Fix atmosphere lighting issue for below ground geo

Author: mate-h

crates/bevy_pbr/src/atmosphere/functions.wgsl

@@ -281,6 +281,17 @@ fn calculate_visible_sun_ratio(atmosphere: Atmosphere, r: f32, mu: f32, sun_angu
 
 // TRANSFORM UTILITIES
 
+/// Clamp a position to the planet surface (with a small epsilon) to avoid underground artifacts.
+fn clamp_to_surface(atmosphere: Atmosphere, position: vec3<f32>) -> vec3<f32> {
+    let min_radius = atmosphere.bottom_radius + EPSILON;
+    let r = length(position);
+    if r < min_radius {
+        let up = normalize(position);
+        return up * min_radius;
+    }
+    return position;
+}
+
 fn max_atmosphere_distance(r: f32, mu: f32) -> f32 {
     let t_top = distance_to_top_atmosphere_boundary(atmosphere, r, mu);
     let t_bottom = distance_to_bottom_atmosphere_boundary(r, mu);
@@ -291,18 +302,7 @@ fn max_atmosphere_distance(r: f32, mu: f32) -> f32 {
 /// Returns the observer's position in the atmosphere
 fn get_view_position() -> vec3<f32> {
     var world_pos = view.world_position * settings.scene_units_to_m + vec3(0.0, atmosphere.bottom_radius, 0.0);
-    
-    // If the camera is underground, clamp it to the ground surface along the local up.
-    let r = length(world_pos);
-    // Nudge r above ground to avoid sqrt cancellation, zero-length segments where 
-    // r is equal to bottom_radius, which show up as black pixels
-    let min_radius = atmosphere.bottom_radius + EPSILON;
-    if r < min_radius {
-        let up = normalize(world_pos);
-        world_pos = up * min_radius;
-    }
-
-    return world_pos;
+    return clamp_to_surface(atmosphere, world_pos);
 }
 
 // We assume the `up` vector at the view position is the y axis, since the world is locally flat/level.

crates/bevy_pbr/src/render/pbr_lighting.wgsl

@@ -3,7 +3,7 @@
 #import bevy_pbr::{
     mesh_view_types::POINT_LIGHT_FLAGS_SPOT_LIGHT_Y_NEGATIVE,
     mesh_view_bindings as view_bindings,
-    atmosphere::functions::calculate_visible_sun_ratio,
+    atmosphere::functions::{calculate_visible_sun_ratio, clamp_to_surface},
     atmosphere::bruneton_functions::transmittance_lut_r_mu_to_uv,
 }
 #import bevy_render::maths::PI
@@ -862,8 +862,9 @@ color *= (*light).color.rgb * texture_sample;
     let O = vec3(0.0, atmosphere.bottom_radius, 0.0);
     let P_scaled = P * vec3(view_bindings::atmosphere_data.settings.scene_units_to_m);
     let P_as = P_scaled + O;
-    let r = length(P_as);
-    let local_up = normalize(P_as);
+    let P_clamped = clamp_to_surface(atmosphere, P_as);
+    let r = length(P_clamped);
+    let local_up = normalize(P_clamped);
     let mu_light = dot(L, local_up);
 
     // Sample atmosphere