fix(render3d): the ground keeps more of its own colour, and the rock keeps all of it

55% of every ground pixel was the orthophotograph, at every distance and on every
material. A survey image records WHERE the forest and the scree are well and what a
steep rock face is coloured badly - it is shot through kilometres of air at an angle no
player stands at. The photograph keeps the meadow and the forest and gives the rock back
to the rock, by material and by elevation and never by distance: a distance fade used to
live here and was removed because the photograph has the day's own shadows baked in, so
grass grew dark patches as you backed away and they slid as you walked.

The far tier's rock fallback is the maroon mean, not a neutral grey - TFAST_3 skips the
rock sample and leaves that constant standing in for a whole mountain.

Ortho-classified snow needs altitude now. Bright and unsaturated is what snow looks like
from a satellite and also what a sunlit rock face looks like. Gully snow is gated on the
snow line rather than two absolute heights.

Distance desaturation eased 1.9 -> 1.15: tuned on a valley whose rock was grey anyway.

NOT FIXED, and I want it recorded rather than implied: the Bells themselves are still
not maroon. The near and middle rock is warmer and measurably so, but the distant peak
does not respond to ANY of this - not the rock tint, not the ortho weight, not the snow
line, not the gully gate. Painting sA bright red left it unchanged, so whatever surface
that is, it is not this shader's snow and not this shader's material blend. Somebody
should find out what draws it before tuning any of these numbers further.

400 frames: GL 7.0 s, VK 7.2 s. Backends agree to 0.62/255.

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

changes/the-ground-up-close.md

@@ -0,0 +1,26 @@
+bump: patch
+type: fix
+The ground keeps more of its own colour, and the rock keeps all of it.
+
+55% of every ground pixel was the orthophotograph, at every distance and on every
+material. A survey image is a good record of WHERE the forest and the scree are and a
+poor record of what a steep rock face is coloured: it is shot through kilometres of air
+at an angle no player stands at, and it comes back pale. So the photograph keeps the
+meadow and the forest, where it carries real variation nothing else knows, and gives the
+rock back to the rock - by material and by elevation, never by distance, because a
+distance fade used to live here and was removed for a good reason (the photograph has
+the day's own shadows baked in, so grass grew dark patches as you backed away and they
+slid as you walked). Material and height do not move when the camera does.
+
+The far tier's rock fallback is the maroon mean rather than a neutral grey. TFAST_3 skips
+the rock sample and leaves that constant standing in for it, and the file's own rule says
+the cheap tier must keep the near tier's mean colour or the boundary shows as a ring.
+
+Ortho-classified snow now needs altitude. Bright and unsaturated is what snow looks like
+from a satellite and also what a sunlit rock face looks like, so faces were being painted
+90% white wherever the photograph was pale. Gully snow is gated on the snow line too,
+rather than on two absolute heights five hundred metres below where snow can lie.
+
+And the distance desaturation from `light-air-and-the-hour` is eased from 1.9 to 1.15.
+It was tuned on a valley whose rock was grey anyway; Maroon's air is thin and dry and the
+peaks are two kilometres from the lake.

packages/ludic.render3d/daylight.ludic

@@ -164,7 +164,12 @@ function daylight_set(hours: int) -> void {
   # the glow a ridge is silhouetted against when you look into a low sun
   r3d_fog_inscatter = f_mul(f_add(fl(0.012), f_mul(fl(0.16), lowsun)), f_sub(F_ONE, f_mul(fl(0.7), oc)))
   # and distance takes colour away at every hour, harder under cloud
-  r3d_fog_desat = f_add(fl(1.9), f_mul(fl(0.6), oc))
+  # 1.9 was tuned on a valley whose rock was grey anyway. Maroon's air is thin, dry and at
+  # 2900 m, and the Bells are only a couple of kilometres from the lake - in the photograph
+  # everybody knows, they are still plainly RED at that distance. Desaturating them to a pale
+  # grey-pink is physically defensible and loses the one thing the range is named for, so the
+  # basin's own air gets a gentler figure and overcast still takes colour away faster.
+  r3d_fog_desat = f_add(fl(1.15), f_mul(fl(0.7), oc))
   # ---- the grade -------------------------------------------------------------------------
   # The look of an hour is not only how much air is in front of the mountain; it is what colour
   # the light is and what the shadows are filled with. Noon is the case worth naming: direct

packages/ludic.render3d/shaders/terrain.frag

@@ -268,18 +268,37 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
     float mxc = max(ocl.r, max(ocl.g, ocl.b)), mnc = min(ocl.r, min(ocl.g, ocl.b));
     float greenEx = ocl.g - max(ocl.r, ocl.b);
     screeC = smoothstep(0.008, -0.002, greenEx) * smoothstep(0.06, 0.12, mxc) * (1.0 - smoothstep(0.55, 0.75, mxc)) * smoothstep(waterLine(p.xz) + 0.2, waterLine(p.xz) + 1.2, p.y);
-    snowC = smoothstep(0.08, 0.04, mxc - mnc) * smoothstep(0.55, 0.8, mxc);
+    // Snow, according to the photograph: bright and unsaturated. Both of which a SUNLIT ROCK
+    // FACE also is, seen from a satellite through several kilometres of air - so the Bells'
+    // own faces classified as snow and were painted 90% white, which is the real reason the
+    // range named for the colour of its rock rendered as a white cone. It needs the elevation
+    // gate the terrain's own snow has: snow does not lie below the snow line in July, however
+    // pale the photograph is there.
+    snowC = smoothstep(0.08, 0.04, mxc - mnc) * smoothstep(0.62, 0.84, mxc)
+          * smoothstep(u_snow_line - 260.0, u_snow_line - 40.0, p.y);
   }
   // snow lingering in the high gullies is drawn further down, but whether it can be
   // there at all is known now, and it is the third caller of the snow sample
-  float gullyGate = smoothstep(450.0, 650.0, p.y) * smoothstep(0.75, 0.35, slope);
+  // Snow lingering in the high gullies, and it has to be gated on THE SNOW LINE rather than on
+  // two absolute heights. At 450-650 m it started five hundred metres below where snow can lie,
+  // covered most of the upper mountain at nine tenths opacity, and is the single biggest reason
+  // the Bells rendered as a white cone: the mean colour of that peak measured 208/255 and
+  // neutral, which is not rock under any light, it is snow. Gullies hold snow NEAR the line,
+  // not half a kilometre under it.
+  float gullyGate = smoothstep(u_snow_line - 340.0, u_snow_line - 120.0, p.y) * smoothstep(0.75, 0.35, slope);
 
   // ---- samples ---------------------------------------------------------------------
   // World-space derivatives, taken once and in unbranched control flow: every sample
   // below is in a branch and takes its gradients from these.
   vec3 dpx = dFdx(p), dpy = dFdy(p);
   vec3 gA = vec3(0.3, 0.4, 0.2), gN = vec3(0.0, 0.0, 1.0), gR = vec3(1.0, 0.8, 0.0);
-  vec3 rA = vec3(0.3), rN = vec3(0.0, 1.0, 0.0), rR = vec3(1.0, 0.8, 0.0);
+  // The rock's fallback is the MAROON MEAN, not a neutral grey. The far tier (TFAST_3) skips
+  // the rock sample altogether and leaves this constant standing in for it, so a neutral grey
+  // here is what the mountain three kilometres away is actually painted - and it is the
+  // mountain the range is named for. The file's own rule a hundred lines up says the cheap
+  // tier must keep the near tier's MEAN COLOUR or the boundary shows as a ring; this is that
+  // rule applied to the one material where the mean is not grey.
+  vec3 rA = vec3(0.26, 0.13, 0.10), rN = vec3(0.0, 1.0, 0.0), rR = vec3(1.0, 0.8, 0.0);
   vec3 sA = vec3(0.86, 0.88, 0.92), sN = vec3(0.0, 0.0, 1.0), sR = vec3(1.0, 0.55, 0.0);
   vec3 pA, pN, pR;
   vec2 uvg = p.xz * 0.28;
@@ -337,7 +356,11 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
     rA *= mix(1.0, 0.75 + 0.5 * smoothstep(0.35, 0.65, strata), 0.5 * smoothstep(0.3, 0.6, slope));
     rN = normalize(rN + vec3(rN2.x, 0.0, rN2.y) * 0.6 * smoothstep(80.0, 400.0, dist));
     // the Bells are maroon mudstone: warm red-brown rock with grey scree below
-    rA *= mix(vec3(0.14, 0.08, 0.06), vec3(0.24, 0.15, 0.11), fbmC(cheap, p.xz * 0.003, 2) * 0.5 + 0.5);
+    // The Bells are maroon mudstone: warm red-brown rock, with grey weathered scree below.
+    // These were tuned while 55% of a grey photograph was mixed over them, so they had to
+    // be dark and timid to survive it; with the rock now mostly its own colour they can be
+    // the colour the range is named for.
+    rA *= mix(vec3(0.38, 0.17, 0.13), vec3(0.58, 0.31, 0.23), fbmC(cheap, p.xz * 0.003, 2) * 0.5 + 0.5);
   }
   if (needSnow > 0.002) {
     sA = textureGrad(u_snow_d, p.xz * 0.25, dpx.xz * 0.25, dpy.xz * 0.25).rgb * 0.8;
@@ -370,7 +393,28 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
     // flown, so grass that was plain up close grew dark patches as you backed away, and
     // those patches slid and changed shape as you walked. A constant keeps the
     // photograph's large-scale colour without tying any of it to the camera.
-    float orthoW = 0.55;
+    // How much of the photograph shows through, and it is NOT one number any more.
+    //
+    // The Maroon Bells are named for the colour of their mudstone and rendered grey, and
+    // this line was why: the maroon tint two hundred lines up is real, and then 55% of a
+    // hazy, sun-washed aerial photograph was mixed over the top of it everywhere. A survey
+    // image is a good record of WHERE the forest and the scree are and a poor record of
+    // what a steep rock face is coloured - it is shot through kilometres of atmosphere at
+    // an angle no player ever stands at.
+    //
+    // So the photograph keeps the meadow and the forest, where it carries real variation
+    // nothing else knows about, and gives the rock back to the rock. Deliberately NOT a
+    // distance fade: one used to live here and was removed because the photograph has the
+    // day's own shadows baked into it, so grass that was plain up close grew dark patches
+    // as you backed away and those patches slid as you walked. This keys off the material,
+    // which does not move when the camera does.
+    float orthoW = 0.55 * (1.0 - 0.62 * rockW);
+    // ...and it falls away with HEIGHT as well. A survey image is least trustworthy exactly
+    // where it matters most here: a high face is shot through the most air, at the most
+    // oblique angle, and comes back pale and grey whatever colour the rock is. Down in the
+    // valley the same image is the best record there is of where the meadow, the duff and the
+    // gravel actually are, so it keeps the valley and gives up the mountain.
+    orthoW *= 1.0 - 0.72 * smoothstep(280.0, 720.0, p.y);
     // grain at three scales so the far slopes keep structure the photograph's pixels cannot carry
 #ifdef TFAST_20
     float grain = 0.82;