Revision: 41418
http://brlcad.svn.sourceforge.net/brlcad/?rev=41418&view=rev
Author: starseeker
Date: 2010-11-22 19:47:12 +0000 (Mon, 22 Nov 2010)
Log Message:
-----------
Enhance the rtarea man page description of exposed and presented area.
Modified Paths:
--------------
brlcad/trunk/doc/docbook/system/man1/en/rtarea.xml
Modified: brlcad/trunk/doc/docbook/system/man1/en/rtarea.xml
===================================================================
--- brlcad/trunk/doc/docbook/system/man1/en/rtarea.xml 2010-11-22 14:12:18 UTC
(rev 41417)
+++ brlcad/trunk/doc/docbook/system/man1/en/rtarea.xml 2010-11-22 19:47:12 UTC
(rev 41418)
@@ -63,18 +63,111 @@
<refsect1 id='description'>
<title>DESCRIPTION</title>
<para>
- <emphasis remap='I'>Rtarea</emphasis> calculates the exposed, presented
and centers of
+ The <emphasis remap='I'>rtarea</emphasis> command calculates the
exposed, presented and centers of
surface areas of specified geometry objects from a particular view. The
summary
output includes details for individual regions, assemblies (combinations
that contain
- regions), and summarized totals. The exposed area is the occluded 2D
- projection of the specified geometry. That is, if another
- object is in front, it will reduce the amount of exposed area. The
- presented area is the unoccluded 2D projection of the geometry, where
- hidden objects will report their presented area regardless of any
- objects that are in front.
+ regions), and summarized totals.
</para>
+ <para>
+ The <emphasis remap='B'>exposed </emphasis> area corresponds to what a
human observer would see looking at the object(s)
+ in question, if all the objects were non-transparent. For example, say
rays were cast towards
+ two pieces of sheet metal, one in front of the other:
+ <screen>
+------------------------------- Sheet 2
+ ------------------- Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+
+ The exposed area of sheet 1 corresponds to the total area of the sheet,
since it is not
+ occluded by any other object:
+
+ <screen>
+------------------------------- Sheet 2
+ ******************* Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+
+ Sheet 2, on the other hand, IS occluded by sheet 1 and would thus report
only a fraction
+ of its area as exposed:
+
+ <screen>
+*****-------------------******* Sheet 2
+ ------------------- Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+
+ An object that combines both of these sheets would thus report as its
exposed area the
+ sum of the exposed areas of the individual sheets:
+
+ <screen>
+*****-------------------******* Sheet 2
+ ******************* Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+ </para>
+ <para>
+ <emphasis remap='B'>Presented</emphasis> area, unlike exposed area, does
not take occlusion into account. It is the
+ equalivent of taking each object and considering it in isolation. So
for the previous
+ case of two sheets of metal, sheet 1 would report the same area:
+
+
+ <screen>
+------------------------------- Sheet 2
+ ******************* Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+
+ Sheet 2, on the other hand, ALSO reports its full area:
+
+ <screen>
+******************************* Sheet 2
+ ------------------- Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+
+ An object that combines both of these sheets reports as its presented
area the
+ sum of the full presented areas of the individual sheets:
+
+ <screen>
+******************************* Sheet 2
+ ******************* Sheet 1
+
+ ^
+ | Ray Direction
+ |
+ </screen>
+ Thus, presented area will always be greater than or equal to exposed area.
+
+ </para>
+
<para>
+ It should be noted that rtarea only counts surfaces with a surface
normal that
+ has a component looking back in the direction of the ray. So, for
example, a
+ cube with only one face looking "back toward" the ray will not report
area from
+ other faces as "presented", since their surface normals will indicate
they are
+ not surfaces being presented to the ray.
+ </para>
+
+ <para>
WARNING: <emphasis remap='I'>Rtarea</emphasis> may not correctly report
area or
center when instancing is done at the group level. Using
<command>xpush</command>
can be a workaround for this problem.
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