add FractalBrownianMotion2D function

needs testing

simplex.lua

@@ -556,4 +556,21 @@ function simplex.Seed(seed)
     end
 end
 
+-- TODO test functionality
+function simplex.FractalBrownianMotion2D(x, y, octaves, lacunarity, gain)
+    octaves = octaves or 4
+    lacunarity = lacunarity or 2
+    gain = gain or 0.5
+
+    local total, amplitude, frequency, peak_amplitude = 0, 1, 1, 0
+    for i = 1, octaves do
+        total = total + amplitude * simplex.Noise2D(x * frequency, y * frequency)
+        peak_amplitude = peak_amplitude + amplitude
+        amplitude = amplitude * gain
+        frequency = frequency * lacunarity
+    end
+
+    return total / peak_amplitude   -- normalize to [-1, 1]
+end
+
 return simplex