arrays contain elements which typically represent the values of a
physical quantity at some coordinate location. Consequently they
need not contain any pixel rendering information in the form of
transfer functions, and there is no mechanism for color look-up
tables. An application SHOULD provide this functionality, either
statically using a more or less sophisticated algorithm, or
interactively allowing a user various degrees of choice.
Furthermore, the elements in a FITS data array may be integers or
floating-point numbers. The dynamic range of the data array values
may exceed that of the display medium and the eye, and their
distribution may be highly nonuniform. Logarithmic, square-root, and
quadratic transfer functions along with histogram equalization
techniques have proved helpful for rendering FITS data arrays. Some
elements of the array may have values which indicate that their data
are undefined or invalid; these should be rendered distinctly. Via
WCS Paper I [WCS1] the standard permits "CTYPEnnn = 'COMPLEX'" to
assert that a data array contains complex numbers (future revisions
might admit other elements such as quaternions or general tensors).
Three-dimensional data arrays (NAXIS=3 with NAXIS1, NAXIS2 and NAXIS3
> 1) are of special interest. Applications intended to handle
"image/fits" MAY default to displaying the first 2D plane of such an
image cube, or they MAY default to presenting such an image in a
fashion akin to that used for an animated GIF, or they MAY present
the data cube as a mosaic of "thumbnail" images. Even in the absence
of WCS indication of a temporal axis the time-lapse movie-looping
display technique can be effective, and application writers SHOULD
consider offering it for all three-dimensional arrays.
An "image/fits" PHDU with NAXIS=1 is describing a one-dimensional
entity such as a spectrum or a time series. Applications intended to
handle "image/fits" MAY default to displaying such an image as a
graphical plot rather than as a two-dimensional picture with a single
row.
An application that cannot handle an image with dimensionality other
than 2 SHOULD gracefully indicate its limitations to its users when
it encounters NAXIS=1 or NAXIS=3 cases, while still providing access
to the keyword/value pairs.
FITS files with degenerate axes (i.e., one or more NAXISn=1) MAY be
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