kdu_codestream::set_persistent

The persistent mode is important for interactive applications. By default, tiles, precincts and code-blocks will be discarded as soon as it can be determined that the user will no longer be accessing them. Moreover, after each code-block has been opened and closed, it will automatically be discarded. This behaviour minimizes memory consumption when the image (or some region of interest) is to be decompressed only once. For interactive applications, however, it is often desirable to leave the code-stream intact and permit multiple accesses to the same information so that a new region or resolution of interest can be defined and decompressed at will. For these applications, you must invoke this member function before any attempt to access (i.e., open) an image tile.

Evidently, persistence is irrelevant for output codestreams. For interchange codestreams, this function also has no effect. Interchange codestreams have some persistent properties in that any tile may be opened and closed as often as desired. They also have some of the properties of the non-persistent mode, in that compressed data is discarded as soon as the relevant precinct is closed (but not when the containing tile is closed!). For more on these issues, see the description of kdu_resolution::open_precinct.