Fix for 687591 Overprint Preview is not documented

Fix for 687591 Overprint Preview is not documented.  This change adds
documentaion for how Ghostscript handles overprinting and spot colors
to Use.htm.

Index: doc/Use.htm
===================================================================
RCS file: /cvs/ghostscript/gs/doc/Use.htm,v
retrieving revision 1.109
diff -u -r1.109 Use.htm
--- doc/Use.htm 29 May 2004 01:21:54 -0000      1.109
+++ doc/Use.htm 23 Jul 2004 06:05:05 -0000
@@ -45,6 +45,7 @@
 </ul>

 <li><a href="#EPS">Using Ghostscript with EPS files</a>
+<li><a href="#SPOT">Using Ghostscript with overprinting and spot colors</a>
 <li><a href="#Finding_files">How Ghostscript finds files</a>
 <ul>
 <li><a href="#PS_resources">Finding PostScript Level 2 resources</a>
@@ -144,7 +145,7 @@
 Ghostscript on unix-like systems type:

 <blockquote>
-       <b><tt><pre>gs</tt></b> [options] {filename 1} ... [options] {filename <em>N</em>} ...
+       <pre><b><tt>gs</tt></b> [options] {filename 1} ... [options] {filename <em>N</em>} ...
 </pre></blockquote>

 <p>Here are some basic examples. The details of how these work are described below.
@@ -625,6 +626,79 @@
 class="offsite">
 http://partners.adobe.com/asn/developer/technotes/postscript.html</a>

+<h2><a name="SPOT"></a>Using Ghostscript with overprinting and spot colors</h2>
+
+<p>
+In general with PostScript and PDF interpreters, the handling of
+<b>overprinting</b> and <b>spot colors</b> depends upon the
+process color model of the <a href="#Output_device">output device</a>. Devices
+that produce gray or RGB output have an <b>additive</b> process color model.
+Devices which produce CMYK output have a <b>subtractive</b> process color model.
+Devices may, or may not, have support for spot colors.
+
+<blockquote><i>
+Note: The differences in appearance of files with overprinting and spot colors
+caused by the differences in the color model of the output device are part of the
+PostScript and PDF specifications. They are not due to a limitation in the
+implementation of Ghostscript or its output devices.
+</i></blockquote>
+
+<p>
+With devices which use a subtractive process color model, both PostScript
+and PDF allow the drawing of objects using colorants (inks) for one or more planes
+without affecting the data for the remaining colorants. Thus the inks for one
+object may <b><tt>overprint</tt></b> the inks for another object. In some cases
+this produces a transparency like effect. (The effects of overprinting should
+not be confused with the PDF 1.4 blending operations which are supported for
+all output devices.) Overprinting is not allowed for devices with an additive
+process color model. With files that use overprinting, the appearance of the
+resulting image can differ between devices which produce RGB output versus devices
+which produce CMYK output. Ghostscript automatically overprints (if needed)
+when the output device uses a subtractive process color model. For example,
+if the file is using overprinting, differences can be seen in the appearance
+of the output from the <a href="Devices.htm#TIFF">tiff24nc and tiff32nc devices</a>
+which use an RGB and a CMYK process color models.
+
+<p>
+Most of the Ghostscript <a href="Devices.htm">output devices</a> do not have
+file formats which support spot colors. Instead spot colors are converted using
+the tint transform function contained within the color space definition.. However
+there are several devices which have support for spot colors. The PSD format
+(Adobe Photoshop) produced by the <a href="Devices.htm#PSD">psdcmyk device</a>
+contains both the raster data plus an equivalent CMYK color for each spot color.
+This allows Photoshop to simulate the appearance of the spot colors. The <a href="De
vices.htm#display_device">display
+device (MS Windows, OS/2, gtk+)</a> can be used with different color models:
+Gray, RGB, CMYK only, or CMYK plus spot colors (separation). The display device,
+when using its CMYK plus spot color (separation) mode, also uses an equivalent
+CMYK color to simulate the appearance of the spot color. The
+<a href="Devices.htm#TIFF">tiffsep
+device</a> creates output files for each separation (CMYK and any spot colors
+present). It also creates a composite CMYK file using an equivalent CMYK color
+to simulate the appearance of spot colors. The
+<a href="Devices.htm#XCF">xcfcmyk device</a>
+creates output files with spot colors placed in separate alpha channels. (The
+XCF file format does not currently directly support spot colors.)
+
+<p>
+Overprinting with spot colors is not allowed if the tint transform function
+is being used to convert spot colors. Thus if spot colors are used with overprinting
,
+then the appearance of the result can differ between output devices. One result
+would be obtained with a CMYK only device and another would be obtained with
+a CMYK plus spot color device. In a worst case situation where a file has overprinti
ng
+with both process (CMYK) and spot colors, it is possible to get three different
+appearances for the same input file using the
+<a href="Devices.htm#TIFF">tiff24nc</a> (RGB),
+<a href="Devices.htm#TIFF">tiff32nc</a> (CMYK), and
+<a href="Devices.htm#TIFF">tiffsep</a> (CMYK plus spot colors) devices.
+
+<blockquote><i>
+In Adobe Acrobat, viewing of the effects of overprinting is enabled by the
+'Overprint Preview' item in the 'Advanced' menu. This feature is not available
+in the free Acrobat Reader. The free Acrobat Reader also uses the tint transform
+functions to convert spot colors to the appropriate alternate color space.
+</i></blockquote>
+
+
 <hr>

 <h2><a name="Finding_files"></a>How Ghostscript finds files</h2>