From 75740d214de60e03cd2bcd738186a508c1c4d2f3 Mon Sep 17 00:00:00 2001
From: yum <yum.food.vr@gmail.com>
Date: Tue, 11 Nov 2025 21:46:04 -0800
Subject: implement hazel "Spherical Harmonics for Lighting"

---
 lighting.cginc | 72 +++++++++++++++++++++++++++++++---------------------------
 1 file changed, 39 insertions(+), 33 deletions(-)

(limited to 'lighting.cginc')

diff --git a/lighting.cginc b/lighting.cginc
index 5a31a25..87a3dfc 100644
--- a/lighting.cginc
+++ b/lighting.cginc
@@ -96,6 +96,33 @@ float3 getIndirectSpecular(v2f i, float roughness, float3 view_dir, float3 refle
   return env_refl;
 }
 
+// Geomerics SH evaluation
+// https://community.arm.com/cfs-file/__key/telligent-evolution-components-attachments/01-2066-00-00-00-01-27-70/Simplifying_2D00_Spherical_2D00_Harmonics_2D00_for_2D00_Lighting.pdf
+float shEvaluateDiffuseL1Geomerics(float L0, float3 L1, float3 n) {
+  // average energy
+  float R0 = max(L0, 0);
+
+  // avg direction of incoming light
+  float3 R1 = 0.5f * L1;
+
+  // directional brightness
+  float lenR1 = length(R1);
+
+  // linear angle between normal and direction 0-1
+  float q = dot(normalize(R1), n) * 0.5 + 0.5;
+  q = saturate(q);
+
+  // power for q
+  // lerps from 1 (linear) to 3 (cubic) based on directionality
+  float p = 1.0f + 2.0f * lenR1 / R0;
+
+  // dynamic range constant
+  // should vary between 4 (highly directional) and 0 (ambient)
+  float a = (1.0f - lenR1 / R0) / (1.0f + lenR1 / R0);
+
+  return R0 * (a + (1.0f - a) * (p + 1.0f) * pow(q, p));
+}
+
 float3 yumSH9(float4 n, float3 worldPos, inout LightIndirect light) {
   [branch]
   if (_UdonLightVolumeEnabled) {
@@ -106,44 +133,23 @@ float3 yumSH9(float4 n, float3 worldPos, inout LightIndirect light) {
         dot(light.L01b, n.xyz));
   }
 
-  // No light volumes - use SH9
-  // Unity gives us the first three bands (L0-L2) of SH coefficients as follows:
-  //   unity_SHA*.w:   L0 coefficients
-  //   unity_SHA*.xyz: L1 coefficients
-  //   unity_SHB*:     first four of the L2 coefficients
-  //   unity_SHC:      last L2 coefficient
-
-  // L0 band
+  // Geomerics SH evaluation
+  // Extract L0 and optionally incorporate part of L2
   float3 L0 = float3(unity_SHAr.w, unity_SHAg.w, unity_SHAb.w);
+  float3 L0L2 = float3(unity_SHBr.z, unity_SHBg.z, unity_SHBb.z) / 3.0;
+  L0 = L0 + L0L2; // useL2 = true
 
-  // L1 band
-  float3x3 L1m = float3x3(unity_SHAr.xyz, unity_SHAg.xyz, unity_SHAb.xyz);
-  float3 L1 = mul(L1m, n);
-
-  // L2 band
-  float4 v = float4(n.x * n.y, n.y * n.z, n.z * n.z, n.x * n.z);
-  float3 L2 =
-    float3(dot(unity_SHBr.xyzw, v), dot(unity_SHBg.xyzw, v), dot(unity_SHBb.xyzw, v)) +
-    unity_SHC.xyz * (n.x * n.x - n.y * n.y);
-
-  // TODO expose this as a parameter
-  float wrap_term = 0.0f;
-
-  // Original coefficients: 1, 2/3, 1/4.
-  // Wrapped coefficients: 1, (2-w)/3, ((1-w)^2)/4.
-  float l1_wrap = 1.0f - wrap_term * 0.75f;
-  L1 *= l1_wrap;
-
-  float l2_wrap_base = 1.0f - wrap_term;
-  float l2_wrap = l2_wrap_base * l2_wrap_base;
-  L2 *= l2_wrap;
+  float3 finalSH;
+  finalSH.r = shEvaluateDiffuseL1Geomerics(L0.r, unity_SHAr.xyz, n.xyz);
+  finalSH.g = shEvaluateDiffuseL1Geomerics(L0.g, unity_SHAg.xyz, n.xyz);
+  finalSH.b = shEvaluateDiffuseL1Geomerics(L0.b, unity_SHAb.xyz, n.xyz);
 
   light.L00 = L0;
-  light.L01r = unity_SHAr.xyz * l1_wrap;
-  light.L01g = unity_SHAg.xyz * l1_wrap;
-  light.L01b = unity_SHAb.xyz * l1_wrap;
+  light.L01r = unity_SHAr.xyz;
+  light.L01g = unity_SHAg.xyz;
+  light.L01b = unity_SHAb.xyz;
 
-  return L0 + L1 + L2;
+  return finalSH;
 }
 
 float4 getIndirectDiffuse(v2f i, Pbr pbr, inout LightIndirect light) {
-- 
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