OpenGL: GL_QUADS does not draw square

Question

I'm trying to draw a square on the screen but it clearly draws a rectangle.

This is my render code:

glClear(GL_COLOR_BUFFER_BIT);
glTranslatef(0,0,-0.1);
glBegin(GL_QUADS);
    glVertex3f(0,0,0);
    glVertex3f(1,0,0);
    glVertex3f(1,1,0);
    glVertex3f(0,1,0);
glEnd();

SDL_GL_SwapBuffers();

And OpenGL Init code:

glClearColor(0,0,0,0.6f);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(30,640.0/480.0,.3f,200.0);
glMatrixMode(GL_MODELVIEW);

Why is this happening?

maybe your screens horizontal dpi does not match vertical dpi ? — huseyin tugrul buyukisik, Aug 15 '12 at 14:31
The dpi of the screen would not affect his screenshot though. — Tim, Aug 15 '12 at 14:33
I don't know what I did but it's now drawing the square normally. I didn't change anything so I don't know what was wrong. Just the universe trolling me I guess :( — Mertcan Ekiz, Aug 15 '12 at 14:34

tiguero · Accepted Answer · 2012-08-15T15:11:25.743

6

I don't see anywhere in your code where you have set-up the glViewport. I will rather write something like this in your init method:

glViewport(0,0,640,480);        // Reset The Current Viewport

glMatrixMode(GL_PROJECTION);    // Select The Projection Matrix
glLoadIdentity();               // Reset The Projection Matrix

// Calculate The Aspect Ratio Of The Window
gluPerspective(30.0f,(GLfloat)640/(GLfloat)480,0.3f,200.0f);

glMatrixMode(GL_MODELVIEW);     // Select The Modelview Matrix
glLoadIdentity();

also check the second Nehe tutorial it will help you to start with OpenGL for very basic stuff like drawing primitives such as triangle, square etc...

edited Aug 15 '12 at 15:11

answered Aug 15 '12 at 15:03

tiguero

11,477
5
43
61

Thanks. I don't know what glViewPort does but casting to GLfloat did the trick for me. – Mertcan Ekiz Aug 15 '12 at 21:03
Ok i have just read that when an OpenGL context is first attached to a window, width and height are set to the dimensions of that window so it might be not necessary. As for the glViewport command it defines a transformation from normalized device coordinates (that is, post-projection matrix coordinates with the perspective divide applied) to window coordinates – tiguero Aug 15 '12 at 21:35

score 0 · Answer 2 · answered Aug 15 '12 at 15:15

0

Try using gluOrtho2D to generate a correct orthogonal projection matrix, in your case gluOrtho2D(0,640,0,480), this is assuming you want a square in 2D and not 3D.

This will of course change your coordinate system from (0,1),(0,1) to (0,640),(0,480).

answered Aug 15 '12 at 15:15

Necrolis

25,836
3
63
101

OpenGL: GL_QUADS does not draw square

2 Answers2