C++ Programming Code Examples C++ > Computer Graphics Code Examples Apply Above-Below-on Test to Find the Position of a Point with respect to a Line Apply Above-Below-on Test to Find the Position of a Point with respect to a Line This is a C++ Program to check whether point lies above, below or on the line. For any point t (xt, yt) on the plane, its position with respect to the line L connecting p and q is found by calculating the scalar s: s = A xt + B yt + C If s < 0, t lies in the clockwise halfplane of L; if s > 0, t lies on the counter-clockwise halfplane; if s = 0, t lies on L. For example, the equation of the line connecting points (2, 2) and (4, 5) is -3x + 2y + 2 = 0. The point (6, 3) lies in the clockwise halfplane of this line, because (-3)(6) + (2)(3) + 2 = -10. Conversely, the point (0, 5) lies in the other halfplane as (-3)(0) +(2)(5) +2 = 12. #include<time.h> #include<stdlib.h> #include<iostream> #include<math.h> using namespace std; const int LOW = 0; const int HIGH = 10; int main(int argc, char **argv) { time_t seconds; time(&seconds); srand((unsigned int) seconds); int x1, x2, y1, y2; x1 = rand() % (HIGH - LOW + 1) + LOW; x2 = rand() % (HIGH - LOW + 1) + LOW; y1 = rand() % (HIGH - LOW + 1) + LOW; y2 = rand() % (HIGH - LOW + 1) + LOW; cout << "The Equation of the 1st line is : (" << (y2 - y1) << ")x+(" << (x1 - x2) << ")y+(" << (x2 * y1 - x1 * y2) << ") = 0\n"; int x, y; cout << "\nEnter the point:"; cin >> x; cin >> y; int s = (y2 - y1) * x + (x1 - x2) * y + (x2 * y1 - x1 * y2); if (s < 0) cout << "The point lies below the line or left side of the line"; else if (s > 0) cout << "The point lies above the line or right side of the line"; else cout << "The point lies on the line"; return 0; }