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<H1><A NAME="SECTION002650000000000000000"></A>
<A NAME="32235"></A>
<A NAME="32236"></A>
<BR>
I.5 Artificial illumination
</H1>
<A NAME="tex2html1876"
  HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> uses the HSV system to
achieve artificial illumination of colored images (e.g. <B>-I</B>
option in <A NAME="tex2html1877"
  HREF="../man/grdimage.html"><I><B>grdimage</B></I></A><A NAME="32292"></A>) by changing the saturation <I>s</I> and value <I>v</I>
coordinates of the color.  When the intensity is zero (flat illumination), the data
are colored according to the cpt file.  If the intensity is
non-zero, the color is either lightened or darkened depending on the illumination.
The color is first converted to HSV (if necessary) and then
darkened by moving (<IMG
 WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img258.png"
 ALT="$s$">,<IMG
 WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img259.png"
 ALT="$v$">) toward (<B>HSV_MIN_SATURATION</B>,
<B>HSV_MIN_VALUE</B>) if the intensity is negative, or lightened by sliding (<IMG
 WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img258.png"
 ALT="$s$">,<IMG
 WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img259.png"
 ALT="$v$">) toward
(<B>HSV_MAX_SATURATION</B>, <B>HSV_MAX_VALUE</B>) if the illumination is positive.
The extremes of the <IMG
 WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img258.png"
 ALT="$s$"> and <IMG
 WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img259.png"
 ALT="$v$"> are defined in the <U>.gmtdefaults4</U> file and are usually
chosen so the corresponding points are nearly black (<IMG
 WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img258.png"
 ALT="$s$"> = 1,
<IMG
 WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img259.png"
 ALT="$v$"> = 0) and white (<IMG
 WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img258.png"
 ALT="$s$"> = 0, <IMG
 WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
 SRC="img259.png"
 ALT="$v$"> = 1).
The reason this works is that the HSV system allows movements in
color space which correspond more closely to what we mean by
``tint'' and ``shade''; an instruction like ``add white'' is
easy in HSV and not so obvious in RGB.

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<ADDRESS>
Paul Wessel
2011-02-27
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