When choosing a value for the DomainDomain attribute of a FrameFrame, it
obviously makes sense to avoid generic names which might clash with
those used for similar (but subtly different!) purposes by other
programmers. If you are developing software for an instrument, for
example, and want to identify an instrumental coordinate system, then
it is sensible to add a distinguishing prefix. For instance, you might
use INST
_FOCAL_PLANE, where
INST
(e.g. an
acronym) identifies your instrument.
For some purposes, however, a standard choice of Domain name is desirable so that different items of software can communicate. For this purpose, the following Domain names are reserved by AST and the use recommended below should be carefully observed:
If data are copied or transformed to a new data grid (by whatever means), or a subset of the original grid is extracted, then the same rules apply to the copy or subset. Its first element therefore has GRID coordinate values of unity at its centre. Note that this means that GRID coordinates remain attached to the first element of the data grid and not to its data content (e.g. the features in an image).
Because the amount of shift is unspecified, the PIXEL domain is distinct from the GRID domain. The relationship between them contains a degree of uncertainty, such as typically arises from the different conventions used by different software systems. For instance, in some software the first pixel is regarded as being centred at (1,1), while in other software it is at (0.5,0.5). In addition, some software packages implement a “pixel origin” which allows pixel coordinates to start at an arbitrary value.
The GRID domain (which corresponds with the pixel-numbering convention used by FITS) is a special case of the PIXEL domain and avoids this uncertainty. In general, additional information is required in order to convert from one to the other.
Although we have drawn a necessary distinction here between the GRID and PIXEL domains, we will continue to refer in general terms to image “pixels” and “pixel coordinates” whenever this distinction is not important. This should not be taken to imply that the GRID convention for numbering pixels is excluded—in fact, it is usually to be preferred (at the level of data handling being discussed in this document) and we recommend it.