Question

This simple program was designed to draw 13 even, red and white stripes placed alternately, as in the US flag.

// A Simple OpenGL Project
// Author: Michael Hall
//
// This C++ code and project are provided "as is" without warranty of any kind.
//
// Copyright 2010 XoaX - For personal use only, not for distribution
// 
// Elaborated by me :)

#include <glut.h>

void DrawStripes(const int quantity)
{
    glBegin(GL_QUADS);

    for(int i=1; i <= quantity ; i++)
    {
        if(i%2)
            glColor3f(1.0,0.0,0.0);
        else 
            glColor3f(1.0,1.0,1.0);

            glVertex2f(0,static_cast<float>(i-1)/static_cast<float>(quantity));
            glVertex2f(1.0,static_cast<float>(i-1)/static_cast<float>(quantity));

            glVertex2f(0,static_cast<float>(i)/static_cast<float>(quantity));
            glVertex2f(1.0,2*static_cast<float>(i)/static_cast<float>(quantity));
    }
}



void Draw() 
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.0, 0.0, 1.0);
    DrawStripes(13);


    glEnd();
    glFlush();
}

void Initialize() 
{
    glClearColor(0.0, 0.0, 102.0/255.0, 0.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}

int main(int iArgc, char** cppArgv) 
{
    glutInit(&iArgc, cppArgv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(950,500);
    glutInitWindowPosition(200, 200);
    glutCreateWindow("Rough draft");
    Initialize();


    glutDisplayFunc(Draw);
    glutMainLoop();

    return 0;
}

This is how it looks in practice:

Why there are blue triangles on the foreground? I’ve specified only “QUADS” inside Draw/DrawStripes functions.

Answer 1

I think you’re drawing the quad vertices in the wrong order. They should be counterclockwise, but you’re drawing a ‘twisted’ quad.

Try swapping the 3rd and 4th vertex in your for loop.