I’m using OS X and Cocoa to create a viewport that has a transparent background. This is achieved by setting the NSOpenGLCPSurfaceOpacity parameter of a Cocoa NSOpenGLContext instance to 0.
The effect is that my gl drawing is nicely performed over the top of my desktop i.e. I can see right through the viewport and have my gl objects drawn on top - this is exactly what I want.
Unfortunately, performance is miserable on my old 500mhz G3. However if I set the opaqueness back to 1 then performance is great on my 500mhz G3.
I thus conclude that something interesting happens with OS X/OpenGL when a transparent background is being used.
Any thoughts on this anyone? Is there a better way to capure what is underneath my viewport and use that as a background image or something? I read somewhere that textures can be used to achieve this effect so any sample code would be highly appreciated.
Thanks for any help.