From 965be72ea477ccbe391c53d4b8a348e4bc22cb55 Mon Sep 17 00:00:00 2001
From: Georg Tomitsch <georg@tomitsch.net>
Date: Sat, 6 Jun 2026 10:41:00 +0200
Subject: [PATCH] Tune WebGL book bounce lighting

---
 public/js/webgl-book-lab.js | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/public/js/webgl-book-lab.js b/public/js/webgl-book-lab.js
index bc29491..ad15bf6 100644
--- a/public/js/webgl-book-lab.js
+++ b/public/js/webgl-book-lab.js
@@ -547,17 +547,20 @@ function configureBookShadowReceiver(material, strength) {
                     return clamp(max(max(shadow0, shadow1), shadow2), 0.0, 1.0);
                 }
 
-                vec3 bookLocalBounce(vec3 worldPosition, vec3 worldNormal, float shadow) {
+                vec3 bookLocalBounce(vec3 worldPosition, vec3 worldNormal, float shadow, vec3 albedo) {
                     float tableDistance = max(0.0, worldPosition.y - bookTableTopY);
-                    float tableReach = 1.0 - smoothstep(0.02, 0.24, tableDistance);
+                    float tableReach = 1.0 - smoothstep(0.0, 0.34, tableDistance);
+                    float sideReach = 1.0 - smoothstep(0.02, 0.62, tableDistance);
                     float grazingSide = 1.0 - pow(abs(worldNormal.y), 0.65);
-                    float underside = smoothstep(0.12, 0.82, -worldNormal.y);
-                    float pageGlow = smoothstep(0.02, 0.18, worldPosition.y - bookTableTopY) *
-                        (1.0 - smoothstep(0.18, 0.34, worldPosition.y - bookTableTopY));
-                    float bounce = tableReach * (0.42 + grazingSide * 0.34 + underside * 0.32) + pageGlow * 0.16;
-                    vec3 tableWarmth = vec3(0.055, 0.029, 0.014);
-                    vec3 pageWarmth = vec3(0.03, 0.021, 0.012);
-                    return (tableWarmth * bounce + pageWarmth * pageGlow) * mix(1.0, 0.62, shadow);
+                    float underside = smoothstep(0.16, 0.88, -worldNormal.y);
+                    float sideFill = grazingSide * sideReach;
+                    float tableFill = tableReach * (0.22 + underside * 0.24);
+                    float pageFill = smoothstep(0.02, 0.2, tableDistance) * (1.0 - smoothstep(0.24, 0.72, tableDistance));
+                    vec3 tableWarmth = vec3(0.08, 0.045, 0.024) * tableFill;
+                    vec3 roomWarmth = vec3(0.045, 0.033, 0.024) * sideFill;
+                    vec3 pageWarmth = vec3(0.05, 0.041, 0.029) * pageFill * grazingSide;
+                    vec3 indirect = tableWarmth + roomWarmth + pageWarmth;
+                    return albedo * indirect * mix(1.0, 0.72, shadow);
                 }
 
                 float spineClothThread(float coordinate, float frequency, float sharpness) {
@@ -606,7 +609,7 @@ function configureBookShadowReceiver(material, strength) {
                 ${isHeadband ? 'outgoingLight = headbandCreviceLight(vBookSurfaceUv, outgoingLight);' : ''}
                 float bookReceiverShadow = bookReceiverShadowField(vBookReceiverWorldPosition) * bookShadowReceiverStrength;
                 outgoingLight *= mix(vec3(1.0), ${isHeadband ? 'vec3(0.16, 0.095, 0.055)' : 'vec3(0.38, 0.29, 0.2)'}, bookReceiverShadow);
-                outgoingLight += bookLocalBounce(vBookReceiverWorldPosition, normalize(vBookReceiverWorldNormal), bookReceiverShadow);
+                outgoingLight += bookLocalBounce(vBookReceiverWorldPosition, normalize(vBookReceiverWorldNormal), bookReceiverShadow, diffuseColor.rgb);
                 #include <opaque_fragment>`
             );
     };