OpenGL - Scanline filling algorithm

Scanline.c

#include<stdio.h>
#include<GL/glut.h>
#include<math.h>

int le[500], re[500], flag=0 ,m;


	void init()
	{
		gluOrtho2D(0, 500, 0, 500);
	}


	void edge(int x0, int y0, int x1, int y1)
	{
		if (y1<y0)
		{
			int tmp;
			tmp = y1;
			y1 = y0;
			y0 = tmp;
			tmp = x1;
			x1 = x0;
			x0 = tmp;
		}
		int x = x0;
		m = (y1 - y0) / (x1 - x0);
		for (int i = y0; i<y1; i++)
		{
			if (x<le[i])
				le[i] = x;
			if (x>re[i])
				re[i] = x;
			x += (1 / m);
		}
	}

	void display()
	{
		glClearColor(1, 1, 1, 1);
		glClear(GL_COLOR_BUFFER_BIT);
		
		glColor3f(0, 0, 1);
		glBegin(GL_LINE_LOOP);
		glVertex2f(200, 100);
		glVertex2f(100, 200);
		glVertex2f(200, 300);
		glVertex2f(300, 200);
		glEnd();

		for (int i = 0; i<500; i++)
		{
			le[i] = 500;
			re[i] = 0;
		} 
		edge(200, 100, 100, 200);
		edge(100, 200, 200, 300);
		edge(200, 300, 300, 200);
		edge(300, 200, 200, 100);

		if (flag == 1)
		{
			for (int i = 0; i < 500; i++)
			{
				if (le[i] < re[i])
				{
					for (int j = le[i]; j < re[i]; j++)
					{
						glColor3f(1, 0, 0);
						glBegin(GL_POINTS);
						glVertex2f(j, i);
						glEnd();

					}

				}
			}
		}
		glFlush();
	}


	void ScanMenu(int id) {
		if (id == 1) 
			flag = 1;
		else if (id == 2) 
			flag = 0;
		else 
     			 exit(0);
		
		glutPostRedisplay();
	}


	void main(int argc, char **argv)
	{
		glutInit(&argc, argv);
		glutInitWindowPosition(100, 100);
		glutInitWindowSize(500, 500);
		glutCreateWindow("scan line");
		init();

		glutDisplayFunc(display);
		glutCreateMenu(ScanMenu);
		glutAddMenuEntry("scanfill", 1);
		glutAddMenuEntry("clear", 2);
		glutAddMenuEntry("exit", 3);
		glutAttachMenu(GLUT_RIGHT_BUTTON);
		glutMainLoop();
	}