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PyFITS Documentation, Release 3.4.dev
Uncompressed image data
header : Header instance, optional
Header to be associated with the image; when reading the HDU from a file
(data=DELAYED), the header read from the file
name : str, optional
The EXTNAME value; if this value is None, then the name from the input image header
will be used; if there is no name in the input image header then the default name
COMPRESSED_IMAGE is used.
compression_type : str, optional
Compression algorithm: one of ’RICE_1’, ’RICE_ONE’, ’PLIO_1’, ’GZIP_1’,
’GZIP_2’, ’HCOMPRESS_1’
tile_size : int, optional
Compression tile sizes. Default treats each row of image as a tile.
hcomp_scale : float, optional
HCOMPRESS scale parameter
hcomp_smooth : float, optional
HCOMPRESS smooth parameter
quantize_level : float, optional
Floating point quantization level; see note below
quantize_method : int, optional
Floating point quantization dithering method; can be either NO_DITHER (-1),
SUBTRACTIVE_DITHER_1 (1; default), or SUBTRACTIVE_DITHER_2 (2); see
note below
dither_seed : int, optional
Random seed to use for dithering; can be either an integer in the range 1 to 1000 (inclusive), DITHER_SEED_CLOCK (0; default), or DITHER_SEED_CHECKSUM (-1); see
note below
Notes
The pyfits module supports 2 methods of image compression.
1.The entire FITS file may be externally compressed with the gzip or pkzip utility programs, producing a
*.gz or *.zip file, respectively. When reading compressed files of this type, pyfits first uncompresses
the entire file into a temporary file before performing the requested read operations. The pyfits module
does not support writing to these types of compressed files. This type of compression is supported in the
_File class, not in the CompImageHDU class. The file compression type is recognized by the .gz or
.zip file name extension.
2.The CompImageHDU class supports the FITS tiled image compression convention in which the image is
subdivided into a grid of rectangular tiles, and each tile of pixels is individually compressed. The details
of this FITS compression convention are described at the FITS Support Office web site. Basically, the
compressed image tiles are stored in rows of a variable length arrray column in a FITS binary table. The
pyfits module recognizes that this binary table extension contains an image and treats it as if it were an
image extension. Under this tile-compression format, FITS header keywords remain uncompressed. At
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Chapter 1. Introduction