GL Lighting POSITION

[Kurt Akeley]

In article <17450005 at hpfcdj.HP.COM>, kuba at hpfcdj.HP.COM (Kathy Kuba) writes:
|> I recently received my SGI 210GTX workstation (running 3.3) and began
|> porting code from my 120GTX (3.2).  All went well, except that my light
|> sources did not illuminate my objects.  Upon further investigation, I
|> discovered that moving my POSITION from 0 0 1 0 to 0 0 -1 0 fixed the
|> problem.  Has any one else seen this??  I really don't want to do keep
|> different versions of code for each machine.  The SGI Hotline graphics
|> expert I talked with said they "fixed" the light sources so backface
|> lighting would work properly.  All other SGI graphics workstations I've 
|> used have the positive z axis pointing out of the screen, so a light
|> at 0 0 1 0 is  shining from behind to the origin.  If they believe this
|> is really a "fix", then why didn't they make it transparent to the user?  
|> The answer I received, "change your code", is not acceptable.  Comments 
|> suggestions, and alternate solutions welcome!
|> 
|> 
|> Kathy

You have received some misinformation.  The object coordinate system has been
and remains right handed - positive Z will always extend out of the screen
toward you.  The default light position is and will remain 0,0,1,0.  You don't
need to specify this to get it.

Probably the most common problem encountered with lighting is that light
positions are transformed by the current ModelView matrix when the light is
bound (i.e. when lmbind(LIGHTn,foo) is called).  As a result you must be
very careful about what ModelView matrix is present when a light is bound.
In releases previous to 3.3, the ModelView matrix was undefined until it was
first loaded (typically with an identity matrix).  In release 3.3, the
ModelView matrix is initialized to identity when mmode(MVIEWING) is first
called (putting the GL into multimatrix mode, required for lighting and many
other advanced GL features).  It is likely that some error has been made in
the matrix/lmbind sequence, and that this error is affected by the differences
between the 3.2 and 3.3 software releases.

Of course there really may be a bug in the GTX 3.3 lighting code, but I haven't
noticed any problems with my machine (a 3.3 GTX), so it's worth going over your
code carefully.

-- kurt