Algorithm:
Step 1: Start
Step 2: Given a line segment with endpoint P1 = (x1, y1) and P2 = (x2, y2).
Step 3: Compute the 4-bit codes for each endpoint. If both codes are 0000, (bitwise OR of the codes yields 0000) line lies completely inside the window: pass the endpoints to the draw routine. If both codes have a 1 in the same bit position (bitwise AND of the codes is not 0000), the line lies outside the window. It can be trivially rejected.
Step 4: If a line cannot be trivially accepted or rejected, at least one of the two end points must lie outside the window and the line segment crosses a window edge. This line must be clipped at the window edge before being passed to the drawing routine.
Step 5: Examine one of the endpoints, say P1 = (x1, y1). Read P1’s 4-bit code in order: Left-to-Right, Bottom-to-Top.
Step 6: When a set bit (1) is found, compute the intersection I of the correspondingwindow edge with the line from P1 to P2.
Step 7: Stop
/* Cohen-Sutherland Line Clipping Algorithm with Window to viewport Mapping */
#include <stdio.h>
#include <GL/glut.h>
#define outcode int
double xmin=50,ymin=50, xmax=100,ymax=100;
// Window boundaries //bit codes for the top,bottom,right & left
const int TOP = 1;
const int BOTTOM = 2;
const int RIGHT = 4;
const int LEFT = 8;
//used to compute bit codes of a point
outcode ComputeOutCode (double x, double y);
//Cohen-Sutherland clipping algorithm clips a line from
P0 = (x0, y0) 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)
{
//Outcodes for P0, P1, and whatever point lies outside the clip rectangle
outcode outcode0, outcode1, outcodeOut;
bool accept = false, done = false;
//compute outcodes
outcode0 = ComputeOutCode (x0, y0);
outcode1 = ComputeOutCode (x1, y1);
do
{
if (!(outcode0 | outcode1)) //logical or is 0 Trivially accept & exit
{
accept = true;
done = true;
}
else if (outcode0 & outcode1)
//logical and is not 0. Trivially reject & exit
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;
//At least one endpoint is outside the clip rectangle; pick it.
outcodeOut = outcode0? outcode0: outcode1;
//Now find the intersection point;
//use formulas y = y0 + slope * (x - x0), x = x0 + (1/slope)* (y - y0)
if (outcodeOut & TOP) //point is above the clip rectangle
{
x = x0 + (x1 - x0) * (ymax - y0)/(y1 - y0);
y = ymax;
}
else if (outcodeOut & BOTTOM) //point is below the clip rectangle
{
x = x0 + (x1 - x0) * (ymin - y0)/(y1 - y0);
y = ymin;
}
else if (outcodeOut & RIGHT)
//point is to the right of clip rectangl
{
y = y0 + (y1 - y0) * (xmax - x0)/(x1 - x0);
x = xmax;
}
else //point is to the left of clip rectangle
{
y = y0 + (y1 - y0) * (xmin - x0)/(x1 - x0);
x = xmin;
} //Now we move outside point to intersection point to clip //and gets 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)
{
//draw a red colored viewport
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINE_LOOP);
glVertex2f(xmin+200, ymin+200);
glVertex2f(xmax+200, ymin+200);
glVertex2f(xmax+200, ymax+200);
glVertex2f(xmin+200, ymax+200);
glEnd();
glColor3f(0.0,0.0,1.0);
// draw blue colored clipped line
glBegin(GL_LINES);
glVertex2d (x0+200, y0+200);
glVertex2d (x1+200, y1+200);
glEnd();
}
}
//Compute the bit code for a point (x, y) using the clip rectangle
//bounded diagonally by (xmin, ymin), and (xmax, ymax)
outcode ComputeOutCode (double x, double y)
{
outcode code = 0;
if (y > ymax)
//above the clip window
code |= TOP;
else if (y < ymin)
//below the clip window
code |= BOTTOM;
if (x > xmax)
//to the right of clip
window code |= RIGHT;
else if (x < xmin)
//to the left of clip
window code |= LEFT;
return code;
}
void display()
{
double x0=120,y0=10,x1=40,y1=130;
glClear(GL_COLOR_BUFFER_BIT);
//draw the line with red color
glColor3f(1.0,0.0,0.0);
glBegin(GL_LINES);
glVertex2d (x0, y0);
glVertex2d (x1, y1);
glVertex2d (10,70);
glVertex2d (250,90); glEnd(); //draw a blue colored window
glColor3f(0.0, 0.0, 1.0);
glBegin(GL_LINE_LOOP);
glVertex2d(xmin, ymin);
glVertex2d(xmax, ymin);
glVertex2d(xmax, ymax);
glVertex2d(xmin, ymax);
glEnd();
CohenSutherlandLineClipAndDraw(x0,y0,x1,y1);
CohenSutherlandLineClipAndDraw(10,70,250,90);
glFlush();
}
void myInit() {
glClearColor(1.0,1.0,1.0,1.0);
glColor3f(1.0,0.0,0.0);
glPointSize(1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0,499.0,0.0,499.0);
}
int main(int argc, char** argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(0,0);
glutCreateWindow("Cohen Sutherland Line Clipping Algorithm");
glutDisplayFunc(display);
myInit();
glutMainLoop();
return 0;
}
