Portable Network Graphics (PNG) format
(see the PNG home page at the following URL
http://libpng.org/pub/png)
is used for the display of pixel images in the PPS products.
PNG is an extensible file format for the loss-less, portable,
well-compressed storage of images. PNG provides a patent-free
replacement for GIF as the graphics file format of choice and can also
replace many common uses of
TIFF. Indexed-colour, grey-scale, and true-colour images are supported, plus an
optional alpha channel for transparency. Sample depths range from 1 to 16
bits.
As with PDF graphics, in some cases, PNG graphics files are derived, at least in part, from PPS product files in FITS format. In these cases the PNG files are significantly smaller than their FITS files and/or contain annotations providing a user-friendly summary of the FITS file.