[Plib-devel] Annoying SSG bug.

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

SSG uses a frame counter for a couple of jobs.

    ssgGetFrameCounter();

...the counter is incremented in ssgCullAndDraw() - but if you use
multiple calls to ssgCullAndDraw each frame (eg to do multipass
rendering - or to render some objects in a separate rendering pass)
then the counter is incremented too many times.

This is rather nasty to fix.

SSG has no way to know when you call Swapbuffers - so it can't tell
the difference between true multipass rendering and two consecutive
legitimate frames.

In retrospect, we should probably have an SSG call that you're
required to make before each swapbuffers - but we don't have it
and if I add one and use it to increment the frame counter then
I'll break existing applications.

I suspect this problem will affect FlightGear...it explains a couple
of odd things I've seen in some of my work.

The only way I can think to fix this in a backwards compatible
way is to write:

ssgFrameUpdate () { ssgSetFrameCounter( ssgGetFrameCounter() + 1 ) ; }

ssgImprovedCullAndDraw ( ssgEntity *e )
{
   int x = ssgGetFrameCounter() ;
   ssgCullAndDraw( something ) ;
   ssgSetFrameCounter ( x ) ;
}

...and to tell people:

   1) Call ssgFrameUpdate() at the end of every frame - just before
      Swapbuffers.

   2) Use ssgImprovedCullAndDraw instead of ssgCullAndDraw - which is
      now deprecated.

We'd need a better name than ssgImprovedCullAndDraw.

Can anyone think of a cleaner way to fix this?

---------------------------- Steve Baker -------------------------
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