1 // util.cxx - general-purpose utility functions.
2 // Copyright (C) 2002 Curtis L. Olson - curt@me.umn.edu
4 // This program is free software; you can redistribute it and/or
5 // modify it under the terms of the GNU General Public License as
6 // published by the Free Software Foundation; either version 2 of the
7 // License, or (at your option) any later version.
9 // This program is distributed in the hope that it will be useful, but
10 // WITHOUT ANY WARRANTY; without even the implied warranty of
11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 // General Public License for more details.
14 // You should have received a copy of the GNU General Public License
15 // along with this program; if not, write to the Free Software
16 // Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
25 // Originally written by Alex Perry.
27 fgGetLowPass (double current, double target, double timeratio)
29 if ( timeratio < 0.0 ) {
30 if ( timeratio < -1.0 ) {
31 // time went backwards; kill the filter
34 // ignore mildly negative time
36 } else if ( timeratio < 0.2 ) {
37 // Normal mode of operation; fast
38 // approximation to exp(-timeratio)
39 current = current * (1.0 - timeratio) + target * timeratio;
40 } else if ( timeratio > 5.0 ) {
41 // Huge time step; assume filter has settled
44 // Moderate time step; non linear response
45 double keep = exp(-timeratio);
46 current = current * keep + target * (1.0 - keep);