##
Re-Defining a FrameSet Coordinate System

As discussed earlier (§13.4), an important application of a FrameSetFrameSet is to allow coordinate system information to be attached to entities such as images in order to calibrate them. In addition, one of the main objectives of AST is to simplify the propagation of such information through successive stages of data processing, so that it remains consistent with the associated image data.

In such a situation, the FrameSet's base FrameFrame would correspond with the image's data grid coordinates and its other Frames (if any) with the various alternative world coordinate systems associated with the image. If the data processing being performed does not change the relationship between the image's data grid coordinates and any of the associated world coordinate systems, then propagation of the WCS information is straightforward and simply involves copying the FrameSet associated with the image.

If any of these relationships change, however, then corresponding changes must be made to the way Frames within the FrameSet are inter-related. By far the most common case occurs when the image undergoes some geometrical transformation resulting in “re-gridding” on to another data grid, but the same principles can be applied to any re-definition of a coordinate system.

To pursue the re-gridding example, we would need to modify our FrameSet to account for the fact that the image's data grid coordinate system (corresponding to the FrameSet's base Frame) has changed. Looking at the steps needed in detail, we might proceed as follows:

1. Create a MappingMapping which represents the relationship between the original data grid coordinate system and the new one.

2. Obtain a Frame to represent the new data grid coordinate system (we could re-use the original base Frame here, using astGetFrameastGetFrame to obtain a pointer to it).

3. Add the new Frame to the FrameSet, related to the original base Frame by the new Mapping. This Frame now represents the new data grid coordinate system and is correctly related to all the other Frames present.

4. Remove the original base Frame (representing the old data grid coordinate system).

5. Make the new Frame the base Frame and restore the original current Frame.

The effect of these steps is to change the relationship between the base Frame and all the other Frames present. It is as if a new Mapping has been interposed between the Frame we want to alter and all the other Frames within the FrameSet (Figure 17).

Figure 17: The effect of astRemapFrameastRemapFrame is to interpose a Mapping between a nominated Frame within a FrameSet and the remaining contents of the FrameSet. This effectively “re-defines” the coordinate system represented by the affected Frame. It may be used to compensate (say) for geometrical changes made to an associated image. The inter-relationships between all the other Frames within the FrameSet remain unchanged.
[width=0.7]sun211_figures/fsremap

Performing the steps above is rather lengthy, however, so the astRemapFrame function is provided to perform all of these operations in one go. A practical example of its use is given below (§14.5).