glXChooseVisual does not handle GLX_RGBA correctly

Geoff Levner
Mon May 14 12:43:00 GMT 2007

Larry Hall (Cygwin X) wrote:
> Geoff Levner wrote:
>> I have an OpenGL-based application executing on a Sun, but displaying on
>> a PC running Windows XP and an X server. The app requires a TrueColor
>> visual, so when it calls glXChooseVisual, it passes GLX_RGBA in the
>> attribute list. This works fine with another X server (Exceed), but when
>> using Cygwin/X, glXChooseVisual fails to return a visual.
>> Running xdpyinfo would seem to indicate that the server has *only*
>> TrueColor visuals (see the output below). And in fact if I remove
>> GLX_RGBA from the attribute list, glXChooseVisual does return one of
>> those visuals. If I ask for information about the visual with
>> glXGetConfig, it responds that color buffers store color indices
>> (GLX_RGBA is false), yet the application works correctly.
>> Anybody have a clue what is going on here?
> Looks to me like you're app is looking for 32 bits while Cygwin-X is
> only providing 24 bits.  Unless you know you absolutely need transparency,
> I think you'll find everything works fine with the visuals you can get.
> Just ask for GLX_RGB instead.
That sounds reasonable, but GLX_RGB is not defined, and I can't find any 
reference to it in the GLX 1.4 specifications. According to the 
documentation, GLX_RGBA just means you want TrueColor or DirectColor 
visuals; it says nothing about alpha. And we do not ask for any alpha 


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