Re: [Plib-users] PLIB vs OpenGL axes

[Ed J. <ed....@or...>]

> Well, it's been a while since I answered this question! 


Yeah, sorry, I'm a bit behind the times.
;-)

Thanks for the reply and for the history lesson. Very interesting.

> So - the first thing SSG does is to push a 90 degree pitch rotation
> onto the matrix stack.  This handily converts from 'Flight Simulation'
> coordinates to 'OpenGL coordinates' - and voila! Z is UP. 

So presumably, if I move everything in Plib around as if it's using 
OpenGL coordinates and
stick a -90 degree pitch rotation right at the top of the scene-graph, 
then my Z-UP Plib stuff
will integrate perfectly with my Y-UP terrain engine?

Cheers,
Ed.


Steve Baker wrote:

> Ed Jones wrote:
>
>> OK, so this is obviously a dopey newbie Plib 101 question but;
>>
>> What's the relationship between the axes that Plib uses and those of 
>> OpenGL?
>>
>> I'm used to OpenGL having X going left-right, Y going up-down and Z 
>> going in-out. This doesn't seem to be what Plib does, so why the 
>> difference; "Since SG is designed to work with OpenGL"? (from the docs)
>
>
> Well, it's been a while since I answered this question!
>
> The reason is "Steve likes it that way" - which is perhaps an
> unfortunate reason (well, it's not unfortunate for me - but that's
> because I'm Steve!).
>
> I wrote the original version of PLIB's SSG library back when I didn't
> expect anyone else to use it but me.
>
> I design flight simulator graphics systems - and have been doing 3D
> graphics since the early 1980's...LONG before OpenGL was even thought
> of.
>
> In flight simulation, the early systems (which were around before 
> computers
> were invented - let alone 3D graphics) were purely two-dimensional
> map viewing systems - so the X/Y coordinate system lay naturally along
> the ground.  When we went to 3D, it was natural to assume Z-is-up as a
> convention.  I've been doing flight simulation for so long that this
> convention has become heavily burned into my brain.
>
> On the other hand, when Silicon Graphics first built their IRIS system
> with IrisGL, they were going from a 2D computer screen (running something
> a lot like X-windows) where the X/Y plane was naturally in the plane of
> the screen.  So when THEY added the third dimension, it was natural for
> Z to point into the screen.  OpenGL is a direct descendent of IrisGL.
>
> So - the first thing SSG does is to push a 90 degree pitch rotation
> onto the matrix stack.  This handily converts from 'Flight Simulation'
> coordinates to 'OpenGL coordinates' - and voila! Z is UP.
>
> If I'd known when I wrote SSG that it would be used for all sorts of
> stuff outside of flight simulation, I'd probably have stuck with the
> OpenGL coordinate system.  However, it's *FAR* too late to change now.
>
> So, SSG's coordinate system is:
>
>   X is positive to the right.
>   Y is positive into the screen.
>   Z is positive up.
>
> Having said that - some modern flight simulators DO use Y-is-up...but
> it makes very little practical difference.  One additional matrix push
> per frame is NEGLIGABLE in terms of performance.
>
> ---------------------------- Steve Baker -------------------------
> HomeEmail: <sjb...@ai...>    WorkEmail: <sj...@li...>
> HomePage : http://www.sjbaker.org
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