147 lines
5.2 KiB
HTML
147 lines
5.2 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML>
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<HEAD>
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<META http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<TITLE>A Simple Scene Graph API for OpenGL.</TITLE>
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</HEAD>
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<BODY text="#B5A642" link="#8FFF8F" vlink="#18A515" alink="#20336B"
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bgcolor="#005000" background="../marble.png">
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<H2><code>class ssgEntity</code> - A Node in the Tree.</H2>
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All nodes in the SSG scene graph are ssgEntities.
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<pre>
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clas ssgEntity : public ssgBase
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{
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public:
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ssgEntity (void) ;
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virtual ~ssgEntity (void) ;
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int getTraversalMask () ;
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void setTraversalMask ( int t ) ;
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void setTraversalMaskBits ( int t ) ;
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void clrTraversalMaskBits ( int t ) ;
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virtual void recalcBSphere (void) ;
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int isDirtyBSphere (void) ;
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void dirtyBSphere () ;
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sgSphere *getBSphere () ;
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virtual int getNumKids (void) ;
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int getNumParents () ;
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ssgBranch *getParent ( int p ) ;
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ssgBranch *getNextParent () ;
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virtual void cull ( sgFrustum *f, sgMat4 m, int test_needed ) ;
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virtual void isect ( sgSphere *s, sgMat4 m, int test_needed ) ;
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virtual void hot ( sgVec3 s, sgMat4 m, int test_needed ) ;
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} ;
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</pre>
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<H3>The tree structure.</H3>
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Every entity has a list of parent entities and (conceptually), a list of
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child entities ("kids"). In practice, ssgRoot nodes never have parents
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and ssgLeaf nodes never have kids.
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<p>
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The structure of the scene graph permits the same node to be inserted
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into the graph in multiple locations. This is useful for saving
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space when the same object is needed many times in the scene.
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Hence, any given node may have more than one parent node.
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<p>
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You can traverse the list of parent nodes
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using <code>ssgEntity::getNumParents()</code> to find out the number
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of parents this node has and
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<code>ssgEntity::getParent(n)</code> to locate the n'th parent.
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<p>
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Alternatively, after calling getParent, you can call getNextParent
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to get the N+1'th parent node - it returns NULL when no more parents
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are available.
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<p>
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As a convenience for general tree-walking routines, there is a
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<code>ssgEntity::getNumKids()</code> call - which will always
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return zero on leaf nodes. You cannot actually get kid nodes unless
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the node is some kind of ssgBranch.
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<H3>Traversals</H3>
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Much of the work done on an SSG scene graph entails 'traversing'
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the tree structure. This is done most commonly to display the
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scene using OpenGL - but is also done when doing intersection
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testing and other operations.
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<p>
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It's quite useful to be able to limit the traversal so that
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certain nodes do not get tested. This can save time - or
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prevent undesirable side-effects.
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<p>
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Each entity has a 'traveral mask' - which is a simple integer
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with one bit per kind of traversal. At present, there are
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three kinds of traversal:
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<pre>
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SSGTRAV_CULL -- Culling to the field of view.
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SSGTRAV_ISECT -- General intersection testing.
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SSGTRAV_HOT -- Height-over-terrain testing.
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</pre>
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You can directly set or get the traversal mask with <code>ssgEntity::setTraversalMask(m)</code>
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<code>ssgEntity::getTraversalMask()</code>. You can set an individual traversal bit using
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<code>ssgEntity::setTraversalMaskBits(m)</code> or clear one using
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<code>ssgEntity::clrTraversalMaskBits(m)</code>.
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<H3>Bounding Sphere</H3>
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Quite a few graphics algorithms can be accellerated using a
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bounding sphere. The standard ssgEntity uses bounding
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spheres to do field-of-view and intersection testing.
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<p>
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Clearly one does not want to recompute the bounding sphere
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every frame - just some objects do change their size over
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time. Hence, the bounding sphere is lazily evaluated.
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<p>
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Whenever you do something to change the size or shape of an entity,
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you should call <code>ssgEntity::dirtyBSphere()</code>. This will
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mark this entity's sphere as invalid ("dirty") and also, walk
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backwards up the scene graph tree making all the nodes above
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this one dirty too. The next time SSG needs to know the bounding
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sphere size, it'll recompute it.
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<p>
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If you'd prefer for the bounding sphere recalculation to be
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done immediately, then you can call <code>ssgEntity::recalcBSphere()</code>
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and it will be done immediately. Branch nodes like ssgTransforms will
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automatically dirty their bounding spheres when necessary. Leaf nodes
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generally do not.
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<p>
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When anyone needs to know the bounding sphere size for a node,
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they'll call <code>ssgEntity::getBSphere()</code> - which will
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recaclulate the Bsphere if it needs to.
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<H3>Culling and Drawing</H3>
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The actual tree-traversal, culling and rendering is handled by
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a virtual function <code>ssgEntity::cull()</code> - calling
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this on the root node in the scene graph causes the entire
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scene to be rendered in an efficient manner.
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<hr>
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<table width="100%">
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<tr>
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<td width="33%" align="left"><a href="ssgBase.html"><= previous =</a></td>
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<td width="34%" align="center"><a href="index.html">Return to SSG Index</a></td>
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<td width="33%" align="right"><a href="ssgLeaf.html">= next =></a></td>
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</tr>
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</table>
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<hr>
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<table>
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<tr>
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<td>
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<a href="http://validator.w3.org/check/referer"><img border="0" src="../valid-html40.png" alt="Valid HTML 4.0!" height="31" width="88"></a>
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<td>
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<ADDRESS>
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<A HREF="http://www.sjbaker.org">
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Steve J. Baker.</A>
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<<A HREF="mailto:sjbaker1@airmail.net">sjbaker1@airmail.net</A>>
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</ADDRESS>
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</table>
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</BODY>
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</HTML>
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