Re: [Plib-users] Runway Lights and Undocumented Secrets of Plib

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

David Megginson wrote:

> Plib has callbacks for branches as well as leaves, but the branch
> callbacks are not documented, so I had to dig trough the plib source
> code (they're the same ones I used for the dynamic scenery objects).
> For a branch, you can set pre-traversal and post-traversal callbacks;
> if the pre-traversal callback return false, then the branch will be
> skipped.  Here's a trivial (and not-so-efficient) example:
> 
>   static int cloaking_callback (ssgEntity * e, int mask)
>   {
>     return !fgGetBool("/controls/cloaking-device");
>   }
> 
>   ssgBranch * ship;
> 
>   // ...
> 
>   ship->setTravCallback(SSG_CALLBACK_PRETRAV, cloaking_callback);
> 
> Now, the 'ship' branch will automatically vanish whenever the
> /controls/cloaking-device property is true and reappear when it's
> false.

Yes - I apologise that this isn't documented - but it's vitally
important that you can do this -so it *should* be documented.

Bear in mind though that your callback is only called when the 
object's parent is inside the field of view.  This is both an
advantage and a curse.  It means that expensive callbacks are
only called when they are needed - but also that callbacks with
side-effects are a bit unpredictable!

> For the dynamic objects, I use a much more complicated approach -- the
> branches are actually empty until ssg attempts to traverse them, then
> (at first traversal) FlightGear populates them with pseudo-randomly
> placed objects.

Wow!

> That way, branches that never come into range or into
> the view direction are never populated.  I also have a callback to
> depopulate the branches once they fall out of range.  That turns out
> to be a fairly efficient way to add many objects (such as buildings or
> trees) to the scenery nearby without overwhelming the system.

Interesting.

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