Check out example codes for "Find the intersection point at the window boundary (base on region code)". It will help you in understanding the concepts better.

Code Example 1

// C++ program to implement Cohen Sutherland algorithm 
// for line clipping. 
#include <iostream> 
using namespace std; 
  
// Defining region codes 
const int INSIDE = 0; // 0000 
const int LEFT = 1; // 0001 
const int RIGHT = 2; // 0010 
const int BOTTOM = 4; // 0100 
const int TOP = 8; // 1000 
  
// Defining x_max, y_max and x_min, y_min for 
// clipping rectangle. Since diagonal points are 
// enough to define a rectangle 
const int x_max = 10; 
const int y_max = 8; 
const int x_min = 4; 
const int y_min = 4; 
  
// Function to compute region code for a point(x, y) 
int computeCode(double x, double y) 
{ 
    // initialized as being inside 
    int code = INSIDE; 
  
    if (x < x_min) // to the left of rectangle 
        code |= LEFT; 
    else if (x > x_max) // to the right of rectangle 
        code |= RIGHT; 
    if (y < y_min) // below the rectangle 
        code |= BOTTOM; 
    else if (y > y_max) // above the rectangle 
        code |= TOP; 
  
    return code; 
} 
  
// Implementing Cohen-Sutherland algorithm 
// Clipping a line from P1 = (x2, y2) to P2 = (x2, y2) 
void cohenSutherlandClip(double x1, double y1, 
                         double x2, double y2) 
{ 
    // Compute region codes for P1, P2 
    int code1 = computeCode(x1, y1); 
    int code2 = computeCode(x2, y2); 
  
    // Initialize line as outside the rectangular window 
    bool accept = false; 
  
    while (true) { 
        if ((code1 == 0) && (code2 == 0)) { 
            // If both endpoints lie within rectangle 
            accept = true; 
            break; 
        } 
        else if (code1 & code2) { 
            // If both endpoints are outside rectangle, 
            // in same region 
            break; 
        } 
        else { 
            // Some segment of line lies within the 
            // rectangle 
            int code_out; 
            double x, y; 
  
            // At least one endpoint is outside the 
            // rectangle, pick it. 
            if (code1 != 0) 
                code_out = code1; 
            else
                code_out = code2; 
  
            // Find intersection point; 
            // using formulas y = y1 + slope * (x - x1), 
            // x = x1 + (1 / slope) * (y - y1) 
            if (code_out & TOP) { 
                // point is above the clip rectangle 
                x = x1 + (x2 - x1) * (y_max - y1) / (y2 - y1); 
                y = y_max; 
            } 
            else if (code_out & BOTTOM) { 
                // point is below the rectangle 
                x = x1 + (x2 - x1) * (y_min - y1) / (y2 - y1); 
                y = y_min; 
            } 
            else if (code_out & RIGHT) { 
                // point is to the right of rectangle 
                y = y1 + (y2 - y1) * (x_max - x1) / (x2 - x1); 
                x = x_max; 
            } 
            else if (code_out & LEFT) { 
                // point is to the left of rectangle 
                y = y1 + (y2 - y1) * (x_min - x1) / (x2 - x1); 
                x = x_min; 
            } 
  
            // Now intersection point x, y is found 
            // We replace point outside rectangle 
            // by intersection point 
            if (code_out == code1) { 
                x1 = x; 
                y1 = y; 
                code1 = computeCode(x1, y1); 
            } 
            else { 
                x2 = x; 
                y2 = y; 
                code2 = computeCode(x2, y2); 
            } 
        } 
    } 
    if (accept) { 
        cout << "Line accepted from " << x1 << ", "
             << y1 << " to " << x2 << ", " << y2 << endl; 
        // Here the user can add code to display the rectangle 
        // along with the accepted (portion of) lines 
    } 
    else
        cout << "Line rejected" << endl; 
} 
  
// Driver code 
int main() 
{ 
    // First Line segment 
    // P11 = (5, 5), P12 = (7, 7) 
    cohenSutherlandClip(5, 5, 7, 7); 
  
    // Second Line segment 
    // P21 = (7, 9), P22 = (11, 4) 
    cohenSutherlandClip(7, 9, 11, 4); 
  
    // Third Line segment 
    // P31 = (1, 5), P32 = (4, 1) 
    cohenSutherlandClip(1, 5, 4, 1); 
  
    return 0; 
}

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