Revision: 41420
http://brlcad.svn.sourceforge.net/brlcad/?rev=41420&view=rev
Author: starseeker
Date: 2010-11-22 20:48:14 +0000 (Mon, 22 Nov 2010)
Log Message:
-----------
Thanks Keith - make some changes, better examples for 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 20:26:38 UTC
(rev 41419)
+++ brlcad/trunk/doc/docbook/system/man1/en/rtarea.xml 2010-11-22 20:48:14 UTC
(rev 41420)
@@ -70,80 +70,125 @@
</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:
+ in question, if all the objects were non-transparent. For example, say
rays were cast towards two cubes, each one of which
+ is a region, one in front of the other, grouped into an assembly, with
the ray direction pointed into the page:
<screen>
-------------------------------- Sheet 2
- ------------------- Sheet 1
-
- ^
- | Ray Direction
- |
+|----------------| Rear Cube
+| |
+| |
+| |--------------| Front Cube
+| | | |
+| | | |
+|--------|-------| |
+ | |
+ | |
+ |--------------|
</screen>
- The exposed area of sheet 1 corresponds to the total area of the sheet,
since it is not
+ The exposed area of the Front Cube 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>
+|----------------| Rear Cube
+| |
+| |
+| |--------------| Front Cube
+| |**************|
+| |**************|
+|--------|**************|
+ |**************|
+ |**************|
+ |--------------|
</screen>
- Sheet 2, on the other hand, IS occluded by sheet 1 and would thus report
only a fraction
+
+
+ The Rear Cube, on the other hand, IS occluded by the Front Cube and
would thus report only a fraction
of its area as exposed:
-
<screen>
-*****-------------------******* Sheet 2
- ------------------- Sheet 1
-
- ^
- | Ray Direction
- |
+|----------------| Rear Cube
+|****************|
+|****************|
+|********|--------------| Front Cube
+|********| | |
+|********| | |
+|--------|-------| |
+ | |
+ | |
+ |--------------|
</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
+|----------------| Rear Cube
+|****************|
+|****************|
+|****************|------| Front Cube
+|***********************|
+|***********************|
+|--------|**************|
+ |**************|
+ |**************|
+ |--------------|
+ </screen>
- ^
- | 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:
+ case of two sheets of metal, the Front Cube would report the same area:
-
+ <screen>
+|----------------| Rear Cube
+| |
+| |
+| |--------------| Front Cube
+| |**************|
+| |**************|
+|--------|**************|
+ |**************|
+ |**************|
+ |--------------|
+ </screen>
+
+
+
+ the Rear Cube, on the other hand, ALSO reports its full area:
<screen>
-------------------------------- Sheet 2
- ******************* Sheet 1
-
- ^
- | Ray Direction
- |
+|----------------| Rear Cube
+|****************|
+|****************|
+|****************-------| Front Cube
+|****************| |
+|****************| |
+|--------|-------| |
+ | |
+ | |
+ |--------------|
</screen>
- Sheet 2, on the other hand, ALSO reports its full area:
-
+
+ When reporting the presented area of the group, there is a subtle point
- rtarea does not count area covered
+ by multiple regions multiple times when totaling the presented area. So
for the group containing both cubes,
+ the presented area is NOT the cumulative area of both regions' presented
areas but the area of the union of those
+ two areas. In this case, that works out to be the same as the exposed
area.
<screen>
-******************************* Sheet 2
- ------------------- Sheet 1
-
- ^
- | Ray Direction
- |
+|----------------| Rear Cube
+|****************|
+|****************|
+|****************|------| Front Cube
+|***********************|
+|***********************|
+|--------|**************|
+ |**************|
+ |**************|
+ |--------------|
</screen>
-
+
</para>
<para>
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