The function update_func is of type LqrProgressFuncUpdate, whose prototype is defined by:

typedef LqrRetVal (*LqrProgressFuncUpdate) (gdouble completion_percentage);
                        

Each time that the function lqr_carver_resize(3) performs a rescale operation (recall that it can perform more than one rescale operation at each call) this function will be called at regular intervale, with the current completion percentage completion_percentage as an argument.

The update step can be set with the function lqr_progress_set_update_step(3).