From faf1d70c34d0315e97e2b75ea886859fc6a2b54d Mon Sep 17 00:00:00 2001
From: Kelsi <kelsihates2fa@gmail.com>
Date: Mon, 6 Apr 2026 18:18:14 -0700
Subject: [PATCH] fix(rendering): reduce terrain chunk edge seams (#56)

Two sources of visible chunk-boundary squares:

1. Derivative-based bump mapping (bumpStrength=9) used dFdx/dFdy which
   are invalid across draw-call boundaries, producing strong normal
   discontinuities at every chunk edge.  Fade bump to zero near chunk
   edges using LayerUV as the chunk-space distance metric.

2. sampleAlpha used an abrupt step() to switch between point-sampled
   and 4-tap-blurred alpha, creating a visible ring 2 texels from each
   chunk edge.  Replace with smoothstep transition and a 5-tap average
   that includes the center sample.
---
 assets/shaders/terrain.frag.glsl | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/assets/shaders/terrain.frag.glsl b/assets/shaders/terrain.frag.glsl
index 0fcdd7dd..1a0de9b1 100644
--- a/assets/shaders/terrain.frag.glsl
+++ b/assets/shaders/terrain.frag.glsl
@@ -50,19 +50,21 @@ float sampleShadowPCF(sampler2DShadow smap, vec3 coords) {
 }
 
 float sampleAlpha(sampler2D tex, vec2 uv) {
+    // Smooth 5-tap box near chunk edges to hide alpha-map seams;
+    // blends gradually to avoid a visible ring at the transition.
     vec2 edge = min(uv, 1.0 - uv);
     float border = min(edge.x, edge.y);
-    float doBlur = step(border, 2.0 / 64.0);
-    if (doBlur < 0.5) {
-        return texture(tex, uv).r;
-    }
+    float blurWeight = 1.0 - smoothstep(0.5 / 64.0, 3.0 / 64.0, border);
+    float center = texture(tex, uv).r;
+    if (blurWeight < 0.001) return center;
     vec2 texel = vec2(1.0 / 64.0);
-    float a = 0.0;
-    a += texture(tex, uv + vec2(-texel.x, 0.0)).r;
-    a += texture(tex, uv + vec2(texel.x, 0.0)).r;
-    a += texture(tex, uv + vec2(0.0, -texel.y)).r;
-    a += texture(tex, uv + vec2(0.0, texel.y)).r;
-    return a * 0.25;
+    float avg = center;
+    avg += texture(tex, uv + vec2(-texel.x, 0.0)).r;
+    avg += texture(tex, uv + vec2( texel.x, 0.0)).r;
+    avg += texture(tex, uv + vec2(0.0, -texel.y)).r;
+    avg += texture(tex, uv + vec2(0.0,  texel.y)).r;
+    avg *= 0.2;
+    return mix(center, avg, blurWeight);
 }
 
 void main() {
@@ -87,9 +89,12 @@ void main() {
     vec3 norm = normalize(Normal);
 
     // Derivative-based normal mapping: perturb vertex normal using texture detail.
-    // Fade out with distance — looks noisy/harsh beyond ~100 units.
+    // Fade out with distance and near chunk edges (dFdx/dFdy are invalid across
+    // chunk draw-call boundaries, producing visible seams if not faded).
     float fragDist = length(viewPos.xyz - FragPos);
     float bumpFade = 1.0 - smoothstep(50.0, 125.0, fragDist);
+    float edgeDist = min(min(LayerUV.x, 1.0 - LayerUV.x), min(LayerUV.y, 1.0 - LayerUV.y));
+    bumpFade *= smoothstep(0.0, 0.06, edgeDist);
     if (bumpFade > 0.001) {
         float lum = dot(finalColor.rgb, vec3(0.299, 0.587, 0.114));
         float dLdx = dFdx(lum);