Re: [Plib-users] Newbie question

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

David Megginson wrote:
> Marco Bancale writes:
> 
>  > Sure, changing the far plane was my first idea, but in that way
>  > all the objects would not be culled (by the far plane) anymore!
>  > Isn't it? So, if I have 100 spaceships around the solar system,
>  > I should see all of them... always. I think this is not much
>  > optimized :).
> 
> You can put the planets in a separate scene graph:
> 
>   ssgSetNearFar(1, 100000);
>   ssgCullAndDraw(ships_root);
>   ssgSetNearFar(1000, 100000000000000);
>   ssgCullAndDraw(planets_root);
> 
> The problem is that when the ships get close to planets or the sun,
> they might not be drawn properly (i.e. the planet will block the ship
> when it shouldn't).

That won't work...the values written into the Z buffer when rendering
the ships would be meaningless after you'd changed the near/far planes
for drawing the planets - so it would be quite possible for a planet
that's a million miles away to occlude a ship that's just one mile away.

There are some Z tricks you can play to avoid that (in principle) - but
this way isn't one of them.

You could draw the planets first, then clear the Z buffer, then draw the
ships...but then a ship would never be occluded by a planet - no matter
what.

> As long as you have only 100 ships, you can just put each one under an
> ssgRangeSelector to make sure it's not draw when it's too far away.
> With ssgRangeSelector, you can use the same near/far plane for
> everything and avoid the z-buffer strangeness.

Yes - that's the 'right' answer.

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