Liang-Barsky Line Clipping Algorithm

#include <stdio.h>
#include <graphics.h>
#define X1 20
#define Y1 20
#define X2 80
#define Y2 110
#define XMIN 40
#define YMIN 40
#define XMAX 100
#define YMAX 90
#define M 3
int main(){
    int i, gd = DETECT,gm;
    initgraph(&gd, &gm, NULL);
    line(M*X1, M*Y1, M*X2, M*Y2);
    delay(5000);
    float p[4], q[4], r, t1 = 0, t2 = 1, xx1, yy1, xx2, yy2;
    p[0] = -(X2-X1);  p[1] = (X2-X1);
    p[2] = -(Y2-Y1);  p[3] = (Y2-Y1);
    q[0] = (X1-XMIN); q[1] = (XMAX-X1);
    q[2] = (Y1-YMIN); q[3] = (YMAX-Y1);
    for(i=0; i < 4; i++){
        r = q[i]/p[i];
        if(p[i] == 0 && q[i] < 0){
        printf("Line is completly outside.\n");
        return;
        }
        else if(r > t1 && p[i] < 0) t1 = r;
        else if(r < t2 && p[i] > 0) t2 = r;
    }
    cleardevice();
    if(t1 > t2){
        printf("Line is completly outside.\n");
        return 0;
    } else{
        printf("t1 = %f, t2 = %f\n", t1, t2);
        xx1 = X1 + t1 * (X2 - X1);
        yy1 = Y1 + t1 * (Y2 - Y1);
        xx2 = X1 + t2 * (X2 - X1);
        yy2 = Y1 + t2 * (Y2 - Y1);
        printf("(%.2f, %.2f) to (%.2f, %.2f)\n", xx1, yy1, xx2, yy2);
        rectangle(M*XMIN, M*YMIN, M*XMAX, M*YMAX);
        line(M*xx1, M*yy1, M*xx2, M*yy2);
        delay(5000);
    }
}