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<h3 class="subsection"> 4.9.4 Portal Engine </h3>
<p><em>Written by Jorrit Tyberghein,
<a href="mailto:jorrit.tyberghein@gmail.com">jorrit.tyberghein@gmail.com</a>.</em>
</p>
<p>The world in Crystal Space is defined with <em>Sectors</em>. A sector is
an infinite area of space that itself does not represent geometry but can
contain objects that represent geometry. There are various types of
objects that you can put in a sector (see section <a href="MeshObject.html#0">Mesh Object Plug-In System</a>).
</p>
<p>A sector is in principle an infinite area of space. But usually the
sector has logical bounds which are made from some mesh object. In this
discussion we will use the thing mesh object to make sector boundaries.
</p>
<p>You can define multiple sectors and connect them together by using
portals in a portal container (see section <a href="MeshObject-Portal-Container.html#0">Portal Container Mesh Object</a>).
</p>
<p>Assume that you want to define a large room with a pillar in the middle of the
room. You can do this two ways; either with four sectors, or with one sector.
First let us define it with four sectors.
</p>
<p>As seen from above, the sectors would look something like this:
</p>
<p><img src="usingcs/engine/portal1.png" alt="usingcs/engine/portal1">
</p>
<p>Sector S1 is composed of eight polygons (including the top and bottom polygon
for its roof and floor, as well as the three polygons at the east side). The
two polygons adjacent to sectors S2 and S4 are portals to the respective
sectors. All the other polygons are texture mapped as normal.
</p>
<p>Sectors S2 and S4 have six polygons each. Their west polygons are again
portals to sector S1. Their east polygons are portals to sector S3.
</p>
<p>Sector S3 is defined as is sector S1.
</p>
<p>Another way to define this room using just the same four sectors could be done
as follows:
</p>
<p><img src="usingcs/engine/portal2.png" alt="usingcs/engine/portal2">
</p>
<p>To the person standing in this room this makes no difference at all.
</p>
<p>There are many other ways to define this room using the four sectors. One
important thing to note is that four is the minimum number of Sectors that you
need to define this room if you want to render the polygons using Z-fill
mode (this means that the Z-buffer is updated but not tested). If you want
to be able to render using Z-fill mode a sector has to be <em>convex</em>.
</p>
<p>An easier way to define this room is by using only one Sector and one Thing
to define the pillar, as shown below.
</p>
<p><img src="usingcs/engine/thing.png" alt="usingcs/engine/thing">
</p>
<p>Again this makes no difference for the person standing in this room. For a
simple situation like this you should avoid the overhead of portals as 3D
hardware is fast enough to just render everything anyway. Portals are mainly
useful when you are creating complicated and large worlds. For example, if you
have a large outside world with buildings then you could model the outside
as one sector and the inside of every building as another sector.
</p>
<p>Using Things like this make it easier to define worlds. If they are small
enough they will also enhance performance.
</p>
<p>With Sectors, Portals, and Things you can describe virtually any world that
you want.
</p>
<a name="1"></a>
<h4 class="subsubheading"> Sectors </h4>
<p>In this section I will describe Sectors a bit more thoroughly. As stated
before sectors represent infinite space. You can fill a sector with geometry
by using mesh objects (see section <a href="MeshObject.html#0">Mesh Object Plug-In System</a>).
</p>
<p>With a sector there is always an associated visibility culler. Currently
Crystal Space supports two cullers:
</p><ul class="toc">
<li>
<samp>‘frustvis’</samp>: This is the default visibility culler that is used if no
other is specified. It only does frustum culling. It is the best culler to
use for simple sectors that don't have a lot of objects or else for sectors
that are big but where all objects are small. For such sectors it doesn't
make much sense to try to do more complicated visibility culling so
frustum cullling (testing if an object is visible in the current view)
is more than enough.
</li><li>
<samp>‘dynavis’</samp>: This is a more advanced visibility culler that does
frustum culling in addition to trying to avoid rendering objects that
are obscured by other objects (see section <a href="Visibility-Culling.html#0">Visibility Culling In Detail</a>). This culler
is mainly useful when you have big complex sectors with lots of objects
and when objects have a good chance of occluding (hiding) other objects.
</li></ul>
<a name="2"></a>
<h4 class="subsubheading"> Things </h4>
<p>A Thing is one of the basic building blocks to create geometry in
Crystal Space. A Thing is made from convex polygons.
</p>
<p>Things (and other mesh objects for that matter) can be rendered using
Z-fill or Z-use mode. With Z-fill the previous contents of the Z-buffer
(where that Thing is rendered) is just overwritten. With Z-use the
previous contents is checked in order to see if the Thing is in front
or behind the polygons that are already on screen. If you have a convex
object (like the outer polygons of a room) then you can use Z-fill because
the polygons will not overlap. Things which are then inside those walls
will need to use Z-use in order not to overwrite each other.
</p>
<p>Note that polygons have a visible side and an invisible side (backface
culling).
</p>
<a name="3"></a>
<h4 class="subsubheading"> Polygons </h4>
<p>Things are made of 3D polygons. As mentioned before polygons must
be convex. The vertices of polygons are oriented clockwise. This fact is
used for backface culling; a polygon has only one visible side.
</p>
<p>Polygons are drawn with a texture. How the texture is mapped on the polygon
depends on a transformation matrix. This is general enough so that you can
translate, rotate, scale, and mirror the texture in every possible direction.
The texture is tiled across the polygon surface.
</p>
<p>In a pre-computing stage three light-maps are created for every polygon (this
is explained in more detail later). Lighting is sampled in a grid which is 16
by 16 texture pixels (or <em>texels</em>) in size, by default. Bilinear
interpolation is used by the texture caching machinery to make this lighting
appear smooth. The end result of this is a non-tiled lighted texture that is
mapped across the polygon surface.
</p>
<a name="4"></a>
<h4 class="subsubheading"> Portal Containers </h4>
<p>A special kind of object is the portal container. It also has polygons
but every polygon is a portal to another sector (or the same sector in case of
a mirror or other space warping transformation).
</p>
<a name="5"></a>
<h4 class="subsubheading"> Portals </h4>
<p>A Portal is a special kind of polygon.
</p>
<p>Instead of texture mapping a portal polygon, the engine will recursively
draw the sector that this portal points to. After this, if the texture is
semi-transparent, the texture will be mapped over the already rendered
sector.
</p>
<p>Portals can also transform space. This feature can be used to implement
mirrors or reflecting surfaces.
</p>
<p>Note that when there is a portal from sector A to sector B you should probably
also define a portal from sector B to sector A. Adjacent polygons of
different sectors are not shared so you need to set a portal on each of them.
Otherwise you will have the effect that you can see from sector A to sector B
but not the other way around.
</p>
<p>A special feature of portals is that you could (in theory) have a portal from
sector A to sector B, but instead of going back to sector A from sector B you
could set the portal to point at sector C which (in this example) would be a
sector which has the same world space coordinates as sector A. This is
perfectly possible (although maybe not desirable) with Crystal Space. An
important result of this is that a given world space coordinate can belong to
more than one sector! A corollary of this is that you always need a current
sector reference together with a world space coordinate to really know where
you are.
</p>
<p>Portals in Crystal Space can solve the problem of polygon sorting if you
structure your world strictly with convex sectors (not really recommended for
practical worlds). All polygons
in the current sector are certainly visible (unless they are behind the view
plane) and do not overlap, so they can just be drawn in any order without
overdraw and without conflicts. If a portal polygon is reached, all polygons
in that other sector are behind all the polygons in the current sector. In
fact portals are an explicit form of <small>BSP</small> tree. The advantages of this
approach are summarized below.
</p>
<ul>
<li>
Space warping can be used as described above.
</li><li>
In theory it would be rather easy to make dynamic worlds. Because the portals
are explicit it is easy to define them so that certain sectors can move and
transform in certain ways.
Note that static lighting makes this a bit difficult in some cases.
</li></ul>
<p>One disadvantage a portal engine is that it can be more difficult to define
worlds. If you want to be able to use <small>ZFILL</small> mode you must make all
sectors convex. If you don't care about that then you can use <small>ZUSE</small> too.
</p>
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