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<H2><A NAME="SECTION000316000000000000000"></A><A NAME="ss:howtoplotgrid"></A>
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...Display a Graphical Coordinate Grid
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<P>
A common requirement when displaying image data is to plot an
   associated coordinate grid (<SPAN  CLASS="textit">e.g.</SPAN> Figure&nbsp;<A HREF="#fig:overgrid">9</A>)
   over the displayed image.
   
<DIV ALIGN="CENTER"><A NAME="fig:overgrid"></A><A NAME="602"></A>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 9:</STRONG>
An example of a displayed image with a coordinate grid
   plotted over it.</CAPTION>
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[width=0.7]sun211_figures/overgrid_bw
   
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<P>
The use of AST in such circumstances is independent of the underlying
graphics system, so starting up the graphics system, setting up a
coordinate system, displaying the image, and closing down afterwards
can all be done using the graphics functions you would normally use.

<P>
However, displaying an image at a precise location can be a little
fiddly with some graphics systems, and obviously the grid drawn by AST
will not be accurately registered with the image unless this is done
correctly. In the following template, we therefore illustrate both
steps, basing the image display on the C interface to the PGPLOT
graphics package.<A NAME="tex2html21"
  HREF="footnode.html#foot22050"><SUP><IMG  ALIGN="BOTTOM" BORDER="1" ALT="[*]"
 SRC="/usr/share/latex2html/icons/footnote.png"></SUP></A>Plotting a coordinate grid with AST then becomes a relatively minor
part of what is almost a complete graphics program.

<P>
Once again, we assume that a pointer, ``wcsinfo'', to a suitable
FrameSetFrameSet associated with the image has already been obtained
(&#167;<A HREF="node17.html#ss:howtoreadwcs">3.4</A>).

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<P>
Note that once you have set up a PlotPlot which is aligned with a
displayed image, you may also use it to generate further graphical
output of your own, specified in the image's world coordinate system
(such as markers to represent astronomical objects, annotation,
<SPAN  CLASS="textit">etc.</SPAN>). There is also a range of Plot attributes which gives
control over most aspects of the output's appearance.  For details of
the facilities available, see &#167;<A HREF="node194.html#ss:plots">21</A> and the description of
the Plot class in Appendix&nbsp;<A HREF="node217.html#ss:classdescriptions">D</A>.

<P>
For details of how to build a graphics program which uses PGPLOT, see
&#167;<A HREF="node16.html#ss:howtobuild">3.3</A> and the description of the ast_linkast_link command in
Appendix&nbsp;<A HREF="node218.html#ss:commanddescriptions">E</A>.

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