CODEWITHSUNDEEP
c++opengl
peco
peco

Reputation: 1451

OpenGL - gluOrtho2d and glLoadIdentity?

I'm starting out on OpenGL. I'm not sure how gluOrtho2d and glLoadIdentity play out together. In particular, I have the following code, which is supposed to draw a 2x2 square in the center of a 10x10 display.

int main() {
    glutCreateWindow("Draw A Square");
    glutDisplayFunc(mydisplay);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-5.0, 5.0, -5.0, 5.0);

    glutMainLoop();
}

void mydisplay() {
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    drawAUnitSquare();

    glFlush();
}

void drawAUnitSquare() {
    // glLoadIdentity();
    glBegin(GL_POLYGON);
        glColor3f(1.0, 0.0, 0.0);
        glVertex2f(-1, -1);
        glVertex2f(1, -1);
        glVertex2f(1, 1);
        glVertex2f(-1, 1);
    glEnd();
}

The above code works. However, if I uncomment the glLoadIdentity(); in drawAUnitSquare(), the square then fills the entire viewport. What is happening here?

Upvotes: 1

Views: 629

Answers (1)

Axalo
Axalo

Reputation: 2953

You forgot to switch the matrix mode back to GL_MODELVIEW.

Just add glMatrixMode(GL_MODELVIEW); after gluOrtho2D(-5.0, 5.0, -5.0, 5.0);.

Upvotes: 1

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