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Fix bug in algorithm |
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# For nonzero <math>p_k</math>, <math>u = \frac{q_k}{p_k}</math> gives the intersection point.
# For each line, calculate <math>u_1</math> and <math>u_2</math>. For <math>u_1</math>, look at boundaries for which <math>p_k < 0</math> (outside -> in). Take <math>u_1</math> to be the largest among <math>\left(0,\frac{q_k}{p_k}\right)</math>. For <math>u_2</math>, look at boundaries for which <math>p_k > 0</math> (inside -> out). Take <math>u_2</math> to be the minimum of <math>\left(1, \frac{q_k}{p_k}\right)</math>. If <math>u_1 > u_2</math>, the line is outside and therefore rejected.
'''Implementation of algorithm in c/c++:'''
double xmin=50,ymin=50, xmax=100,ymax=100; // Window boundaries
double xvmin=200,yvmin=200,xvmax=300,yvmax=300; // Viewport boundaries
int cliptest(double p, double q, double *t1, double *t2)
{
double t=q/p;
if(p < 0.0) // potentially enry point, update te
{
if( t > *t1) *t1=t;
if( t > *t2) return(false); // line portion is outside
}
else
if(p > 0.0) // Potentially leaving point, update tl
{
if( t < *t2) *t2=t;
if( t < *t1) return(false); // line portion is outside
}
else
if(p == 0.0)
{
if( q < 0.0) return(false); // line parallel to edge but outside
}
return(true);
}
void LiangBarskyLineClipAndDraw (double x0, double y0,double x1, double y1)
{
double dx=x1-x0, dy=y1-y0, te=0.0, tl=1.0;
if(cliptest(-dx,x0-xmin,&te,&tl)) // inside test wrt left edge
if(cliptest(dx,xmax-x0,&te,&tl)) // inside test wrt right edge
if(cliptest(-dy,y0-ymin,&te,&tl)) // inside test wrt bottom edge
if(cliptest(dy,ymax-y0,&te,&tl)) // inside test wrt top edge
{
if( tl < 1.0 )
{
x1 = x0 + tl*dx;
y1 = y0 + tl*dy;
}
if( te > 0.0 )
{
x0 = x0 + te*dx;
y0 = y0 + te*dy;
}
// Window to viewport mappings
double sx=(xvmax-xvmin)/(xmax-xmin); // Scale parameters
double sy=(yvmax-yvmin)/(ymax-ymin);
double vx0=xvmin+(x0-xmin)*sx;
double vy0=yvmin+(y0-ymin)*sy;
double vx1=xvmin+(x1-xmin)*sx;
double vy1=yvmin+(y1-ymin)*sy;
//draw a red colored viewport
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINE_LOOP);
glVertex2f(xvmin, yvmin);
glVertex2f(xvmax, yvmin);
glVertex2f(xvmax, yvmax);
glVertex2f(xvmin, yvmax);
glEnd();
glColor3f(0.0,0.0,1.0); // draw blue colored clipped line
glBegin(GL_LINES);
glVertex2d (vx0, vy0);
glVertex2d (vx1, vy1);
glEnd();
}
}// end of line clipping
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