The structure dipio_Compression specifies the compression method to use when writing an image file, and contains the following elements:
| Data type | Name | Description |
| dipio_CompressionMethod | method | Compression method |
| dip_int | level | Compression parameter, dependent on method |
dipio_CompressionMethod is an enum with the known compression methods. File formats typically only support one or a few of these, and most of these methods do not have a parameter to set, in which case level is ignored. If an unsupported compression method is selected, no compression is done. The dipio_CompressionMethod has the following values:
| Name | Description |
| DIPIO_CMP_DEFAULT | Default compression method for the file format |
| DIPIO_CMP_NONE | No compression |
| DIPIO_CMP_GZIP | ZIP compression, using zlib. The level parameter is between 0 and 9, 1 being the faster, lesser compression and 9 being the slower, higher compression. 0 indicates no compression. |
| DIPIO_CMP_DEFLATE | Deflate (same as DIPIO_CMP_GZIP) |
| DIPIO_CMP_COMPRESS | Using UNIX's "compress" utility, which uses the LZW algorithm |
| DIPIO_CMP_LZW | LZW compression (same as DIPIO_CMP_COMPRESS) |
| DIPIO_CMP_JPEG | Lossy JPEG compression. The level parameter is between 1 and 100, higher numbers giving better quality output but larger files. |
| DIPIO_CMP_PACKBITS | PackBits |
| DIPIO_CMP_THUNDERSCAN | ThunderScan |
| DIPIO_CMP_NEXT | NeXT |
| DIPIO_CMP_CCITTRLE | CCITT RLE |
| DIPIO_CMP_CCITTRLEW | CCITT RLE/W |
| DIPIO_CMP_CCITTFAX3 | CCITT Group 3 |
| DIPIO_CMP_CCITTFAX4 | CCITT Group 4 |
Thus only DIPIO_CMP_GZIP and DIPIO_CMP_JPEG currently have a level to set.
The TIFF file writer understand the methods DIPIO_CMP_NONE, DIPIO_CMP_DEFLATE, DIPIO_CMP_LZW, DIPIO_CMP_JPEG, DIPIO_CMP_PACKBITS, DIPIO_CMP_THUNDERSCAN, DIPIO_CMP_NEXT, DIPIO_CMP_CCITTRLE, DIPIO_CMP_CCITTRLEW, DIPIO_CMP_CCITTFAX3 and DIPIO_CMP_CCITTFAX4. It defaults to DIPIO_CMP_DEFLATE. The level parameter is currently not used.
The ICS file writer understands DIPIO_CMP_NONE, DIPIO_CMP_GZIP and DIPIO_CMP_COMPRESS, although DIPIO_CMP_COMPRESS is currently not implemented. It defaults to DIPIO_CMP_GZIP.
The GIF file writer only understands DIPIO_CMP_LZW. The compression method selected is simply ignored.
The JPEG file writer only understands DIPIO_CMP_JPEG. The compression method selected is simply ignored.
The PNG file writer only understands DIPIO_CMP_GZIP. The compression method selected is simply ignored. [The PNG writer is not yet implemented!]
All other file writers do not compress, and simply ignore the compression method requested.