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...Display a Graphical Coordinate Grid

A common requirement when displaying image data is to plot an associated coordinate grid (e.g. Figure 9) over the displayed image.

Figure 9: An example of a displayed image with a coordinate grid plotted over it.
[width=0.7]sun211_figures/overgrid_bw

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.

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. Plotting a coordinate grid with AST then becomes a relatively minor part of what is almost a complete graphics program.

Once again, we assume that a pointer, “wcsinfo”, to a suitable FrameSetFrameSet associated with the image has already been obtained (§3.4).

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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, etc.). 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 §21 and the description of the Plot class in Appendix D.

For details of how to build a graphics program which uses PGPLOT, see §3.3 and the description of the ast_linkast_link command in Appendix E.