Trouble drawing Sierpinski with OpenGL - c++

Question

I am trying to generate the Sierpinski Gasket using a function that draws dot patterns and will generate the gasket.
But when I compile and run the program it displays nothing but black screen. What's causing the problem?
Here is my code:
#include <Windows.h>
#include <gl/GL.h>
#include <glut.h>
void myInit(void) {
glClearColor(1.0, 1.0, 1.0, 0.0);
glColor3f(0.0f, 0.0f, 0.0f);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, 640.0, 0.0, 480.0);
}
class GLintPoint {
public:
GLint x, y;
};
int random(int m) {
return rand() % m;
}
void drawDot(GLint x, GLint y)
{
glBegin(GL_POINTS);
glVertex2i(x, y);
glEnd();
}
void Sierpinski(void) {
GLintPoint T[3] = {{ 10, 10 }, {300, 30}, {200, 300}};
int index = random(3);
GLintPoint point = T[index];
for (int i = 0; i < 1000; i++)
{
index = random(3);
point.x = (point.x + T[index].x) / 2;
point.y = (point.y + T[index].y) / 2;
drawDot(point.x, point.y);
}
glFlush();
}
void main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(100, 150);
glutCreateWindow("Sierpinski Gasket");
glutDisplayFunc(drawDot);
myInit();
glutMainLoop();
}

Answer 1

If you want to draw black dots on a white background, then you have to clear glClear the background:
glClear( GL_COLOR_BUFFER_BIT );
Note, glClearColor sets the color which is used to clear the view port, but doesn't clear anything itself.
Your code should look somehow like this:
void drawDot(GLint x, GLint y)
{
glVertex2i(x, y);
}
void Sierpinski(void) {
GLintPoint T[3] = {{ 10, 10 }, {300, 30}, {200, 300}};
int index = random(3);
GLintPoint point = T[index];
glClearColor(1.0, 1.0, 1.0, 1.0); // set up white clear color
glClear( GL_COLOR_BUFFER_BIT ); // clear the back ground (white)
glMatrixMode( GL_MODELVIEW );
glPushMatrix(); // setup model matrix
glScalef( 1.5f, 1.5f, 1.0f ); // scale the point distribution
glColor3f( 0.0f, 0.0f, 0.0f ); // set black draw color
glPointSize( 5.0f ); // set the size of the points
glBegin(GL_POINTS);
for (int i = 0; i < 1000; i++)
{
index = random(3);
point.x = (point.x + T[index].x) / 2;
point.y = (point.y + T[index].y) / 2;
drawDot(point.x, point.y);
}
glEnd();
glPopMatrix(); // reset model matrix
glFlush();
}