Re: [Plib-users] GL context fecal matter

[Steve B. <sjb...@ai...>]

Cameron Moore wrote:

> Why do non-plib apps like Quake3, Rune, and gltron work while plib ones
> do not?  I mean, how can these non-plibs apps render without a valid
> context?  If they can't, how are they getting a valid context when plib
> isn't?  Or if it's possible to render without a valid context, why do we
> care if one exists (playing devil's advocate)?

In OpenGL you are NOT ALLOWED TO MAKE ANY OPENGL CALLS WITHOUT A VALID
RENDERING CONTEXT.

It's a rule - and that's that.

Now, IIRC, in one broken Mesa version, the query function that checks
whether there is a valid OpenGL context or not is BROKEN...it says
"no valid context" even when there is one.  That's a bug in that version
of Mesa - and it needs to be fixed.

So, if your application doesn't CHECK for a valid context - but happens
to have one anyway, it'll work just fine - even on the broken version of
Mesa.  However, if such an application were ever to fail to get a valid
context, it would crash mysteriously and without a good error message.

HOWEVER, there is another class of "bugs" in OpenGL implementations that
you have to watch for.  Some of them tolerate you making *some* OpenGL
calls
before the rendering context has been created.  That's not exactly a bug
because correctly working OpenGL applications don't do that - and broken
OpenGL applications are just broken anyway.

So, in the light of all that, you might ask why PLIB goes and checks that
the rendering context is valid.  Well, before we added that, people would
do all sorts of illegal things like loading texture maps before they'd
opened up an OpenGL window (a rendering context).  In particular they'd
try to call ssgInit() before they had a valid rendering context.  That
was a VERY BAD THING because the 'permissive' OpenGL implementations
would allow this and those programs would seem to work perfectly on the
authors computer - and then crash mysteriously on other people's machines
because they had OpenGL implementations that couldn't tolerate the error.

This made life VERY difficult.  You'd write a program at home, test it
to death on your machine, post it to the internet and get stacks of
complaints from people who would claim that it crashed without an error
message.

So, as a service to mankind, ssgInit (and puInit and IIRC, fntInit) all
check that there is a valid rendering context - and if there isn't they'll
complain and refuse to continue.  That means that even if your OpenGL
implementation is a 'tolerant' one, you'll find the error in your program
and fix it so that other people will be able to run it too.

I think that's "A Good Thing" - and since we've added it, the number of
mysterious crashes inside PLIB on startup have been reduced *GREATLY*
because people now get a good error message instead.

Now, along comes an OpenGL implementation with a bug in it...what should
I do?

My decision:   Tell people to FIX THE BUG!!

I'm not about to remove a valuable bug-tracer that's proved useful (especially
to PrettyPoly which has a very hard time with keeping the startup sequence
right and maintaining a valid OpenGL rendering context at all times).

CONCLUSION:

PLIB is not in error here...it works very well on other versions of Mesa and
on non-Mesa OpenGL implementations.

So - I make no apologies - please bitch and whine to the Mesa team (only
be gentle with Brian Paul - he's not getting paid to maintain Mesa anymore
:-(  ...so we have to be nice to him if we want to get things fixed! :-)

FOOTNOTE:

If you are *desperate*, look for 'glXGetCurrentContext' in src/ssg/ssg.cxx,
src/fnt/fntTXF.cxx and src/pui/pu.cxx and make the necessary *HACK* - but
please don't distribute that hack to all and sundry because it'll make our
lives hell again.  If it were me, I'd go to an earlier or later version of
Mesa - who knows what else is broken as a consequence of this?

----------------------------- Steve Baker -------------------------------
HomeMail : <sjb...@ai...>   WorkMail: <sj...@li...>
HomePage : http://web2.airmail.net/sjbaker1
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