Struct lsst::afw::fits::ImageCompressionOptions

struct ImageCompressionOptions

Options for tile compression of image pixels

Tile compression is a feature provided by cfitsio, where contiguous parts of the image (“tiles”, e.g., rows, multiple rows, blocks or the entire image) are compressed separately. The aim of this struct is to hold the parameters used to configure the compression.

Floating-point images can be losslessly compressed (quantizeLevel=0.0) using (only) the GZIP or GZIP_SHUFFLE compression algorithms, but the compression factor so achieved is modest (e.g., ~ 10% compression). Better compression factors can be achieved if floating-point images are first quantised into integer images. This can be done by cfitsio (through the quantizeLevel parameter) or through use of the ImageScalingOptions.

The Compression is specified by:

• the compression algorithm

• the tile size

• the quantization level (for quantization applied by cfitsio; for floating-point images)

Due to bugs, cfitsio may require setting the quantizeLevel to a value other than zero when compressing integer data, but in this case it should have no effect.