## Can I overlay contours on an image?

Contours can be overlain on images, or on other contours, or even curves and histograms. If there is a current plot, then an add_<object> call will add the new data to this plot. It may not however be visible if the axis range and scales of the existing plot and new data are not similar.

If acis.fits and vla.fits both contain WCS information then the following commands will overlay contours from vla.fits onto the acis.fits image (blue lines are positive values and red lines negative ones):

chips> clear()
chips> make_figure("acis.fits", "image", ["colormap", "heat"])
chips> set_contour(["levels", [-0.02, -0.01], "color", "red"])
chips> set_contour(["levels", [0.01, 0.02, 0.03, 0.04, 0.05], "color", "blue"])


#### The ChIPS Gallery

More examples can be seen in the ChIPS Gallery.

Unsupported WCS projections

The add_contour, add_image, and make_figure commands will warn you if given an image with an unsupported WCS projection (that is, everything but a tangent-plane projection), and fall back to using the physical or logical systems. An example of the warning is:

chips WARNING: Unable to use world transform - RA---SIN, DEC--SIN is an unsupported type. Using physical transform.