Viewing image alignment

The previous sections describe various methods for aligning a set of images in a common coordinate system. Once this is done, you will usually want to resample and combine or compare them. Before, instead of, or after that however, you might want to see the positions of the images in the aligned coordinates. You can do this using the application:

• DRAWNDF

If you give DRAWNDF a list of images which are all aligned in their Current attached coordinate system, it will plot outlines of the regions covered by each image file. In this way it is easy to see which parts of your data cover which parts of the coordinate space.

There are two basic ways of running DRAWNDF, according to whether the parameter CLEAR is set to TRUE or FALSE. If CLEAR is TRUE, then the graphics device will be cleared and the outline of the area covered by each image will be shown. By default, a set of axes will be drawn as well. This will give you a good idea of the alignment of your files relative to each other, and by examining the axes you can see the absolute positions too. It can often be a good idea to do this with a set of data files before you resample and combine them, which may be a time-consuming step, just to check that the alignment looks sensible. You can see an example of this in Figure .

Figure: This image was generated by registering the images and running the command:
drawndf r1059* clear noaxes labname labnum

If CLEAR is FALSE however, DRAWNDF will try to align the outlines with an existing picture on the graphics display. This means that if you have already displayed an image on your graphics device which shares a coordinate system with your data files, you can see where they would fit over it. For instance you could show an image which has SKY coordinates using KAPPA's DISPLAY program and then run DRAWNDF on a set of data files in the same region; if their Current coordinates are also SKY you will be able to see how they map onto the displayed image. Another useful application is to DISPLAY a mosaic which has been made by combining a set of data files, then to run DRAWNDF on the originals so it is clear which file has contributed to which part of the mosaic. To achieve this alignment with previously drawn graphics, DRAWNDF uses the AGI graphics database in the same way as KAPPA programs; this is described more fully in SUN/95. You can see an example of this in Figure .

Figure: DRAWNDF command with CLEAR=FALSE. This picture was generated by displaying the sky region and then running the command:
drawndf "gc6352p1-?" noclear labpos=cf style="width(curves)=2"

Finally, DRAWNDF can display the actual image data for you. Unlike KAPPA's DISPLAY program, which always displays an image as a rectangle whose edges are horizontal and vertical, perhaps with with non-orthogonal image Current coordinate axes drawn over it, DRAWNDF always plots on a surface in which the Current coordinate system has horizontal and vertical axes. The image data array is therefore resampled to fit within the image bounds as they would appear in these coordinates. This can give you a good view of what a group of images will look like when they have been resampled and combined into their Current coordinates. Note however that unlike MAKEMOS, DRAWNDF performs no averaging or sophisticated normalisation of images, so that if two images overlap in their Current coordinates, all but the last-plotted will be obscured, and the relative brightnesses may not be right. When this option is used, the display is always cleared. An example of a set of frames from a mosaic camera displayed in this way is given in Figure .

Figure: DRAWNDF command with IMAGE=TRUE. This display was generated using the command:
drawndf "r10629?" image=yes