Indexing of elements of convolution functions that are used for gridding by `HPG` has been designed to optimize performance of the gridding kernel, and also to support a wide range of uses. To that end, the indexing of convolution function elements has been reduced to the essentials that are needed by the gridding kernel, and consequently some of the context of convolution functions as used for imaging has been lost. However, there is sufficient generality in the indexing scheme to allow all the usual gridding algorithms used for imaging to be realized using the generic indexing scheme. Nevertheless, because of the apparent lack of context, it can be difficult to map the normal schemes used for indexing convolution functions needed for imaging to the scheme implemented by `HPG`. To support use of index schemes used by CASA with `HPG`, various index transformation classes are implemented.
