OpenGL Not Drawing Anything - c++

Question

Just trying to draw a line between two points using glut for openGL version 3.0. Here's the code.
#include <GL/glut.h>
#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
void init(void) {
glClearColor(1.0,1.0,1.0,0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0,30.0,0.0,30.0);
}
void setPixel(GLint x, GLint y) {
glClearColor(1.0,1.0,1.0,0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0,30.0,0.0,30.0);
}
float roundValue(float v) {
return floor(v + 0.5);
}
void lineDDA(int xa, int ya, int xb, int yb) {
double dx = xb-xa, dy = yb-ya, steps;
float xInc, yInc, x = xa, y = ya;
if (abs(dx) > abs(dy))
steps = abs(dx);
else
steps = abs(dy);
xInc = dx/(float)steps;
yInc = dy/(float)steps;
setPixel(x, y);
int k;
for (k = 0; k < steps; ++k) {
/* code */
x += xInc;
y += yInc;
setPixel(roundValue(x), roundValue(y));
}
}
void update() {
glClear(GL_COLOR_BUFFER_BIT);
glPointSize(5.0f);
glColor3f(1, 0, 0);
lineDDA(1, 1, 8, 7);
lineDDA(1, 1, 8, 2);
glFlush();
}
int main(int argc,char **argv){
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowPosition(200,100);
glutInitWindowSize(640,480);
glutCreateWindow("Tutorial-1: Q4");
glLoadIdentity();
init();
glutDisplayFunc(update);
glutMainLoop();
return 0;
}
But this is not drawing anything on the window. It just remains white. Can anyone please suggest a solution?

Answer 1

Your setPixel function doesn't use its parameters x and y. You are not enabling line rendering mode or adding vertices to be drawn.
Fixing would consist of at least these steps:
Call glBegin(GL_LINES); after glPointSize(5.0f);
Call glEnd() before glFlush();
Modify setPixel to look like this:
void setPixel(GLint x, GLint y)
{
glVertex2f(x, y);
}