IFD		Image
Code		259 (hex 0x0103)
Name		Compression
LibTiff name		TIFFTAG_COMPRESSION
Type		SHORT
Count		1
Default		1 (No compression)

Description

Compression scheme used on the image data.

The specification defines these values to be baseline:

1 = No compression
2 = CCITT modified Huffman RLE
32773 = PackBits compression, aka Macintosh RLE

Additionally, the specification defines these values as part of the TIFF extensions:

3 = CCITT Group 3 fax encoding
4 = CCITT Group 4 fax encoding
5 = LZW
6 = JPEG ('old-style' JPEG, later overridden in Technote2)

Technote2 overrides old-style JPEG compression, and defines:

7 = JPEG ('new-style' JPEG)

Adobe later added the deflate compression scheme:

8 = Deflate ('Adobe-style')

The TIFF-F specification (RFC 2301) defines:

9 = Defined by TIFF-F and TIFF-FX standard (RFC 2301) as ITU-T Rec. T.82 coding, using ITU-T Rec. T.85 (which boils down to JBIG on black and white).
10 = Defined by TIFF-F and TIFF-FX standard (RFC 2301) as ITU-T Rec. T.82 coding, using ITU-T Rec. T.43 (which boils down to JBIG on color).

LibTiff supports most of these. (Even though it can be argued that old-style JPEG cannot be really properly supported, and is probably best ignored, there is some attempt at decoding support for some common old-style JPEG interpretations.) Additionally, LibTiff adds support for some compression schemes that are not part of the specification and are somewhat less common. Here's LibTiff's definition of possible values:

COMPRESSION_NONE = 1;
COMPRESSION_CCITTRLE = 2;
COMPRESSION_CCITTFAX3 = COMPRESSION_CCITT_T4 = 3;
COMPRESSION_CCITTFAX4 = COMPRESSION_CCITT_T6 = 4;
COMPRESSION_LZW = 5;
COMPRESSION_OJPEG = 6;
COMPRESSION_JPEG = 7;
COMPRESSION_NEXT = 32766;
COMPRESSION_CCITTRLEW = 32771;
COMPRESSION_PACKBITS = 32773;
COMPRESSION_THUNDERSCAN = 32809;
COMPRESSION_IT8CTPAD = 32895;
COMPRESSION_IT8LW = 32896;
COMPRESSION_IT8MP = 32897;
COMPRESSION_IT8BL = 32898;
COMPRESSION_PIXARFILM = 32908;
COMPRESSION_PIXARLOG = 32909;
COMPRESSION_DEFLATE = 32946;
COMPRESSION_ADOBE_DEFLATE = 8;
COMPRESSION_DCS = 32947;
COMPRESSION_JBIG = 34661;
COMPRESSION_SGILOG = 34676;
COMPRESSION_SGILOG24 = 34677;
COMPRESSION_JP2000 = 34712;