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<H4> 7.6.7.2 Entities </H4>
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<P>
But first lets discuss the type of entities that Crystal Space supports.
We already saw sectors above. These are the main containers for all
the entities below.
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<H4> Sprites or Models </H4>
<!--docid::SEC393::-->
<P>
Crystal Space supports (among others) both 3D and 2D sprites. A 3D sprite
is basically a set of triangles or a triangle mesh. I will not go into much
more detail here because this document is related to visibility. A 2D sprite
is a general polygon which always faces the camera. Both are considered
detail objects (more on that later).
</P><P>
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<H4> Things </H4>
<!--docid::SEC394::-->
<P>
Things are general objects used to define the basic geometry in a level.
They can move and rotate but in general their most important use is to define
level geometry (i.e. walls, floor, ceiling, a painting on the wall, a staircase,
a chair, etc.). Small and detailed things will be tagged as detail
objects. Polygons on things can also be portals to other areas.
</P><P>
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<H4> Curved Surfaces </H4>
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<P>
Curved surfaces are technically part of things but for visibility
they are considered differently. They are always considered as detail
objects as they represent a rather large triangle mesh.
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<H4> Terrain Surfaces </H4>
<!--docid::SEC396::-->
<P>
A terrain surface is a large entity that can be in a sector.
It is not a detail object (obviously). Handling of terrain and visibility
is not ready yet. We need to do more work on this part. At this moment
the terrain engine already has a visibility algorithm inside which is
used for visibility culling of its own triangles but this algorithm should
also be used to cull objects that are on the terrain.
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