public void SetViewportSize(int viewportWidth, int viewportHeight)
{
var aspectRatio = viewportWidth / (float)viewportHeight;
ProjectionMatrix = Matrix4.CreatePerspectiveFieldOfView(MathHelper.PiOver4, aspectRatio, 1.0f, 40000.0f);
// setup projection
GL.Viewport(0, 0, viewportWidth, viewportHeight);
}