Both callback functions are called by TransfromDIB at the beginning and at the end of processing. Callback routine can initialize private data. Sent before beginning processing of a line. Good time to update a progress indicator. A pixel's data is about to be processed.
Conversion is complete.
Temporary data objects can now be released. The application can use this address as a reference value or to store data for use by the callback function. Notes regarding the use of callback routines Callback routines exceeding DLL limits require special handling in applications targetted for Win16 deployment. More information is available in the Windows API documentation for MakeProcInstance. Also see your compiler's documentation regarding smart callbacks or intelligent callbacks .