From 9f425c615998f378dd2a1128c1718e59e0864a5f Mon Sep 17 00:00:00 2001
From: Astrash <astrash@atr.sh>
Date: Wed, 9 Oct 2024 20:39:45 +1100
Subject: [PATCH] Add support for derivatives in FBM

---
 .../noise/fractal/BrownianMotionSampler.java  | 56 +++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/common/addons/config-noise-function/src/main/java/com/dfsek/terra/addons/noise/samplers/noise/fractal/BrownianMotionSampler.java b/common/addons/config-noise-function/src/main/java/com/dfsek/terra/addons/noise/samplers/noise/fractal/BrownianMotionSampler.java
index bc766c731..6c78ce727 100644
--- a/common/addons/config-noise-function/src/main/java/com/dfsek/terra/addons/noise/samplers/noise/fractal/BrownianMotionSampler.java
+++ b/common/addons/config-noise-function/src/main/java/com/dfsek/terra/addons/noise/samplers/noise/fractal/BrownianMotionSampler.java
@@ -7,6 +7,7 @@
 
 package com.dfsek.terra.addons.noise.samplers.noise.fractal;
 
+import com.dfsek.terra.api.noise.DerivativeNoiseSampler;
 import com.dfsek.terra.api.noise.NoiseSampler;
 import com.dfsek.terra.api.util.MathUtil;
 
@@ -52,4 +53,59 @@ public class BrownianMotionSampler extends FractalNoiseFunction {
 
         return sum;
     }
+
+    @Override
+    public boolean isDifferentiable() {
+        return input instanceof DerivativeNoiseSampler dSampler && dSampler.isDifferentiable();
+    }
+
+    @Override
+    public double[] getNoiseDerivativeRaw(long seed, double x, double y) {
+        double[] sum = {0, 0, 0};
+        double amp = fractalBounding;
+
+        for(int i = 0; i < octaves; i++) {
+            // This should only be called after `input` is verified as a `DerivativeNoiseSampler`
+            // so this should be a safe cast
+            double[] noise = ((DerivativeNoiseSampler) input).noised(seed++, x, y);
+            sum[0] += noise[0] * amp;
+
+            // Directional derivative of each octave can be subject to the same addition and product
+            // as per derivative sum and product rules in order to produce the correct final derivative
+            sum[1] += noise[1] * amp;
+            sum[2] += noise[2] * amp;
+
+            amp *= MathUtil.lerp(weightedStrength, 1.0, Math.min(noise[0] + 1, 2) * 0.5);
+
+            x *= lacunarity;
+            y *= lacunarity;
+            amp *= gain;
+        }
+
+        return sum;
+    }
+
+    @Override
+    public double[] getNoiseDerivativeRaw(long seed, double x, double y, double z) {
+        double[] sum = {0, 0, 0};
+        double amp = fractalBounding;
+
+        for(int i = 0; i < octaves; i++) {
+            double[] noise = ((DerivativeNoiseSampler) input).noised(seed++, x, y, z);
+            sum[0] += noise[0] * amp;
+
+            // See comment in 2D version
+            sum[1] += noise[1] * amp;
+            sum[2] += noise[2] * amp;
+
+            amp *= MathUtil.lerp(weightedStrength, 1.0, (noise[0] + 1) * 0.5);
+
+            x *= lacunarity;
+            y *= lacunarity;
+            z *= lacunarity;
+            amp *= gain;
+        }
+
+        return sum;
+    }
 }