12.6 Tile-Compressed Image Format

CFITSIO supports a convention for compressing n-dimensional images and storing the resulting byte stream in a variable-length column in a FITS binary table. The general principle used in this convention is to first divide the n-dimensional image into a rectangular grid of subimages or `tiles'. Each tile is then compressed as a continuous block of data, and the resulting compressed byte stream is stored in a row of a variable length column in a FITS binary table. By dividing the image into tiles it is generally possible to extract and uncompress subsections of the image without having to uncompress the whole image. The default tiling pattern treats each row of a 2-dimensional image (or higher dimensional cube) as a tile, such that each tile contains NAXIS1 pixels (except the default with the HCOMPRESS algorithm is to compress the whole 2D image as a single tile). Any other rectangular tiling pattern may also be defined. In the case of relatively small images it may be sufficient to compress the entire image as a single tile, resulting in an output binary table with 1 row. In the case of 3-dimensional data cubes, it may be advantageous to treat each plane of the cube as a separate tile if application software typically needs to access the cube on a plane by plane basis.

See section 5.6 “Image Compression” for more information on using this tile-compressed image format.