A selection modifier is an operation which only affects the sels member of a sample's sounddata. The undo and redo methods which are registered for a selection modifier do not attempt to retain any information about any other portion of the sample -- they are optimised to only retain information about changes in the selection.

To create a selection modifier, you must provide a function of the form SweepModify, as defined in :

typedef void (*SweepModify) (sw_sample * sample, sw_param_set pset, gpointer custom_data);

This function is expected to modify only the selection data, ie. sample->sounddata->sels. Note that this data may also be accessed by the playback thread, so it is protected by a mutex. You must surround any modifications to the sels member of a sounddata object with calls to sounddata_lock_selection() and sounddata_unlock_selection(), as defined in .

To perform the selection modification, call the following function (also defined in ):

sw_op_instance * perform_selection_op (sw_sample * s, char * desc, SweepModify func, sw_param_set pset, gpointer custom_data);

The first argument is the sample to modify, the second is some short descriptive text for the operation, 'func' is the SweepModify function, and pset and custom_data are those provided by the proc.