Fix T55284: error in Hybrid MultiFractal Musgrave texture

The calculation was revised to address two issues:
* Discontinuities occurring when detail was a non-integer greater than 2.
* Levels of detail in the interval [0,1) repeating the levels of detail in
  the interval [1,2).

This fixes Cycles, Eevee and geometry nodes.

Differential Revision: https://developer.blender.org/D15785

2 files changed, 80 insertions, 56 deletions

diff --git a/source/blender/blenlib/intern/noise.cc b/source/blender/blenlib/intern/noise.cc
index a514c9e5183..3de9b17d3c4 100644
--- a/source/blender/blenlib/intern/noise.cc
+++ b/source/blender/blenlib/intern/noise.cc
@@ -809,15 +809,14 @@ float musgrave_hybrid_multi_fractal(const float co,
 {
   float p = co;
   const float pwHL = std::pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = perlin_signed(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0f;
+  float value = 0.0f;
+  float weight = 1.0f;
 
   const float octaves = CLAMPIS(octaves_unclamped, 0.0f, 15.0f);
 
-  for (int i = 1; (weight > 0.001f) && (i < (int)octaves); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < (int)octaves); i++) {
     if (weight > 1.0f) {
       weight = 1.0f;
     }
@@ -830,8 +829,12 @@ float musgrave_hybrid_multi_fractal(const float co,
   }
 
   const float rmd = octaves - floorf(octaves);
-  if (rmd != 0.0f) {
-    value += rmd * ((perlin_signed(p) + offset) * pwr);
+  if ((rmd != 0.0f) && (weight > 0.001f)) {
+    if (weight > 1.0f) {
+      weight = 1.0f;
+    }
+    float signal = (perlin_signed(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   return value;
@@ -961,15 +964,14 @@ float musgrave_hybrid_multi_fractal(const float2 co,
 {
   float2 p = co;
   const float pwHL = std::pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = perlin_signed(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0f;
+  float value = 0.0f;
+  float weight = 1.0f;
 
   const float octaves = CLAMPIS(octaves_unclamped, 0.0f, 15.0f);
 
-  for (int i = 1; (weight > 0.001f) && (i < (int)octaves); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < (int)octaves); i++) {
     if (weight > 1.0f) {
       weight = 1.0f;
     }
@@ -982,8 +984,12 @@ float musgrave_hybrid_multi_fractal(const float2 co,
   }
 
   const float rmd = octaves - floorf(octaves);
-  if (rmd != 0.0f) {
-    value += rmd * ((perlin_signed(p) + offset) * pwr);
+  if ((rmd != 0.0f) && (weight > 0.001f)) {
+    if (weight > 1.0f) {
+      weight = 1.0f;
+    }
+    float signal = (perlin_signed(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   return value;
@@ -1115,15 +1121,14 @@ float musgrave_hybrid_multi_fractal(const float3 co,
 {
   float3 p = co;
   const float pwHL = std::pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = perlin_signed(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0f;
+  float value = 0.0f;
+  float weight = 1.0f;
 
   const float octaves = CLAMPIS(octaves_unclamped, 0.0f, 15.0f);
 
-  for (int i = 1; (weight > 0.001f) && (i < (int)octaves); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < (int)octaves); i++) {
     if (weight > 1.0f) {
       weight = 1.0f;
     }
@@ -1136,8 +1141,12 @@ float musgrave_hybrid_multi_fractal(const float3 co,
   }
 
   const float rmd = octaves - floorf(octaves);
-  if (rmd != 0.0f) {
-    value += rmd * ((perlin_signed(p) + offset) * pwr);
+  if ((rmd != 0.0f) && (weight > 0.001f)) {
+    if (weight > 1.0f) {
+      weight = 1.0f;
+    }
+    float signal = (perlin_signed(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   return value;
@@ -1269,15 +1278,14 @@ float musgrave_hybrid_multi_fractal(const float4 co,
 {
   float4 p = co;
   const float pwHL = std::pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = perlin_signed(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0f;
+  float value = 0.0f;
+  float weight = 1.0f;
 
   const float octaves = CLAMPIS(octaves_unclamped, 0.0f, 15.0f);
 
-  for (int i = 1; (weight > 0.001f) && (i < (int)octaves); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < (int)octaves); i++) {
     if (weight > 1.0f) {
       weight = 1.0f;
     }
@@ -1290,8 +1298,12 @@ float musgrave_hybrid_multi_fractal(const float4 co,
   }
 
   const float rmd = octaves - floorf(octaves);
-  if (rmd != 0.0f) {
-    value += rmd * ((perlin_signed(p) + offset) * pwr);
+  if ((rmd != 0.0f) && (weight > 0.001f)) {
+    if (weight > 1.0f) {
+      weight = 1.0f;
+    }
+    float signal = (perlin_signed(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   return value;
diff --git a/source/blender/gpu/shaders/material/gpu_shader_material_tex_musgrave.glsl b/source/blender/gpu/shaders/material/gpu_shader_material_tex_musgrave.glsl
index 961fe23e67e..7171c5f2b36 100644
--- a/source/blender/gpu/shaders/material/gpu_shader_material_tex_musgrave.glsl
+++ b/source/blender/gpu/shaders/material/gpu_shader_material_tex_musgrave.glsl
@@ -153,13 +153,12 @@ void node_tex_musgrave_hybrid_multi_fractal_1d(vec3 co,
   float lacunarity = max(lac, 1e-5);
 
   float pwHL = pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = snoise(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0;
+  float value = 0.0;
+  float weight = 1.0;
 
-  for (int i = 1; (weight > 0.001f) && (i < int(octaves)); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < int(octaves)); i++) {
     if (weight > 1.0) {
       weight = 1.0;
     }
@@ -172,8 +171,12 @@ void node_tex_musgrave_hybrid_multi_fractal_1d(vec3 co,
   }
 
   float rmd = octaves - floor(octaves);
-  if (rmd != 0.0) {
-    value += rmd * ((snoise(p) + offset) * pwr);
+  if ((rmd != 0.0) && (weight > 0.001f)) {
+    if (weight > 1.0) {
+      weight = 1.0;
+    }
+    float signal = (snoise(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   fac = value;
@@ -375,13 +378,12 @@ void node_tex_musgrave_hybrid_multi_fractal_2d(vec3 co,
   float lacunarity = max(lac, 1e-5);
 
   float pwHL = pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = snoise(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0;
+  float value = 0.0;
+  float weight = 1.0;
 
-  for (int i = 1; (weight > 0.001f) && (i < int(octaves)); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < int(octaves)); i++) {
     if (weight > 1.0) {
       weight = 1.0;
     }
@@ -394,8 +396,12 @@ void node_tex_musgrave_hybrid_multi_fractal_2d(vec3 co,
   }
 
   float rmd = octaves - floor(octaves);
-  if (rmd != 0.0) {
-    value += rmd * ((snoise(p) + offset) * pwr);
+  if ((rmd != 0.0) && (weight > 0.001f)) {
+    if (weight > 1.0) {
+      weight = 1.0;
+    }
+    float signal = (snoise(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   fac = value;
@@ -597,13 +603,12 @@ void node_tex_musgrave_hybrid_multi_fractal_3d(vec3 co,
   float lacunarity = max(lac, 1e-5);
 
   float pwHL = pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = snoise(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0;
+  float value = 0.0;
+  float weight = 1.0;
 
-  for (int i = 1; (weight > 0.001f) && (i < int(octaves)); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < int(octaves)); i++) {
     if (weight > 1.0) {
       weight = 1.0;
     }
@@ -616,8 +621,12 @@ void node_tex_musgrave_hybrid_multi_fractal_3d(vec3 co,
   }
 
   float rmd = octaves - floor(octaves);
-  if (rmd != 0.0) {
-    value += rmd * ((snoise(p) + offset) * pwr);
+  if ((rmd != 0.0) && (weight > 0.001f)) {
+    if (weight > 1.0) {
+      weight = 1.0;
+    }
+    float signal = (snoise(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   fac = value;
@@ -819,13 +828,12 @@ void node_tex_musgrave_hybrid_multi_fractal_4d(vec3 co,
   float lacunarity = max(lac, 1e-5);
 
   float pwHL = pow(lacunarity, -H);
-  float pwr = pwHL;
 
-  float value = snoise(p) + offset;
-  float weight = gain * value;
-  p *= lacunarity;
+  float pwr = 1.0;
+  float value = 0.0;
+  float weight = 1.0;
 
-  for (int i = 1; (weight > 0.001f) && (i < int(octaves)); i++) {
+  for (int i = 0; (weight > 0.001f) && (i < int(octaves)); i++) {
     if (weight > 1.0) {
       weight = 1.0;
     }
@@ -838,8 +846,12 @@ void node_tex_musgrave_hybrid_multi_fractal_4d(vec3 co,
   }
 
   float rmd = octaves - floor(octaves);
-  if (rmd != 0.0) {
-    value += rmd * ((snoise(p) + offset) * pwr);
+  if ((rmd != 0.0) && (weight > 0.001f)) {
+    if (weight > 1.0) {
+      weight = 1.0;
+    }
+    float signal = (snoise(p) + offset) * pwr;
+    value += rmd * weight * signal;
   }
 
   fac = value;