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Trolltech公司发布的基于C++图形开发环境

The overlay example

This example program shows how to use OpenGL overlays with the Qt OpenGL
Extension. It contains a QGLWidget with a relatively expensive
redrawing operation, and rubber-band drawing in the overlay plane.
Using the overlay has the advantage that the rubber-band may be drawn
and erased without damaging the image in the main plane, so costly
redraws are avoided.