ClippingTestResult CohenSutherlandLineClipper::ClipLine (LineSegment2F & line, Rectangle2F const& clipRect) const
{
// Until we have a trivial accept or reject,
// 1. Compute outcodes for start and end points
// 2. If start | end is 0, trivial accept
// Else if start & end is not 0, trivial reject
// Else,
// a) Find the outcode of the "out" point (the one whose outcode is non-zero),
// b) Update this to the intersection of the line and the appropriate clip rectangle border edge...take advantage of outcode when computing
auto result = ClippingTestResult::NON_TRIVIAL;
while (result == ClippingTestResult::NON_TRIVIAL)
{
auto outcode_start = ComputeOutcode(line.start, clipRect);
auto outcode_end = ComputeOutcode(line.end , clipRect);
if ((outcode_start | outcode_end) == 0) // both points are within clip rectangle
{
result = ClippingTestResult::TRIVIAL_ACCEPT;
}
else if ((outcode_start & outcode_end) != 0) // both points are outside of clip rectangle
{
result = ClippingTestResult::TRIVIAL_REJECT;
}
else // at least one point is within the clip rectangle
{
// Given two points (x0, y0), (x1, y1):
// Slope `m` = (y1 - y0) / (x1 - x0)
// Equation for `x` = x0 + (y - y0)/m
// Equation for `y` = y0 + (x - x0)*m
float x0 = line.start.x, x1 = line.end.x;
float y0 = line.start.y, y1 = line.end.y;
float m = (y1 - y0) / (x1 - x0);
float xmin = clipRect.x, xmax = clipRect.x + clipRect.width;
float ymin = clipRect.y - clipRect.height, ymax = clipRect.y;
auto IsStartOutside = [=](void){ return outcode_start != 0; };
auto outcode_out = IsStartOutside() ? outcode_start : outcode_end;
Point2F & updated = IsStartOutside() ? line.start : line.end;
// Update the point that we determined is outside the clip rect
if (outcode_out & LEFT) // set x = xmin, solve for y
{
updated.x = xmin;
updated.y = y0 + (updated.x - x0) * m;
}
else if (outcode_out & RIGHT) // set x = xmax, solve for y
{
updated.x = xmax;
updated.y = y0 + (updated.x - x0) * m;
}
else if (outcode_out & TOP) // set y = ymax, solve for x
{
updated.y = ymax;
updated.x = x0 + (updated.y - y0) / m;
}
else if (outcode_out & BOTTOM) // set y = ymin, solve for x
{
updated.y = ymin;
updated.x = x0 + (updated.y - y0) / m;
}
}
}
return result;
}
// Cohen-Sutherland clipping algo clips a line from
// P0=(x0,y) to P1=(x1,y1) against a rectangle with
// diagonal from (xmin,ymin) to (xmax,ymax)
void CohenSutherlandLineClipAndDraw(double x0,double y0,double x1,double y1)
{
outcode outcode0,outcode1,outcodeOut;
bool accept=false,done=false;
// compute outcodes
outcode0=ComputeOutcode(x0,y0);
outcode1=ComputeOutcode(x1,y1);
do
{
// logical or is 0 Trivially accept & exit
if(!(outcode0|outcode1))
{
accept=true;
done=true;
}
// logical and is not 0.Trivially reject & exit
else if(outcode0 & outcode1)
{
done=true;
}
else
{
// failed both tests,so calculate the line segment to clip
// from an outside point to an intersection with clip edge
double x,y;
//Atleast one endpoint is outside the clip rectangle,pick it.
outcodeOut=outcode0?outcode0:outcode1;
//Now find the intersection point
//use formula y=y0+slope*(x-x0), x=x0+(1/slope)*(y-y0)
//point is above the rectangular clip
if(outcodeOut & TOP)
{
x=x0+(x1-x0)*(ymax-y0)/(y1-y0);
y=ymax;
}
// point is below the clip rectangle
else if(outcodeOut & BOTTOM)
{
x=x0+(x1-x0)*(ymin-y0)/(y1-y0);
y=ymin;
}
// point lies right of clipping rectangle
else if(outcodeOut & RIGHT)
{
y=y0+(y1-y0)*(xmax-x0)/(x1-x0);
x=xmax;
}
// point lies leftside of clipping rectangle
else
{
y=y0+(y1-y0)*(xmin-x0)/(x1-x0);
x=xmin;
}
// now we move outside point to intersection point to
// clip and get ready for next pass
if(outcodeOut==outcode0)
{
x0=x;
y0=y;
outcode0=ComputeOutcode(x0,y0);
}
else
{
x1=x;
y1=y;
outcode1=ComputeOutcode(x1,y1);
}
}
}
while(!done);
if(accept)
{
// Window to viewport mappings
// scaling parameters
double sx=(xvmax-xvmin)/(xmax-xmin);
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 coloured 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();
// draws blue colour viewport
glColor3f(0.0,0.0,1.0);
glBegin(GL_LINES);
glVertex2d(vx0,vy0);
glVertex2d(vx1,vy1);
glEnd();
}
}
