Coordinates systems.

Hi there,

Being completely new and dummy to OpenGL, i have the following query regarding the use of coordinates systems within OpenGL.


  • Real world coordinates: latitude/longitude.
  • Screen coordinates: 1024x768 (X_SCREENxY_SCREEN)
  • OpenGL coordinates: (X_OPENGLxY_OPENGL)


Let’s suppose that the current area represented on the screen display (1024x768) refers only to the United Kingdom (a UK chart or map represented on an XGA picture).

Let’s suppose that i want to draw a circle with the centre having its latitude / longitude in Germany (i.e. outside the screen area), but the radius of this circle is big enough to have some part of the line of this circle visible on the current screen display, i.e. the UK chart.

How do i do that in terms of coordinates systems? Do i need to define a “view” which is bigger than the screen area and then clip everything which is outside the screen are? Or is there anything else that can be done???

Please reply to:

Thanks very much for your help!

You just draw the circle in Germany and OpenGL clips the stuff that is outside of United Kingdom.