Question

I have a simple OpenGL drawing. When the user changes the window’s size, I want the drawing to maintain it’s aspect ration. I accomplished that by setting the glViewport to the maximum rectangle with the appropriate aspect ration whenever the reshape method is called.

My problem is that I want to draw a square that will always remain in the top right corner of the window, no matter what the size or shape of the window is. Right now, that square moves around the screen whenever the window is reshaped.

Can anyone please explain how to do this?

Thank you,

Answer 1

If I understand correctly, you wish to draw a square at the top right corner of the window, regardless of where your scene viewport is positioned.

The easiest way to do this is to, after having rendered your normal scene with desired aspect, change the gl viewport to the square you want to draw in the top corner. Then draw a “full-screen” quad to fill the square, with full-screen in-fact becoming full-viewport in this case.

Untested semi-pseudo code would go something like this:

// Draw normal scene
glViewport(x, y, w, h);
drawScene();

// Draw top-right red square
glViewport( windowWidth - squareWidth, windowHeight - squareHeight,
            squareWidth, squareHeight );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glColor3f(1.f, 0.f, 0.f);
glRectf(-1.f, -1.f, 1.f, 1.f);

Making sure that the winding of the glRectf matches your current gl cull face configuration. Alternatively, just temporarily disable culling by glDisable(GL_CULL_FACE) / glEnable(GL_CULL_FACE).