The contour plot is drawn over an existing image that is displayed on the chosen graphics workstation or its overlay, provided the displayed image has been recorded in the graphics database. (This will be the case for display routines in KAPPA.) The contour plotting occurs within the current picture only if it is a DATA picture, otherwise contours are overlaid in the last DATA picture within the current picture. This application assumes that the world co-ordinate systems of the data array and the displayed image are both in pixel units, but not to the same origins. Pixel - offsets may be given to match the contour plot with the image, provided some contouring will be visible. These displacements are in the sense image co-ordinate minus the data-array co-ordinate for an arbitrary fiducial point.
The contouring algorithm has only pixel resolution, and so the contours are not smooth, but this makes the processing much faster. There are seven methods for selecting contours.
The best way to use this application is to first display an image on the base plane of an image display, make this the current picture, and then plot contours on the overlay plane, clearing the overlay picture each time. This enables more than one attempt at getting the correct contour heights. The underlying image will not be erased. (Note that if you do not make the underlying image the current picture, the contour plot becomes the last DATA picture, and so any subsequent - offsets should be set to 0,0 to prevent successive contour plots being incorrectly located.)
The suggested default is the current value, which is initially "Free".
If the colour is to remain unaltered as the lookup table is manipulated choose an integer between 0 and 15, or a named colour. This parameter will be ignored if PENROT = TRUE. [The current value, but equals 1 (the foreground colour) if there is no current value.]
Pen rotation takes precedence over colour control through CONCOL.
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