fltk does not work on cygwin version octave-6.4.0

[Jon Turney]

On 21/06/2022 12:18, Takashi Yano wrote:
> On Tue, 21 Jun 2022 19:48:09 +0900 (JST)
> Tatsuro MATSUOKA wrote:
>>> From: "Takashi Yano"
>>> To: "cygwin>
>> Cc: "Tatsuro MATSUOKA"
>>> Date: 2022/06/21 火 18:45
>>> Subject: Re: fltk does not work on cygwin version octave-6.4.0
>>   
>>> On Tue, 21 Jun 2022 14:15:12 +0900 (JST)
>>> Tatsuro MATSUOKA wrote:
>>>> fltk does not work on cygwin version octave-6.4.0
>>>>
>>>> octave:1> graphics_toolkit fltk
>>>> octave:2> plot(1:10);
>>>> Insufficient GL support
>>>> error: unable to plot due to insufficient OpenGL support
>>>>
>>>> plot windows flashes and disappear immediately
>>>
[...]
> 
> However, it works as expected, except that the following error message
> appears.
> libGL error: Windows-DRI extention disabled for GDI Generic renderer
> 
> $ env LIBGL_ALWAYS_SOFTWARE=1 octave
> octave:1> graphics_toolkit fltk
> octave:2> plot(1:10);
> 
> also works.
Unfortunately, LIBGL_ALWAYS_SOFTWARE doesn't behave the same everywhere, 
since the set of GLX visuals available with it is based on those 
available for direct rendering.

This seems similar to [1], which I made some changes in libGL to try to 
address, but unfortunately that seems to never been resolved to a root 
cause.

The "Insufficient GL support" message comes from octave, but since it 
doesn't say what GL support it needs but thinks is missing, it's hard to 
tell what the problem actually is.

I guess finding where that comes from in the octave source code would be 
a good starting place.

[1] https://cygwin.com/pipermail/cygwin/2018-June/237659.html