Add various interpolation routines for perlin

Makefile.am

@@ -44,6 +44,8 @@ UTIL_FILES   =		\
     ip.hpp		\
     json.cpp		\
     json.hpp		\
+    lerp.cpp		\
+    lerp.hpp		\
     maths.cpp		\
     maths.hpp		\
     maths/matrix.cpp	\

lerp.cpp

@@ -0,0 +1,49 @@
+/*
+ * This file is part of libgim.
+ *
+ * libgim is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * libgim is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with libgim.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright 2011 Danny Robson <danny@blubinc.net>
+ */
+
+#include "lerp.hpp"
+
+#include "debug.hpp"
+#include "maths.hpp"
+
+#include <cmath>
+
+
+double
+lerp::sigmoid (double val) {
+    return -1.0 + 2.0 / (1.0 + exp (-2.0 * val));
+}
+
+
+double
+lerp::linear (double a, double b, double weight) {
+    check (weight >= 0 && weight <= 1.0);
+    return (1.0 - weight) * a + weight * b;
+}
+
+
+double
+lerp::cosine (double a, double b, double weight) {
+    check (weight >= 0 && weight <= 1.0);
+    double ft = weight * PI;
+    double f  = (1.0 - cos (ft)) * 0.5;
+
+    return a * (1.0 - f) + b * f;
+}
+

lerp.hpp

@@ -0,0 +1,30 @@
+/*
+ * This file is part of libgim.
+ *
+ * libgim is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later
+ * version.
+ *
+ * libgim is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with libgim.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright 2011 Danny Robson <danny@blubinc.net>
+ */
+
+#ifndef __UTIL_LERP_HPP
+#define __UTIL_LERP_HPP
+
+namespace lerp {
+    double sigmoid (double val);
+
+    double linear (double a, double b, double weight);
+    double cosine (double a, double b, double weight);
+}
+
+#endif