Weird invisible surface

[The Purple Panzer]

I've got a very simple map (will quote below) with a trapezoid - not quite the right term, but it's essentially a block where the top is smaller than the bottom, so the walls slant in. I have a script to create it, and it works fine, but as soon as I specify an "inside" and "outside" point, I notice something really weird: if you grab a Wings flag, you can fly up and land on an invisible surface near the top of the diagonal wall (down a bit from the top surface, still inside the outer, lower boundary). If you remove the inside and outside points then things are fine, but of course there may be spawning problems, etc.

I was going to programmatically add normals, but this seems like something of a showstopper.

Here's the map:

world
size 400
end
material
name std_ground
texture std_ground
notexalpha
end
material
name http://images.bzflag.org/astevens/greywall.png
texture http://images.bzflag.org/astevens/greywall.png
notexalpha
end
define Basic_Trap
mesh
shift 0 0 0
matref http://images.bzflag.org/astevens/greywall.png
vertex -210 210 0
vertex 210 210 0
vertex 210 -210 0
vertex -210 -210 0
vertex -190 190 20
vertex 190 190 20
vertex 190 -190 20
vertex -190 -190 20
face
vertices 7 6 5 4
matref std_ground
endface
face
vertices 0 3 7 4
endface
face
vertices 4 5 1 0
endface
face
vertices 5 6 2 1
endface
face
vertices 6 7 3 2
endface
face
vertices 3 2 1 0
endface
inside 0 0 10
outside 0 0 40
end
enddef
group Basic_Trap
shift 0 0 0
end

[trepan]

you don't have full "outside" coverage.
simplify, remove the "outside" point.

[The Purple Panzer]

That fixes it!

What would full outside coverage involve? An outside point near each face, that is so that a line between the inside and outside points goes through that face?

Is it necessary to put one underground as well?

[Spazzy McGee]

As far as I know, inside points and outside points do the same job. Inside points are a lot easier to work with - you just need all of the volume within the mesh to be in 'line-of-sight' of an inside point.

Whereas, I believe, for outside points to work, you need all of the volume outside of the mesh to be in line of sight of the points.

IS there a situation where outside points would be a better choice than inside points?