
Thanks for the reply though.
I just tested the code and it works ;) well as far as it can...
The intersection just tests the line from ball center along the face-normal-vector therefore the ball can intersect at the edge of the triangle but it won't be recognised. If you're inside a "closed" geometry it doesn't matter (eg pool table )
Your line normal should be inverted. You've calculated it but you don't use it.
Define your "u" like this (note the "-"):
Vector3 u = -nNormalized * b.radius;
That would result in this "nu":
float nu = -b.radius;
I've note some strange cases where it detects a collision where it shouldn't... Something is still wrong :D
$$anonymous$$eh,
I still don't get it. I guess I'll have to retake this assignment next year. I'm just not seeing it anymore.
Thanks for the reply.
Got it :D
Copy&Paste error :D You used sParamTop TWO times. when calculating "t" you should use "tParamTop" ins$$anonymous$$d of "sParamTop"
This is a great post.
Do you have the final working script. I am making a game and need exactly this test. Would really appreciated it if someone can provide a working version.
Cheers.
This was like 2 years ago. But if you look at the answer given and the comments you should be able to copy paste the code above with the adjustments from the answer.
