[Algorithms] Re: Portal VS BSP

[Alen L. <ale...@cr...>]

Neil Stewart writes: 

> Yes, but only if it gives you the runtime performance you require. Dynamic
> visibility isn't much use if you can't realistically use it on, say, PS2.

True, true, it really does depend on your targets. PS2 is the border area - 
we used dynamic visibility on Xbox without any problems. 

> Besides, you could vastly improve artist turnaround times by limiting the
> level of optimisation in your PVS/portal builder, which would probably still
> give you better performance than a dynamic visibility solution for viewing
> and basic testing.

This is questionable. The fact that lot of artists are used to having long 
turnaround times blurs the vision of programers, leading them to believe 
that it is acceptable. Would you like to have your sources to compile in 0 
seconds? I know I would. Why else do people think that "edit-and-continue" 
in VisualC is the best thing since sliced bread? :) Ultimately, artists 
should have 0 turnover time as well. In all our projects, we've been working 
with tools that allow you to edit levels using the same renderer as used in 
game. In our current engine, game-test startup is several seconds (though I 
know a few places that could use some heavy optimization). 

Besides, I hope that you are not underestimating the abilities of dynamic 
algorithms. Yes, they are not free, but they are very fast. Did you have any 
particular algorithm(s) in mind when you said that a quickly patched 
just-for-quick-test PVS table would outperform them? I believe that it would 
have much problems with its over-conservativeness then, wouldn't it? 

And note that complexity of visibility geometry does not increase 
substantially with complexity of scene geometry. At least for indoors 
architectural scenes what is best target for PVS methods . Once you go 
outdoors or want to get advantage of aggregated occlusion (like in 
powerplants, industrial complexes, forests, etc), PVS is mostly useless, 
isn't it? 

I don't want to say that PVS is a thing of past. Perhaps someone devises a 
method to make a PVS builder so fast that its precomputation is negligible. 
When that happens, I'll seriously consider using it. :) 

> Also remember that dynamic visibility doesn't "buy" you doors, because you
> can easily represent doors in a PVS/portal system.

Of course. What I ment, as I said in the other mail, they are a special 
case, whereas in dynamic visibility, they usually Just Work. 

> If you are talking about building the PVS/portals directly from the tree's
> topology, then yes. The question was about what current games were doing and
> I only know of games using a sampling method to calculate the PVS or using
> hand-built portals. There may be games which use the BSP's topology, but I
> don't know of any. Perhaps someone else can think of some?

Hm... I don't have the source code of "vis" tool for Quake1 to check it out, 
but IIRC from some texts I read a long time ago, it is just what it does. It 
makes a BSP, then portals and then uses those portals to make the PVS. Maybe 
I confused that with the old Seth Teller's PVS paper. Feel free to correct 
me if I'm wrong. 

Alen