glXChooseVisual does not handle GLX_RGBA correctly

Larry Hall (Cygwin X)
Wed May 16 22:26:00 GMT 2007

Geoff Levner wrote:
> 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 
> bits...

Yup, you're right.  I was thinking of something else.

Perhaps it's just your combination of attributes.  You might check that.
The combination below has worked for me before, albeit from a Linux
machine to Cygwin/X:

        int visual_attributes[] =
            GLX_RED_SIZE,          1,
            GLX_GREEN_SIZE,        1,
            GLX_BLUE_SIZE,         1,
            GLX_DEPTH_SIZE,        1,
            GLX_STENCIL_SIZE,      1,
            GLX_ACCUM_RED_SIZE,    1,
            GLX_ACCUM_GREEN_SIZE,  1,
            GLX_ACCUM_BLUE_SIZE,   1,

Larry Hall                    
RFK Partners, Inc.                      (508) 893-9779 - RFK Office
216 Dalton Rd.                          (508) 893-9889 - FAX
Holliston, MA 01746

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