fix(shaders): parallax mapping reads its depth from a mip about four texels per pixel coarser, filtered bicubically where the hit is refined, so the relief no longer breaks into stalactites and the edges of stones no longer step along the texel grid of the compressed depth; it takes 16 to 64 layers by the angle of view and six steps of refinement, casts the shadows of the relief from the sun over the sun light and darkens the hollows deeper than the mean depth of the texture while lightening the tops, so the mean brightness of a surface stays the same

shaders/light_common.glsl

@@ -36,6 +36,11 @@ float length2(vec3 v)
         return dot(v, v);
 }
 
+#ifdef PARALLAX
+float parallax_occlusion = 0.0;
+float parallax_shadow = 0.0;
+#endif
+
 float calc_shadow()
 {
 #if SHADOWMAP_ENABLED
@@ -289,6 +294,9 @@ vec3 apply_lights(vec3 fragcolor, vec3 fragnormal, vec3 texturecolor, float refl
     // Sun, headlights and emission are added afterwards so they are NOT
     // attenuated by AMBIENT_SCALE - this only dims the indirect term.
     fragcolor *= AMBIENT_SCALE;
+#ifdef PARALLAX
+    fragcolor *= 1.0 - parallax_occlusion;
+#endif
 
     vec3 emissioncolor = vec3(emission);
 
@@ -331,6 +339,9 @@ vec3 apply_lights(vec3 fragcolor, vec3 fragnormal, vec3 texturecolor, float refl
     float shadow1 = 0.0;
     if (shadowtone < 1.0)
         shadow1 = (1.0 - shadowtone) * clamp(calc_shadow(), 0.0, 1.0);
+#ifdef PARALLAX
+    shadow1 = max(shadow1, parallax_shadow);
+#endif
 
     // Sun HDR scale -> SUN_DIFFUSE_SCALE (default 2.5). Controls how
     // bright sun-lit (unshaded) faces get. Lower this if surfaces in