Re: [Algorithms] Leafy BSP Tree Contruction

[Manolache A. <pro...@ya...>]

The bsp construction is part of a bigger application. Actually it's part of my bachelor's thesis. I chose to implement among others a full BSP PVS 3D engine.This translates in the construction of a solid node leafy bsp tree, automatic portal generation and finally the pvs calculation, rle encoded and further a merge between coplanar adjiacent polygons in every leaf. As a plus i also implemented a portal renderer using the automatically generated portals(it's not lightning fast due to the big number of portals but it was fun to do). 
The BSP/PVS 3D engine works flawlessly until now on 3 levels with tri counts ranging from 1000 to 6000. It seems i have to tweak the EPSILON used when clipping and classifying. I found that an epsilon of 0.01 works well on these three levels because of the range of values in them. The problem arises on the 4th level that has about 15000 triangles on which the construction of the bsp tree fails. I checked and there are a lot of cases when vertices are very close to a lot of planes, and this causes problems when splitting with these planes. The construction of the bsp tree fails because polygons like this one arrive in a back leaf and being used as a splitter in the past:
(-29.467429, 91.150700, -145.998993)
(-29.500788, 91.204792, -146.329036)
(-29.500788, 91.151931, -145.998993)
I have run of ideas trying to tweak the epsilon for this level.

Implementation details:
1.0 Every clipped polygon inherits the normal of the initial polygon
2.0 When clipping edges, Intersection points are calculated in a consistent manner (from front of the plane to back, all the time).
3.0 I treat the planes as being fat.

4.0' I'm using doubles when performing calculations.

I am not quite sure how to tackle the other two things mentioned by Chris (4 and 5).  

> Overall though, WHY are you using a BSP tree?  There are
> some valid uses for them, but for a lot of applications
> they are a bad choice of data structure!

One of my favorite games was quake and i wanted to explore similar techniques used by it and second of all i believe these approaches(or a modification of them) may still be useful on less powerful machines (handheld devices) when dealing with interior, dungeon like environments. I would guess that nowadays portal engines are the norm for interior environments(with portals placed by the artists).