home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Crawly Crypt Collection 1
/
crawlyvol1.bin
/
tt
/
raysh402
/
guide.tex
/
node19_mn.html
< prev
next >
Wrap
Text File
|
1992-02-09
|
3KB
|
63 lines
<H1><A ID="SECTION00620000000000000000">
Shadows</A>
</H1>
<P>
In order to determine the color of a point on the surface
of any object, it is necessary
to determine if that point is in shadow with respect to each
defined light source. If the point is totally in shadow with respect to
a light source, then the light source makes no contribution to the
point's final color.
<P>
This shadowing determination is made by tracing rays from the point
of intersection to each light source. These ``shadow feeler'' rays
can add substantially to the overall rendering time. This is especially
true if extended or area light sources are used. If at any point you
wish to disable shadow determination on a global scale, there is
a command-line option (<#335#><TT>-n</TT><#335#>) that allows you to do so. It is also
possible
to disable the casting of shadows onto given objects through the use
of the <#336#><TT>noshadow</TT><#336#> keyword in surface descriptions. In addition,
the <#337#><TT>noshadow</TT><#337#> keyword may be given following the definition
of a light source, causing the light source to cast no shadows onto
any surface.
<P>
Determining if a point is in shadow with respect to a light source
is relatively simple if all the objects in a scene are opaque. In
this case, one simply traces a ray from the point to the light source.
If the ray hits an object before it reaches the light source, then
the point is in shadow.
<P>
Shadow determination
becomes more complicated if there are one or more objects with
non-zero transparency between the point and the light source.
Transparent objects may not completely block the light from a source,
but merely attenuate it. In such cases, it is necessary to compute the
amount of attenuation at each intersection and to continue
the shadow ray until it either reaches the light source or until
the light is completely attenuated.
<P>
By default, <#338#><#1841#><EM>rayshade</EM><#1841#><#338#> computes shadow attenuation by assuming
that the index of refraction of the transparent object is the
same as that of the medium through which the ray is traveling.
To disable
partial shadowing due to transparent objects, the <#339#><TT>shadowtransp</TT><#339#>
keyword should be given somewhere in the input file.
<P>
<DL>
<DT><STRONG><#4712#><#4712#></STRONG></DT>
<DD><#1274#><TT>shadowtransp</TT><#1274#>
<BR> The intensity of light striking a point is <#343#><EM>not</EM><#343#> affected by
intervening transparent objects.
</DD>
</DL>If you enclose an object behind a transparent surface, and you wish
the inner object to be illuminated, you must not use the <#345#><TT>shadowtransp</TT><#345#>
keyword or the <#346#><TT>-o</TT><#346#> option.