## 10/25/2012

### Inner product(the angle between the two lines) - function source code

If there are 3 point(c, p1, p2), as shown in the above figure.
This function got the angle between the two lines using inner product.
Even if the name changed from (p1, p2) to (p2, p1),  the function get the narrow angle between the two lines.

I hope this code useful to you.
thank you.

```float innerAngle(float px1, float py1, float px2, float py2, float cx1, float cy1)
{

float dist1 = sqrt(  (px1-cx1)*(px1-cx1) + (py1-cy1)*(py1-cy1) );
float dist2 = sqrt(  (px2-cx1)*(px2-cx1) + (py2-cy1)*(py2-cy1) );

float Ax, Ay;
float Bx, By;
float Cx, Cy;

//find closest point to C
//printf("dist = %lf %lf\n", dist1, dist2);

Cx = cx1;
Cy = cy1;
if(dist1 < dist2)
{
Bx = px1;
By = py1;
Ax = px2;
Ay = py2;

}else{
Bx = px2;
By = py2;
Ax = px1;
Ay = py1;
}

float Q1 = Cx - Ax;
float Q2 = Cy - Ay;
float P1 = Bx - Ax;
float P2 = By - Ay;

float A = acos( (P1*Q1 + P2*Q2) / ( sqrt(P1*P1+P2*P2) * sqrt(Q1*Q1+Q2*Q2) ) );

A = A*180/PI;

return A;
}

```