Identifies the different format compression schemes.

Values:

IG_COMPRESSION_UNKNOWN	Unidentified compression.
IG_COMPRESSION_NONE	No compression.
IG_COMPRESSION_PACKED_BITS	Packed bits compression.
IG_COMPRESSION_HUFFMAN	Huffman encoding.
IG_COMPRESSION_CCITT_G3	CCITT Group 3.
IG_COMPRESSION_CCITT_G4	CCITT Group 4.
IG_COMPRESSION_CCITT_G32D	CCITT Group 3 2D.
IG_COMPRESSION_JPEG	JPEG compression.
IG_COMPRESSION_RLE	Run length encoding.
IG_COMPRESSION_LZW	LZW compression.
IG_COMPRESSION_ABIC_BW	IBM ABIC compression.
IG_COMPRESSION_ABIC_GRAY	IBM ABIC compression.
IG_COMPRESSION_JBIG	IBM JBIG compression.
IG_COMPRESSION_FPX_SINCOLOR	Single color compression.
IG_COMPRESSION_FPX_NOCHANGE	Save with the same compression as loaded.
IG_COMPRESSION_DEFLATE	Deflate compression.
IG_COMPRESSION_IBM_MMR	IBM MMR compression.
IG_COMPRESSION_ABIC	IBM ABIC compression.
IG_COMPRESSION_PROGRESSIVE	Progressive compression (Progressive JPEG and may be PNG in future).
IG_COMPRESSION_EQPC	PowerSDK EQPC(Wavelet) compression.
IG_COMPRESSION_JBIG2	JBIG2 compression.
IG_COMPRESSION_LURAWAVE	This value has been deprecated and will be removed from the public API in a future release.
IG_COMPRESSION_LURADOC	This value has been deprecated and will be removed from the public API in a future release.
IG_COMPRESSION_LURAJP2	This value has been deprecated and will be removed from the public API in a future release.
IG_COMPRESSION_ASCII	Image data is converted to ASCII text.
IG_COMPRESSION_RAW	Image data is stored directory in binary raw format.
IG_COMPRESSION_JPEG2K	JPEG2K compression.
IG_COMPRESSION_HDP	HD Photo compression.
IG_COMPRESSION_HVEC	High Efficiency Video Coding. Used for HEIF images.
IG_COMPRESSION_WEBP_LOSSLESS	WEBP lossless compression. Used for WEBP images.
IG_COMPRESSION_WEBP_LOSSY	WEBP lossy compression. Used for WEBP images.