Data compression. 0x0103 ∙ 259
IG_ISIS_TAG_COMPRESSION allows you to set or determine the compression being used by a scanner.
IG_ISIS_TAG_TYPE_SHORT
All source and destination drivers.
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. |
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
IG_ISIS_TAG_PHOTOMETRICINTERPRETATION