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point inside triangle function - in C

Started by voidless Dec 29, 2008 at 5:47 PM 4 replies 3k views

Original Post

Dec 29, 2008 at 5:47 PM

hello everyone. this is my first post on the gamedev forums. ive recently started to code a smaller 3d application, and looked around the net after collision detection mechanisms. i have found this, and tried to implement the code in C (as the application is written in C. now my problem comes from the checkPointInTriangle functions return part (also here, a bit modified but still C plus plus). ive made a C code but im not sure if it does the same:


#include <stdio.h>
typedef float VECTOR[3];

#define VECTOR_DOT(v1,v2) (v1[0]*v2[0]+v1[1]*v2[1]+v1[2]*v2[2])

#define VECTOR_SUB(vector,v1,v2) vector[0]= v1[0]-v2[0]; vector[1]= v1[1]-v2[1]; vector[2]= v1[2]-v2[2];

typedef unsigned int uint32;

#define in(a) (*((uint32*)&a))

int check_point_in_triangle(VECTOR point, VECTOR v0, VECTOR v1, VECTOR v2){
 VECTOR e1;
 VECTOR e2;

 VECTOR_SUB(e1,v1,v0);
 VECTOR_SUB(e2,v2,v0);

 float a= VECTOR_DOT(e1,e1);
 float b= VECTOR_DOT(e1,e2);
 float c= VECTOR_DOT(e2,e2);

 float ac_bb= (a*c)-(b*b);

 VECTOR vp;

 VECTOR_SUB(vp,point,v0);

 float d= VECTOR_DOT(vp,e1);
 float e= VECTOR_DOT(vp,e2);
 float x= (d*c)-(e*b);
 float y= (e*a)-(d*b);
 float z= x+y-ac_bb;

 return ((in(z)& ~(in(x)|in(y)))& 0x80000000);
}

it sometimes even gives wrong results, for example for the triangle: (1,0,0) (0,1,1) (0,0,1) and point (1,1,1) it does not return 0. thank you

apatriarca

Dec 30, 2008 at 4:44 AM

I always use a function like that only after a point in the plane test. I think the point you have tested isn't in the plane defined by the triangle.

[Edited by - apatriarca on December 30, 2008 5:44:48 AM]

voidless

Dec 30, 2008 at 6:25 AM

thanks i was thinking about something like that too, but my main concern is the RETURN line.
if is that equivalent of the C plus plus functions return line (as there is no reference type in C)

samster581

Dec 31, 2008 at 8:57 PM

I would check to see if this is a Point in 3D triangle test, or a Point In 2D triangle. If it's 2D only, then it probably won't work for 3D points.

Halifax2

Dec 31, 2008 at 9:23 PM

Quote:
Original post by voidless
thanks i was thinking about something like that too, but my main concern is the RETURN line.
if is that equivalent of the C plus plus functions return line (as there is no reference type in C)

That isn't the reference operator, but rather the bitwise AND operator. It performs as such:

11010010 &01101011----------01000010

It's great for masking out data.

voidless

Jan 1, 2009 at 10:35 AM

Quote:
Original post by Halifax2
Quote:
Original post by voidless
thanks i was thinking about something like that too, but my main concern is the RETURN line.
if is that equivalent of the C plus plus functions return line (as there is no reference type in C)

That isn't the reference operator, but rather the bitwise AND operator. It performs as such:
11010010 &01101011----------01000010
It's great for masking out data.

i know that mainly its the bitwise AND, but in the original C plusplus contect, that line looked like:

typedef unsigned int unit32

#define in(a) ((uint32&)a)

and i rewrote that the way i posted it above in C:

typedef unsigned int unit32

#define in(a) (*((uint32*)&a))

and the return line is common between the C and Cplusplus implementation:

return ((in(z)& ~(in(x)|in(y)))& 0x80000000);

my question wast about the and operator, but rather about the park where in that define it casts a to an unsigned int reference type, and if is my code equivalent to that

oh and by the way in a previous function, it checks if the point is in the triangle plane before calling that function

thanks

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