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Important: This document is part of the Legacy section of the ADC Reference Library. This information should not be used for new development.

Current information on this Reference Library topic can be found here:

Correct Setup of an AGLDrawable

Q: How do I correctly set up and AGLDrawable?

A. One uses aglSetDrawable to attach an AGLDrawable to an AGLContext. This function is declared as follows:

    GLboolean aglSetDrawable (AGLContext context, AGLDrawable drawable);
- where context is a valid context return from aglCreateContext.

drawable is an AGLDrawable, which is defined in <AGL.h> as a CGrafPtr. This AGLDrawable must be a CGrafPtr obtained from a valid window. Under Mac OS 9 one can directly cast a WindowPtr to an AGLDrawable. Carbon is more restrictive due to it's opaque data references. One must use the function GetWindowPort to properly obtain a AGLDrawable from an opaque WindowPtr or WindowRef.

The following example code can be used, on Mac OS 9 without Carbon and on Mac OS 9 and Mac OS X with Carbon, to ensure the proper type is passed to the aglSetDrawable function:
        aglSetDrawable (context, GetWindowPort (pWindow));
        aglSetDrawable (context, (AGLDrawable) pWindow);

Q. What about DrawSprocket's front buffer? Isn't that really just a CGrafPtr?

A. Not really, DrawSprocket's front buffer is a non-standard CGrafPtr that causes problems for OpenGL when trying to identify the correct display device for rendering. This front buffer should never be used as an AGLDrawable. Instead one should create a window on top of the DrawSprocket blanking window and use this window to get your AGLDrawable.

Q. So I've done all this and my application is still crashing on the call to AGLSetDrawable. How can I fix this?

A. If you're using pre-compiled headers such as <MacHeaders.h>, you will need to instead use MacHeadersCarbon.h. Since the pre-compiled headers set the compiler flags when they are pre-compiled not when you compile your code, there is no way for these headers to know if you are targeting Carbon or not. When not compiling for Carbon GetWindowPort is defined in <MacWindows.h> as:
    #define GetWindowPort(w) ((CGrafPtr) (w))
This results in a simple cast which does not work with Carbon's opaque data structures. To avoid this problem ensure you are using the correct pre-compiled headers for the environment you are building.

Updated: 4-October-2000

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