These are all the possible values for the IG_ISIS_TAG_COMPRESSION tag.
| IG_ISIS_COMPRESSION_NONE | No compression, each line is packed to one byte. |
| IG_ISIS_COMPRESSION_MG3 | Modified (no EOL) CCITT Group 3 1-Dimensional Huffman run-length encoding, each line starts on a new byte. |
| IG_ISIS_COMPRESSION_G3 | Standard CCITT Group 3 coding, 1 Dimensional Huffman run-length encoding. |
| IG_ISIS_COMPRESSION_G4 | Standard CCITT Group 4 encoding. |
| IG_ISIS_COMPRESSION_LZW | LZW coding as described in the TIFF specification. |
| IG_ISIS_COMPRESSION_JPEG | JPEG coding typically used for grayscale and color only. |
| IG_ISIS_COMPRESSION_PACK | Packbits compression. |
| IG_ISIS_COMPRESSION_32771 | No compression, each line padded to two bytes. |
| IG_ISIS_COMPRESSION_JBIG | JBIG compression. |
| IG_ISIS_COMPRESSION_PCX | Compression scheme used in PCX files. |
| IG_ISIS_COMPRESSION_4BYTE | No compression, each line padded to four bytes. |
| IG_ISIS_COMPRESSION_RUN | Runlist compression; internal use only. |
| IG_ISIS_COMPRESSION_GIF | Compression scheme used in GIF files. |
| IG_ISIS_COMPRESSION_BMP | Compression scheme used in BMP files. |
| IG_ISIS_COMPRESSION_GIFINTERLACED | Compression scheme used in interlaced GIF files. |
| IG_ISIS_COMPRESSION_WANGJPEG | Wang format JPEG encoding, typically used for grayscale and color only. |
| IG_ISIS_COMPRESSION_JPEG_PROGRESSIVE | Progressive JPEG. |
| IG_ISIS_COMPRESSION_JP2 | JPEG2 Compression. |