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How would one get the relative size of the viewing plane in opengl's own units? I need to find out the width and height in "opengl units". Is there a function which will retrieve this information?

I assume that one unit (let us say 1.0f) in Z would be equivalent to one unit in X, even if conversion to a real measurement system in meaningless.

I know I can get the screen size either by use of GetSystemMetrics(SM_CXSCREEN) or glutGet(GLUT_SCREEN_WIDTH), but this is in pixels.

To handle the graphical window calls, I am using freeglut on non-windows OSes and the WinAPI on Windows.

Derek 朕會功夫
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user2063669
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    What exactly are you trying to do here – Aesthete Feb 12 '13 at 06:26
  • The question is invalid as written. Assuming you're using a perspective projection, there is no "viewing plane". The width of the screen depends on what Z distance you measure it at. If you're talking about a fixed, known distance (such as -1.0 Z), then that's answerable. – Nicol Bolas Feb 12 '13 at 06:36
  • I guess another way of saying it would be that I'm trying to find out the largest line I can draw parallel to the X axis in the screen plane that doesn't exceed the view-port’s dimensions. – user2063669 Feb 12 '13 at 06:36
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    It would be two units. – Aki Suihkonen Feb 12 '13 at 06:51
  • That doesn't make any sense to me, Aki Suihkonen. Why would it always be two? Is there at least a way to set the value? – user2063669 Feb 12 '13 at 07:27
  • It's two, because the screen range goes from -1 to 1. The difference between them is two. – Skalli Feb 12 '13 at 07:38
  • I'm not an expert, but you probably want to look into 'spaces' (e.g. clip space, camera space, world space) and how and why one transforms between them. Another useful thing to understand is projection. Once you understand those you will see why your question may not make sense. – Samuel Harmer Feb 12 '13 at 08:15

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Assuming you want to draw something like a UI, set your projection matrix to an Orthographic matrix with glOrtho, then you don't have any perspective and have a direct orthographic mapping between world coordinates and screen coordinates. The arguments to your glOrtho call determine how wide/high your view port is in world coordinates.

If you want to draw both a UI and a 3D scene, draw the UI with glOrtho and draw the scene with glPerspective using a clipping mask to make sure you don't ruin your UI.

If on the other hand you want to know the width of the view port in a 3D scene with perspective, so that you know how big to draw your object then you'll have to deal with the perspective projection. You need to know at which Z coordinate you want to know the witdh/height of the view port. You can use gluUnProject to calculate the world coordinate corresponding to a given screen coordinate and Z plane.

However it would probably be better to do it the other way around, always draw your object with a given size and then calculate what your projection matrix should be to have that object appear properly in your view port.

wich
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