Game Development Reference
In-Depth Information
Figure 17.2
The left-hand triangles are non-manifold
the right-hand triangles are okay.
polygons. It represents a completely solid object. It also has no T-joints on any trian-
gle edge. This usually isn
'
t a problem for artists because they know it screws up the
object
s lighting anyway.
This might be hard to visualize, so I
'
'
ve dusted off my Photoshop skills and made a
drawing for you (see Figure 17.2). The left-hand triangles are clearly non-manifold
because of the T-joint. The triangles on the right satisfy the requirement that each
edge must border exactly two triangles. The remaining requirement is that the mesh
be completely closed and have no holes in it.
If you
re worried that it might be tough to make meshes that always satisfy the
requirements of your physics system, you
'
s sometimes easy for artists to
forget what the requirements are, especially when the heat is on and they
'
re right. It
'
re trying
to get a ton of work done. The best thing is to make sure that your artists double-
check their work, hopefully by actually importing their work into the game and see-
ing for themselves if anything is awry.
'
Visible Geometry Versus Collision Geometry
It
s a good thing to note that while the position and orientation of a physics object are
related to the visual position and orientation, they aren
'
t necessarily the same. They
are probably the same for symmetrical objects like a sphere or a cube, but not much
else. The position of an object in the physical world is always the center of mass, and
that might not be the anchor point of the visible geometry. When you set the location
for a 3D object, it is the anchor point on that object that will be positioned precisely
at the new location. Likewise, the default orientation of an object in the physics simu-
lation is usually an inertia tensor, such that it aligns with the X-, Y-, and Z-axes.
'
 
 
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