feat(render3d): rain that leaves something behind

Rain fell and the ground did not change: the weather was a curtain of particles in front
of a dry valley.

u_wet, 0 dry to 1 soaked, does the three things a wet surface does. Darkens the albedo.
Drops the roughness hard - which is what makes a soaked meadow read as soaked rather than
merely dark, because the sky glances off it. And pools: water finds the low places and
flat ground holds what a slope sheds, so the puddle mask is the macro noise the materials
already use gated on slope, and a puddle takes the world's up for its normal rather than
the ground's relief.

It joins the shore's existing wet band rather than fighting it - the same term from a
different cause.

34.1% of the frame changes between dry and soaked. GL 7.1 s either way over 400 frames.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

changes/rain-that-leaves-something.md

@@ -0,0 +1,20 @@
+bump: minor
+type: feat
+Rain that leaves something behind.
+
+Rain fell and the ground did not change: the weather was a curtain of particles in front
+of a dry valley, and a shower was something you waited out rather than something that
+happened to the place.
+
+One scalar, u_wet, 0 dry to 1 soaked, doing the three things a wet surface does. It
+darkens the albedo. It drops the roughness hard, which is what makes a soaked meadow read
+as SOAKED rather than merely dark - the sky glances off it. And it pools: a wet ground
+does not darken evenly, water finds the low places and the flat ground holds what a slope
+sheds, so the puddle mask is the macro noise the materials already use gated on slope,
+and a puddle takes the world's up for its normal rather than the ground's relief.
+
+It joins the shore's existing wet band rather than fighting it - the same term, from a
+different cause.
+
+Measured: 34.1% of the frame changes between dry and soaked, and no cost - GL 7.1 s
+either way over 400 frames.

packages/ludic.render3d/shaders/spv/manifest.txt

@@ -2566,7 +2566,7 @@ U 2e2908c8 vert u_node 176 vec3 1 0
 U 2e2908c8 vert u_morph 192 vec2 1 0
 U 2e2908c8 vert u_grid 200 float 1 0
 I 2e2908c8 a_xz 0 vec2
-B 2e2908c8 frag 1 952
+B 2e2908c8 frag 1 968
 U 2e2908c8 frag u_cascade_vp 0 mat4 5 64
 U 2e2908c8 frag u_cascade_split 320 float 5 16
 U 2e2908c8 frag u_cascade_range 400 float 5 16
@@ -2607,12 +2607,13 @@ U 2e2908c8 frag u_ortho_on 828 float 1 0
 U 2e2908c8 frag u_carpet_on 832 float 1 0
 U 2e2908c8 frag u_snow_line 836 float 1 0
 U 2e2908c8 frag u_lake_level 840 float 1 0
-U 2e2908c8 frag u_sea_level 844 float 1 0
-U 2e2908c8 frag u_lake 848 vec4 1 0
-U 2e2908c8 frag u_origin 864 vec2 1 0
-U 2e2908c8 frag u_view 880 mat4 1 0
-U 2e2908c8 frag u_far_split 944 float 1 0
-U 2e2908c8 frag u_far_band 948 float 1 0
+U 2e2908c8 frag u_wet 844 float 1 0
+U 2e2908c8 frag u_sea_level 848 float 1 0
+U 2e2908c8 frag u_lake 864 vec4 1 0
+U 2e2908c8 frag u_origin 880 vec2 1 0
+U 2e2908c8 frag u_view 896 mat4 1 0
+U 2e2908c8 frag u_far_split 960 float 1 0
+U 2e2908c8 frag u_far_band 964 float 1 0
 T 2e2908c8 u_brdf 2
 T 2e2908c8 u_carpet 3
 T 2e2908c8 u_grass_a 4
@@ -2641,7 +2642,7 @@ U 1f95f31b vert u_node 176 vec3 1 0
 U 1f95f31b vert u_morph 192 vec2 1 0
 U 1f95f31b vert u_grid 200 float 1 0
 I 1f95f31b a_xz 0 vec2
-B 1f95f31b frag 1 952
+B 1f95f31b frag 1 968
 U 1f95f31b frag u_cascade_vp 0 mat4 5 64
 U 1f95f31b frag u_cascade_split 320 float 5 16
 U 1f95f31b frag u_cascade_range 400 float 5 16
@@ -2682,12 +2683,13 @@ U 1f95f31b frag u_ortho_on 828 float 1 0
 U 1f95f31b frag u_carpet_on 832 float 1 0
 U 1f95f31b frag u_snow_line 836 float 1 0
 U 1f95f31b frag u_lake_level 840 float 1 0
-U 1f95f31b frag u_sea_level 844 float 1 0
-U 1f95f31b frag u_lake 848 vec4 1 0
-U 1f95f31b frag u_origin 864 vec2 1 0
-U 1f95f31b frag u_view 880 mat4 1 0
-U 1f95f31b frag u_far_split 944 float 1 0
-U 1f95f31b frag u_far_band 948 float 1 0
+U 1f95f31b frag u_wet 844 float 1 0
+U 1f95f31b frag u_sea_level 848 float 1 0
+U 1f95f31b frag u_lake 864 vec4 1 0
+U 1f95f31b frag u_origin 880 vec2 1 0
+U 1f95f31b frag u_view 896 mat4 1 0
+U 1f95f31b frag u_far_split 960 float 1 0
+U 1f95f31b frag u_far_band 964 float 1 0
 T 1f95f31b u_brdf 2
 T 1f95f31b u_carpet 3
 T 1f95f31b u_grass_a 4
@@ -2716,7 +2718,7 @@ U f4382918 vert u_node 176 vec3 1 0
 U f4382918 vert u_morph 192 vec2 1 0
 U f4382918 vert u_grid 200 float 1 0
 I f4382918 a_xz 0 vec2
-B f4382918 frag 1 952
+B f4382918 frag 1 968
 U f4382918 frag u_cascade_vp 0 mat4 5 64
 U f4382918 frag u_cascade_split 320 float 5 16
 U f4382918 frag u_cascade_range 400 float 5 16
@@ -2757,12 +2759,13 @@ U f4382918 frag u_ortho_on 828 float 1 0
 U f4382918 frag u_carpet_on 832 float 1 0
 U f4382918 frag u_snow_line 836 float 1 0
 U f4382918 frag u_lake_level 840 float 1 0
-U f4382918 frag u_sea_level 844 float 1 0
-U f4382918 frag u_lake 848 vec4 1 0
-U f4382918 frag u_origin 864 vec2 1 0
-U f4382918 frag u_view 880 mat4 1 0
-U f4382918 frag u_far_split 944 float 1 0
-U f4382918 frag u_far_band 948 float 1 0
+U f4382918 frag u_wet 844 float 1 0
+U f4382918 frag u_sea_level 848 float 1 0
+U f4382918 frag u_lake 864 vec4 1 0
+U f4382918 frag u_origin 880 vec2 1 0
+U f4382918 frag u_view 896 mat4 1 0
+U f4382918 frag u_far_split 960 float 1 0
+U f4382918 frag u_far_band 964 float 1 0
 T f4382918 u_brdf 2
 T f4382918 u_carpet 3
 T f4382918 u_grass_a 4
@@ -2791,7 +2794,7 @@ U f2952361 vert u_node 176 vec3 1 0
 U f2952361 vert u_morph 192 vec2 1 0
 U f2952361 vert u_grid 200 float 1 0
 I f2952361 a_xz 0 vec2
-B f2952361 frag 1 952
+B f2952361 frag 1 968
 U f2952361 frag u_cascade_vp 0 mat4 5 64
 U f2952361 frag u_cascade_split 320 float 5 16
 U f2952361 frag u_cascade_range 400 float 5 16
@@ -2832,12 +2835,13 @@ U f2952361 frag u_ortho_on 828 float 1 0
 U f2952361 frag u_carpet_on 832 float 1 0
 U f2952361 frag u_snow_line 836 float 1 0
 U f2952361 frag u_lake_level 840 float 1 0
-U f2952361 frag u_sea_level 844 float 1 0
-U f2952361 frag u_lake 848 vec4 1 0
-U f2952361 frag u_origin 864 vec2 1 0
-U f2952361 frag u_view 880 mat4 1 0
-U f2952361 frag u_far_split 944 float 1 0
-U f2952361 frag u_far_band 948 float 1 0
+U f2952361 frag u_wet 844 float 1 0
+U f2952361 frag u_sea_level 848 float 1 0
+U f2952361 frag u_lake 864 vec4 1 0
+U f2952361 frag u_origin 880 vec2 1 0
+U f2952361 frag u_view 896 mat4 1 0
+U f2952361 frag u_far_split 960 float 1 0
+U f2952361 frag u_far_band 964 float 1 0
 T f2952361 u_brdf 2
 T f2952361 u_carpet 3
 T f2952361 u_grass_a 4
@@ -2866,7 +2870,7 @@ U a0f2dbac vert u_node 176 vec3 1 0
 U a0f2dbac vert u_morph 192 vec2 1 0
 U a0f2dbac vert u_grid 200 float 1 0
 I a0f2dbac a_xz 0 vec2
-B a0f2dbac frag 1 952
+B a0f2dbac frag 1 968
 U a0f2dbac frag u_cascade_vp 0 mat4 5 64
 U a0f2dbac frag u_cascade_split 320 float 5 16
 U a0f2dbac frag u_cascade_range 400 float 5 16
@@ -2907,12 +2911,13 @@ U a0f2dbac frag u_ortho_on 828 float 1 0
 U a0f2dbac frag u_carpet_on 832 float 1 0
 U a0f2dbac frag u_snow_line 836 float 1 0
 U a0f2dbac frag u_lake_level 840 float 1 0
-U a0f2dbac frag u_sea_level 844 float 1 0
-U a0f2dbac frag u_lake 848 vec4 1 0
-U a0f2dbac frag u_origin 864 vec2 1 0
-U a0f2dbac frag u_view 880 mat4 1 0
-U a0f2dbac frag u_far_split 944 float 1 0
-U a0f2dbac frag u_far_band 948 float 1 0
+U a0f2dbac frag u_wet 844 float 1 0
+U a0f2dbac frag u_sea_level 848 float 1 0
+U a0f2dbac frag u_lake 864 vec4 1 0
+U a0f2dbac frag u_origin 880 vec2 1 0
+U a0f2dbac frag u_view 896 mat4 1 0
+U a0f2dbac frag u_far_split 960 float 1 0
+U a0f2dbac frag u_far_band 964 float 1 0
 T a0f2dbac u_brdf 2
 T a0f2dbac u_carpet 3
 T a0f2dbac u_grass_a 4
@@ -2941,7 +2946,7 @@ U 5e4589a1 vert u_node 176 vec3 1 0
 U 5e4589a1 vert u_morph 192 vec2 1 0
 U 5e4589a1 vert u_grid 200 float 1 0
 I 5e4589a1 a_xz 0 vec2
-B 5e4589a1 frag 1 952
+B 5e4589a1 frag 1 968
 U 5e4589a1 frag u_cascade_vp 0 mat4 5 64
 U 5e4589a1 frag u_cascade_split 320 float 5 16
 U 5e4589a1 frag u_cascade_range 400 float 5 16
@@ -2982,12 +2987,13 @@ U 5e4589a1 frag u_ortho_on 828 float 1 0
 U 5e4589a1 frag u_carpet_on 832 float 1 0
 U 5e4589a1 frag u_snow_line 836 float 1 0
 U 5e4589a1 frag u_lake_level 840 float 1 0
-U 5e4589a1 frag u_sea_level 844 float 1 0
-U 5e4589a1 frag u_lake 848 vec4 1 0
-U 5e4589a1 frag u_origin 864 vec2 1 0
-U 5e4589a1 frag u_view 880 mat4 1 0
-U 5e4589a1 frag u_far_split 944 float 1 0
-U 5e4589a1 frag u_far_band 948 float 1 0
+U 5e4589a1 frag u_wet 844 float 1 0
+U 5e4589a1 frag u_sea_level 848 float 1 0
+U 5e4589a1 frag u_lake 864 vec4 1 0
+U 5e4589a1 frag u_origin 880 vec2 1 0
+U 5e4589a1 frag u_view 896 mat4 1 0
+U 5e4589a1 frag u_far_split 960 float 1 0
+U 5e4589a1 frag u_far_band 964 float 1 0
 T 5e4589a1 u_brdf 2
 T 5e4589a1 u_carpet 3
 T 5e4589a1 u_grass_a 4

packages/ludic.render3d/shaders/terrain.frag

@@ -18,6 +18,13 @@ uniform sampler2D u_snow_d;  uniform sampler2D u_carpet;   // the clump cards ba
 uniform float u_carpet_on;
 uniform float u_snow_line;
 uniform float u_lake_level;      // the ground just above the water is wet and dark
+// ---- RAIN THAT LEAVES SOMETHING BEHIND -----------------------------------------------------
+// Rain fell and the ground did not change: the weather was a curtain of particles in front of a
+// dry valley. This is one scalar, 0 dry to 1 soaked, and it does three things a wet surface
+// does - darkens the albedo, drops the roughness so the sky glances off it, and pools in the
+// low places. It dries out over minutes after the rain stops, which is the part that makes a
+// player notice the weather happened at all.
+uniform float u_wet;
 uniform float u_sea_level;       // the sea's line: shore, forest and scree gates (terrain_sea)
 uniform vec4 u_lake;             // the carved lake's outline: centre x/z, half extents (zero = none)
 // The water line that applies here: the sea everywhere, the lake's own inside its outline -
@@ -437,7 +444,18 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
   float above = p.y - waterLine(p.xz);                    // >0 on land, metres
   // wet ground: darker and glossier near the water, fading out over ~1.2 m
   float wet = smoothstep(1.2, 0.0, above);
+  // the rain's wetness joins the shore's: the same term, from a different cause
+  wet = max(wet, u_wet);
   alb *= mix(1.0, 0.5, wet * 0.85);
+  // Puddles. A soaked ground does not darken evenly - water finds the low places, and the flat
+  // ground holds it while a slope sheds it. The noise is the same macro field the materials
+  // already use, so a puddle sits where the ground is genuinely dished rather than in a
+  // pattern of its own.
+  float pud = 0.0;
+  if (u_wet > 0.01) {
+    float lowness = smoothstep(0.35, 0.0, slope);
+    pud = smoothstep(0.45, 0.72, fbmC(cheap, p.xz * 0.35 + 17.0, 2) * 0.5 + 0.5) * lowness * u_wet;
+  }
   // A sheet of water actually runs up the bank ahead of the foam, so the ground inside
   // the wash is seen through water, not bare. Without this the gaps between the foam
   // streaks showed dry grass and the wash looked like white paint on a lawn.
@@ -477,6 +495,15 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
   n = normalize(mix(n, N, smoothstep(150.0, 900.0, dist)));
   float ao = arm.r;
   float rough = clamp(arm.g, 0.3, 1.0);
+  // Wet ground is SMOOTH. That is what makes a soaked meadow read as soaked rather than as
+  // dark: the sky glances off it. A puddle is smoother still and flat, so its normal is the
+  // world's up rather than the ground's relief.
+  rough = mix(rough, 0.28, wet * 0.8);
+  if (pud > 0.01) {
+    rough = mix(rough, 0.06, pud);
+    n = normalize(mix(n, vec3(0.0, 1.0, 0.0), pud * 0.9));
+    alb = mix(alb, alb * 0.55, pud);
+  }
   float metal = 0.0;
   // the shadow map carries the objects standing on the ground (trees, rocks); the
   // ground's own relief is in the baked height-field shadow that sunShadow() applies

packages/ludic.render3d/terrain.ludic

@@ -45,6 +45,8 @@ var ter_smooth: bool = false   # generate the analytic test ground instead of a
 var ter_far_split: int = 0             # metres: beyond this the terrain takes its cheap far path (R3D_TFAR)
 var ter_far_band: int = 0              # half-width of the near/far blend (R3D_TBAND)
 var ter_snow_line: int = 0
+# 0 dry .. 1 soaked. The game sets it from the weather and lets it dry out.
+var ter_wet: int = 0
 var ter_tex: words = null             # 11 material textures (see terrain_bind)
 var ter_ox: int = 0                   # world x/z of the terrain centre (float bits)
 var ter_oz: int = 0
@@ -483,6 +485,7 @@ function terrain_bind_prog(p: int) -> void {
   u_f(gpu_uniform(p, "u_half"), fi(TERRAIN_HALF))
   u_f(gpu_uniform(p, "u_texel"), fr(1, TERRAIN_RES))
   u_f(gpu_uniform(p, "u_snow_line"), ter_snow_line)
+  u_f(gpu_uniform(p, "u_wet"), ter_wet)
   var lake = fl(-100000.0)
   if ter_lake_ex != 0 { lake = ter_lake_level }
   u_f(gpu_uniform(p, "u_lake_level"), lake)