<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <!--Converted with LaTeX2HTML 2002-2-1 (1.71) original version by: Nikos Drakos, CBLU, University of Leeds * revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan * with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --> <HTML> <HEAD> <TITLE>I.5 Artificial illumination</TITLE> <META NAME="description" CONTENT="I.5 Artificial illumination"> <META NAME="keywords" CONTENT="GMT_Docs"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META NAME="Generator" CONTENT="LaTeX2HTML v2002-2-1"> <META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css"> <LINK REL="STYLESHEET" HREF="GMT_Docs.css"> <LINK REL="next" HREF="node214.html"> <LINK REL="previous" HREF="node212.html"> <LINK REL="up" HREF="node208.html"> <LINK REL="next" HREF="node214.html"> </HEAD> <BODY bgcolor="#ffffff"> <!--Navigation Panel--> <A NAME="tex2html5684" HREF="node214.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next.png"></A> <A NAME="tex2html5678" HREF="node208.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up.png"></A> <A NAME="tex2html5672" HREF="node212.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="prev.png"></A> <A NAME="tex2html5680" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="contents.png"></A> <A NAME="tex2html5682" HREF="node256.html"> <IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="index.png"></A> <BR> <B> Next:</B> <A NAME="tex2html5685" HREF="node214.html">I.6 Thinking in RGB</A> <B> Up:</B> <A NAME="tex2html5679" HREF="node208.html">I. Color Space: The</A> <B> Previous:</B> <A NAME="tex2html5673" HREF="node212.html">I.4 Color interpolation</A> <B> <A NAME="tex2html5681" HREF="node1.html">Contents</A></B> <B> <A NAME="tex2html5683" HREF="node256.html">Index</A></B> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION002650000000000000000"></A> <A NAME="31726"></A> <A NAME="31727"></A> <BR> I.5 Artificial illumination </H1> <A NAME="tex2html1806" 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="tex2html1807" HREF="../man/grdimage.html"><I><B>grdimage</B></I></A><A NAME="31783"></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="img257.png" ALT="$s$">,<IMG WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img258.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="img257.png" ALT="$s$">,<IMG WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img258.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="img257.png" ALT="$s$"> and <IMG WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img258.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="img257.png" ALT="$s$"> = 1, <IMG WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img258.png" ALT="$v$"> = 0) and white (<IMG WIDTH="10" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img257.png" ALT="$s$"> = 0, <IMG WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0" SRC="img258.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. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html5684" HREF="node214.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="next.png"></A> <A NAME="tex2html5678" HREF="node208.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="up.png"></A> <A NAME="tex2html5672" HREF="node212.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="prev.png"></A> <A NAME="tex2html5680" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="contents.png"></A> <A NAME="tex2html5682" HREF="node256.html"> <IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="index.png"></A> <BR> <B> Next:</B> <A NAME="tex2html5685" HREF="node214.html">I.6 Thinking in RGB</A> <B> Up:</B> <A NAME="tex2html5679" HREF="node208.html">I. Color Space: The</A> <B> Previous:</B> <A NAME="tex2html5673" HREF="node212.html">I.4 Color interpolation</A> <B> <A NAME="tex2html5681" HREF="node1.html">Contents</A></B> <B> <A NAME="tex2html5683" HREF="node256.html">Index</A></B> <!--End of Navigation Panel--> <ADDRESS> Paul Wessel 2010-07-14 </ADDRESS> </BODY> </HTML>