Data compression. 0x0103 ∙ 259

IG_ISIS_TAG_COMPRESSION allows you to set or determine the compression being used by a scanner.

Type:

IG_ISIS_TAG_TYPE_SHORT

Drivers:

All source and destination drivers.

Values:

The application must get choices (using IG_ISIS_choice_get_long) to be aware of the number of possible compression settings available in a scanner. Most scanners either return uncompressed only or uncompressed and Group 4 compressed data. To determine the compression format that exists in a file or other image data, use IG_ISIS_tag_get_long. The following values define the various compression choices that were available when this manual was last revised:

1	IG_ISIS_COMPRESSION_NONE	No compression, each line is packed to one byte.
2	IG_ISIS_COMPRESSION_MG3	Modified (no EOL) CCITT Group 3 1-Dimensional Huffman run-length encoding, each line starts on a new byte. BitsPerSample and SamplesPerPixel must be 1, since this type of compression is defined only for binary images.
3	IG_ISIS_COMPRESSION_G3	Standard CCITT Group 3 coding, 1 Dimensional Huffman run-length encoding. BitsPerSample and SamplesPerPixel must be 1, since this type of compression is defined only for binary images.
4	IG_ISIS_COMPRESSION_G4	Standard CCITT Group 4 encoding. BitsPerSample and SamplesPerPixel must be 1, since this type of compression is defined only for binary images.
5	IG_ISIS_COMPRESSION_LZW	LZW coding as described in the TIFF specification.
6	IG_ISIS_COMPRESSION_JPEG	JPEG coding typically used for grayscale and color only.
32771	IG_ISIS_COMPRESSION_32771	No compression, each line padded to two bytes. (Erroneously called IG_ISIS_ COMPRESSION_N16 in previous documentation.)
32773	IG_ISIS_COMPRESSION_PACK	Packbits compression.
34661	IG_ISIS_COMPRESSION_JBIG	JBIG compression, typically used for binary and grayscale only and returned by very few scanners, is a lossless multi-plane compression format that yields high compression for most business documents. This data can be written to a TIFF file that can be uncompressed and converted into another format.
50000	IG_ISIS_COMPRESSION_PCX	Compression scheme used in PCX files.
50001	IG_ISIS_COMPRESSION_4BYTE	No compression, each line padded to four bytes.
50002	IG_ISIS_COMPRESSION_RUN	Runlist compression; internal use only.
50005	IG_ISIS_COMPRESSION_GIF	Compression scheme used in GIF files.
50006	IG_ISIS_COMPRESSION_BMP	Compression scheme used in BMP files.
50007	IG_ISIS_COMPRESSION_RUNBLOCK	A block of runlists; internal use only.
50014	IG_ISIS_COMPRESSION_WANGJPEG	Wang format JPEG encoding, typically used for grayscale and color only.

Tag Dependencies and Dependent Tags:

Some scanner drivers may reset the value of IG_ISIS_TAG_COMPRESSION based on the values of the color format tags. For example, it does not make sense to use JPEG compression on a binary PCX file, because there are no applications that will open such a file. Therefore, you should be sure to set IG_ISIS_TAG_COMPRESSION after setting the desired color format.

IG_ISIS_TAG_SAMPLESPERPIXEL ∙ IG_ISIS_TAG_BITSPERSAMPLE ∙ IG_ISIS_TAG_PHOTOMETRICINTERPRETATION ∙ IG_ISIS_TAG_COMPRESSION

See Also

IG_ISIS_TAG_BITSPERSAMPLE

IG_ISIS_TAG_SAMPLESPERPIXEL

IG_ISIS_TAG_PHOTOMETRICINTERPRETATION