Add organic S-curve meandering to tile road texture shader

shaders/simple.frag

@@ -24,6 +24,7 @@ uniform float roadRadius;
 uniform float roadBorderWidth;
 uniform float roadTextureScale;
 uniform float roadOpacity;
+uniform float roadMeander;
 uniform vec3 roadColor;
 uniform vec3 roadBorderColor;
 
@@ -102,6 +103,49 @@ float bezierCurveDist(vec2 p, vec2 p0, vec2 p2) {
     return min(min(d0, d1), min(d2, d3));
 }
 
+float meanderingBezierDist(vec2 p, vec2 p0, vec2 p2, float amp, float s1, float s2) {
+    vec2 chord = p2 - p0;
+    float cLen = length(chord);
+    vec2 perp = (cLen > 1e-4) ? vec2(-chord.y / cLen, chord.x / cLen) : vec2(0.0, 1.0);
+
+    // 6-segment meandering curve from p0 (t=0) to p2 (t=1).
+    // At t=0 and t=1, sin(pi*t) == 0 and sin(2*pi*t) == 0, so endpoints remain locked at p0 and p2!
+    const float PI_VAL = 3.14159265;
+    vec2 prevPt = p0;
+    float minDist = 999.0;
+    for (int i = 1; i <= 6; ++i) {
+        float t = float(i) / 6.0;
+        float omt = 1.0 - t;
+        // Base quadratic Bezier through (0, 0)
+        vec2 basePt = (omt * omt) * p0 + (t * t) * p2;
+        // Asymmetric S-curve + bend wave perpendicular to the road chord
+        float wave = amp * (0.65 * s1 * sin(PI_VAL * t) + 0.85 * s2 * sin(2.0 * PI_VAL * t));
+        vec2 curPt = basePt + perp * wave;
+        minDist = min(minDist, distToSegment(p, prevPt, curPt));
+        prevPt = curPt;
+    }
+    return minDist;
+}
+
+float meanderingBranchDist(vec2 p, vec2 pe, vec2 centerHub, float amp, float s) {
+    vec2 dir = centerHub - pe;
+    float dLen = length(dir);
+    vec2 perp = (dLen > 1e-4) ? vec2(-dir.y / dLen, dir.x / dLen) : vec2(0.0, 1.0);
+    const float PI_VAL = 3.14159265;
+
+    vec2 prevPt = pe;
+    float minDist = 999.0;
+    for (int i = 1; i <= 4; ++i) {
+        float t = float(i) / 4.0;
+        vec2 basePt = mix(pe, centerHub, t);
+        float wave = amp * 0.65 * s * sin(PI_VAL * t);
+        vec2 curPt = basePt + perp * wave;
+        minDist = min(minDist, distToSegment(p, prevPt, curPt));
+        prevPt = curPt;
+    }
+    return minDist;
+}
+
 float getRiverDistance(vec2 p, int mask) {
     if (mask == 0) return 999.0;
 
@@ -126,7 +170,13 @@ float getRiverDistance(vec2 p, int mask) {
     }
 }
 
-float getRoadDistance(vec2 p, int mask) {
+float getRoadDistance(vec2 p, int mask, vec2 tileCenterWorld, float amp) {
+    // Deterministic per-tile random signs so each tile meanders uniquely
+    float h1 = fract(sin(dot(tileCenterWorld, vec2(12.9898, 78.233))) * 43758.5453);
+    float h2 = fract(sin(dot(tileCenterWorld, vec2(39.3468, 11.135))) * 24634.6345);
+    float s1 = (h1 > 0.5) ? 1.0 : -1.0;
+    float s2 = (h2 > 0.5) ? 1.0 : -1.0;
+
     int edges[6];
     int count = 0;
     for (int e = 0; e < 6; ++e) {
@@ -137,18 +187,21 @@ float getRoadDistance(vec2 p, int mask) {
     }
 
     if (count == 0) {
-        // Unconnected road on a single tile: short natural country track segment at tile center
-        return distToSegment(p, vec2(-0.14, -0.05), vec2(0.14, 0.05));
+        // Unconnected road on a single tile: short winding country track at tile center
+        return meanderingBezierDist(p, vec2(-0.16, -0.06 * s1), vec2(0.16, 0.06 * s1), amp * 0.6, s1, s2);
     } else if (count == 1) {
-        return distToSegment(p, EDGE_MIDPOINTS[edges[0]], vec2(0.0));
+        vec2 centerHub = vec2(s1, s2) * (amp * 0.25);
+        return meanderingBranchDist(p, EDGE_MIDPOINTS[edges[0]], centerHub, amp, s1);
     } else if (count == 2) {
-        return bezierCurveDist(p, EDGE_MIDPOINTS[edges[0]], EDGE_MIDPOINTS[edges[1]]);
+        return meanderingBezierDist(p, EDGE_MIDPOINTS[edges[0]], EDGE_MIDPOINTS[edges[1]], amp, s1, s2);
     } else {
+        vec2 centerHub = vec2(s1, s2) * (amp * 0.22);
         float minDist = 999.0;
         for (int i = 0; i < count; ++i) {
-            minDist = min(minDist, distToSegment(p, EDGE_MIDPOINTS[edges[i]], vec2(0.0)));
+            float branchSign = ((edges[i] % 2) == 0) ? s1 : -s2;
+            minDist = min(minDist, meanderingBranchDist(p, EDGE_MIDPOINTS[edges[i]], centerHub, amp * 0.75, branchSign));
             for (int j = i + 1; j < count; ++j) {
-                minDist = min(minDist, bezierCurveDist(p, EDGE_MIDPOINTS[edges[i]], EDGE_MIDPOINTS[edges[j]]));
+                minDist = min(minDist, meanderingBezierDist(p, EDGE_MIDPOINTS[edges[i]], EDGE_MIDPOINTS[edges[j]], amp * 0.65, s1, s2));
             }
         }
         return minDist;
@@ -319,7 +372,17 @@ void main() {
 
     // 6b. Road Texture Integration (blended directly into the tile surface texture)
     if (hasRoad == 1 && oceanWaterFac < 0.95) {
-        float rawRoadDist = getRoadDistance(fragVert.xy, roadMask);
+        vec2 tileCenterWorld = vWorldPos.xy - fragVert.xy;
+        float amp = max(0.0, roadMeander);
+
+        // Additional subtle world-space domain warp inside the tile (fades to 0 at hex border so edges stay locked)
+        float borderFade = 1.0 - smoothstep(0.24, 0.42, length(fragVert.xy));
+        vec2 microWarp = vec2(
+            sin(vWorldPos.y * 13.0 + vWorldPos.x * 5.0),
+            cos(vWorldPos.x * 13.0 - vWorldPos.y * 5.0)
+        ) * (amp * 0.22 * borderFade);
+
+        float rawRoadDist = getRoadDistance(fragVert.xy + microWarp, roadMask, tileCenterWorld, amp);
         float rScale = max(0.5, roadTextureScale);
         vec2 uvRoad = vWorldPos.xy * rScale;