Stretched Drawing in OpenGL ES - iOS

Question

I'm trying to draw a simple circle using OpenGL ES. The problem is that the circle is stretched vertically. It looks more like an ellipse than a circle. Could someone point out where the things are going wrong?

I played around with glViewPort to fix this but was not successful. As someone else suggested here on Stackoverflow, I also tried loading a different matrix instead of the identity matrix and that doesn't work too...

Here's the code of drawFrame:

- (void)drawFrame
{
    [(EAGLView *)self.view setFramebuffer];

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();       

    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);   

    GLfloat vertices[720];
    for (int i = 0; i < 720; i += 2) 
    {
        vertices[i]   = (cos(degreesToRadians(i)) * 1);
        vertices[i+1] = (sin(degreesToRadians(i)) * 1);
    }

    glVertexPointer(2, GL_FLOAT, 0, vertices);
    glEnableClientState(GL_VERTEX_ARRAY);
    glColor4f(1.0f, 0.0f, 0.0f, 1.0f);
    glDrawArrays(GL_TRIANGLE_FAN, 0, 360);

    [(EAGLView *)self.view presentFramebuffer];
}

score 2 · Accepted Answer · answered Aug 09 '11 at 20:02

The code you are showing will draw a perfect circle in world co-ordinates. What you need to consider is how those world co-ordinates transform into window co-ordinates i.e. pixels.

If the glViewport is set to always match the window then it's the aspect ratio of the window that will determine what you see using the code sample you have shown. If the window is square it will work i.e. you will see a perfect circle. If the window is taller than it is wide then the circle will be stretched vertically.

To preserve the perfect circle you can use a projection matrix that gives you a viewing volume of the same aspect ratio as the viewport/window. I noticed that before your first edit you had a call to glOrthof in there. Set the aspect ratio to match there and that will do the job for you. If you want a perspective projection instead of an orthographic projection then use glFrustum.

Thanks. I combined your solution with the one I got at http://stackoverflow.com/questions/1067833/why-opengles-stretch-my-3d-model and it helped me. Although I could not get the glViewPort/glOrthof thing working, I created a new matrix with values (as below) and loaded it: GLfloat aspectRatio = self.view.bounds.size.height/self.view.bounds.size.width; float m[16] = { aspectRatio, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1 }; glMatrixMode(GL_PROJECTION); glLoadMatrixf(m); It's crazy -> it works, but I don't understand how. — Ravi, Aug 09 '11 at 21:17

Stretched Drawing in OpenGL ES - iOS

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