This is to announce that a revised 2.1 version of the document
describing the FITS Tiled Image Compression Convention has been placed
on the FITS Support office web site at

http://fits.gsfc.nasa.gov/registry/tilecompression.html

The main changes in this version of the document are in Section 4 and in
Appendix A, which describe a "subtractive dithering" technique for
quantizing floating point FITS images into scaled integer values, prior
to compressing the integers with a choice of algorithm (usually Rice).


The problem with the previous simple quantization technique, in which
all the image pixels are simply quantized into a fixed grid of discrete
values, is that this can bias estimates of the background flux level in
an image when using relatively coarse quantization. For example, if the
actual background level in an image has a mean value of 0.4, but all the
pixels are quantized into levels 0, 0.5, 1.0, ..., then the background
estimate (e.g., using the median of the pixel values, or the mean after
several N sigma rejection cycles) will be biased towards the nearest
quantized level (0.5 in this example).

The subtractive dithering technique (which has been widely used in
signal processing applications) reduces this bias problem by randomly
dithering the zero point of the quantization grid for each pixel. So,
continuing with previous example, all the pixels are still quantized
into levels that have a spacing of 0.5, but the whole grid is shifted up
or down by a random amount on a pixel by pixel basis. Thus, the median
of the quantized pixel values will more closely match that of the
original image. The big advantage of this subtractive dithering
technique is that the randomization is accomplished without introducing
any extra noise into the image.

In collaboration with Rick White (STScI) and Rob Seaman (NOAO),
this new dithering technique has been implemented in the latest 3.20
version of the CFITSIO library and in the new versions of the fpack and
funpack FITS image compression utilities. It should be noted that this
implementation is backwards compatible with any existing FITS images
that were quantized using the previous quantization technique.

Bill Pence
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__________________________________________________ __________________
Dr. William Pence
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
Greenbelt MD 20771 +1-301-286-1684 (fax)