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[fitsbits] Update to the FITS Tiled Image Compression Convention
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 -- __________________________________________________ __________________ Dr. William Pence NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
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