# "Preventing" collision detection

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Alessandro    302
Hello folks, I've created a basic collision detection routine following this document: http://www.lighthouse3d.com/tutorials/maths/ray-triangle-intersection/

I'd like to monitor the distance from the point P and the triangle being tested for intersection, so that I can stop the routine when the distance falls below a certain value.

I tried modifying the 0.00001 values in the code thinking that it was meant to be a tolerance value for such collision "distance", but it seems to be ineffective.

What do you think? There is a way to do it? Here below is the routine derived from the document above:

[code]
int rayIntersectsTriangle(vector3_t orig, vector3_t dir, vector3_t v0, vector3_t v1, vector3_t v2) {
float a,f,u,v,t;
vector3_t e1= v1-v0;
vector3_t e2= v2-v0;
vector3_t h=dir^e2;
a = e1*h;
if (a > -0.00001 && a < 0.00001) return(false);
f = 1.0/a;
vector3_t s=orig-v0;
u = f * (s*h);
if (u < 0.0 || u > 1.0) return(false);
vector3_t q=s^e1;
v = f * (dir*q);
if (v < 0.0 || u + v > 1.0) return(false);
// at this stage we can compute t to find out where the intersection point is on the line
t = f * (e2*q);
if (t > 0.00001) // ray intersection
return(true);
else // this means that there is a line intersection
// but not a ray intersection
return (false);
}
[/code]

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C0lumbo    4411
Can you try to describe more clearly what you mean by "I'd like to monitor the distance from the point P and the triangle being tested for intersection, so that I can stop the routine when the distance falls below a certain value."

FWIW, the line if (a > -0.00001 && a < 0.00001) return(false); is testing if the ray is parallel to the triangle face, if it is, then the function is deciding that there's no collision.

Is it possible that what you want is for your ray to have some thickness? If so, then you need to look at swept/moving sphere vs triangle intersections.

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Alessandro    302
Sorry for not being more precise. Take a look at the attached image. You see there is a point [size=5][b]p[/b][/size] the is being moved in direction [size=5][i][b]v[/b][/i][/size], and the collision point C. I'd like to "monitor" the distance between the point [size=5][b]p[/b][/size] and the collision point [b][size=5]C[/size][/b], so that I can stop the routine before it actually collides.

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C0lumbo    4411
In your function, t is the distance between point p and point c (assuming dir is normalised)

So, changing "if (t > 0.00001)" to "if (t > 1.0)" would make the function ignore collisions that occur within one unit of the origin of the ray.

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Alessandro    302
Sorry for not being more precise. Take a look at the attached image. You see there is a point [size=5][b]p[/b][/size] the is being moved in direction [size=5][i][b]v[/b][/i][/size], and the collision point [size=5][b]C[/b][/size].
I'd like to "monitor" the distance between the point [size=5][b]p[/b][/size] and the collision point [b][size=5]C[/size][/b], so that I can stop the routine before it actually collides.

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Alessandro    302
Also, I'd like to ask if there are faster algorithms that do ray triangle intersection, or this is just the way to go.

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SimonForsman    7642
To get the distance between two points(p and c) just treat them as vectors and you get: distance = ||c-p||

are you trying to prevent collision [b]detection[/b] like the post title says or are you trying to stop actors before a collision occurs ? Edited by SimonForsman

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Alessandro    302
[quote name='SimonForsman' timestamp='1353003308' post='5001297']
To get the distance between two points(p and c) just treat them as vectors and you get: distance = ||c-p||

are you trying to prevent collision [b]detection[/b] like the post title says or are you trying to stop actors before a collision occurs ?
[/quote]

Actually the 2nd option would be perhaps a better option for my needs.

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Alessandro    302
[quote name='C0lumbo' timestamp='1352970376' post='5001156']
In your function, t is the distance between point p and point c (assuming dir is normalised)

So, changing "if (t > 0.00001)" to "if (t > 1.0)" would make the function ignore collisions that occur within one unit of the origin of the ray.
[/quote]

Thank you very much, that was successful.

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