From 9f643149a69a149612cffd65a953527630e95647 Mon Sep 17 00:00:00 2001
From: Valentin Popov <valentin@popov.link>
Date: Sat, 31 Oct 2026 19:13:43 +0400
Subject: [PATCH] fix(render): use native squared distance for vertex fog

---
 .../shaders/triangle.vert                     |   9 ++--
 .../shaders/triangle.vert.spv                 | Bin 20096 -> 20196 bytes
 .../src/ffi/smoke_types.rs                    |  40 +++++++++++++++++-
 apps/fparkan-game/src/main.rs                 |   7 +--
 docs/appendices/knowledge-boundaries.md       |   4 ++
 docs/reference/render-frame.md                |  20 +++++++++
 docs/tomes/05-render.md                       |   3 ++
 7 files changed, 75 insertions(+), 8 deletions(-)

diff --git a/adapters/fparkan-render-vulkan/shaders/triangle.vert b/adapters/fparkan-render-vulkan/shaders/triangle.vert
index d1e2e04..078a0d1 100644
--- a/adapters/fparkan-render-vulkan/shaders/triangle.vert
+++ b/adapters/fparkan-render-vulkan/shaders/triangle.vert
@@ -130,12 +130,13 @@ float native_specular_power(float cosine, float power) {
 }
 
 float native_fog_factor(vec3 world_position) {
-    float fog_span = frame.fog_distances.y - frame.fog_distances.x;
-    float distance_to_camera = distance(world_position, frame.camera_position.xyz);
-    if (fog_span <= 0.000001 || distance_to_camera <= frame.fog_distances.x) {
+    float fog_span_squared = frame.fog_distances.w - frame.fog_distances.z;
+    vec3 to_camera = world_position - frame.camera_position.xyz;
+    float distance_squared = dot(to_camera, to_camera);
+    if (fog_span_squared <= 0.000001 || distance_squared <= frame.fog_distances.z) {
         return 0.0;
     }
-    return clamp((distance_to_camera - frame.fog_distances.x) / fog_span, 0.0, 1.0);
+    return clamp((distance_squared - frame.fog_distances.z) / fog_span_squared, 0.0, 1.0);
 }
 
 vec3 normalized_or(vec3 value, vec3 fallback) {
diff --git a/adapters/fparkan-render-vulkan/shaders/triangle.vert.spv b/adapters/fparkan-render-vulkan/shaders/triangle.vert.spv
index f74b1f81e5a1360c88f6f3752ec1e3d8e9ae40ff..4fc9a1b5a2843a9d466379f569fb8f1d0c7fd504 100644
GIT binary patch
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diff --git a/adapters/fparkan-render-vulkan/src/ffi/smoke_types.rs b/adapters/fparkan-render-vulkan/src/ffi/smoke_types.rs
index 278243b..0996fa1 100644
--- a/adapters/fparkan-render-vulkan/src/ffi/smoke_types.rs
+++ b/adapters/fparkan-render-vulkan/src/ffi/smoke_types.rs
@@ -284,6 +284,9 @@ impl VulkanFrameUniforms {
             .chain([self.fog_start, self.fog_end])
             .chain(self.camera_position)
             .all(f32::is_finite)
+            && (self.fog_end <= self.fog_start
+                || ((self.fog_start * self.fog_start).is_finite()
+                    && (self.fog_end * self.fog_end).is_finite()))
     }
 
     /// Serializes one frame slot in the exact `std140` member order.
@@ -344,7 +347,17 @@ impl VulkanFrameUniforms {
             self.fog_color[2],
             0.0,
         ]);
-        values[84..88].copy_from_slice(&[self.fog_start, self.fog_end, 0.0, 0.0]);
+        let squared_fog_range = if self.fog_end > self.fog_start {
+            [self.fog_start * self.fog_start, self.fog_end * self.fog_end]
+        } else {
+            [0.0; 2]
+        };
+        values[84..88].copy_from_slice(&[
+            self.fog_start,
+            self.fog_end,
+            squared_fog_range[0],
+            squared_fog_range[1],
+        ]);
         values[88..92].copy_from_slice(&[
             self.camera_position[0],
             self.camera_position[1],
@@ -874,6 +887,31 @@ mod static_mesh_tests {
         assert_eq!(mesh.validate(), Ok(()));
     }
 
+    #[test]
+    fn fog_uniforms_match_live_native_squared_distance_range() {
+        // Terrain ShadeIndexedStrided: far=0x442389e6, far²=0x48d0f1ca;
+        // distance²=0x47707b04 gives visibility=0x3f5b2b94.
+        let frame = VulkanFrameUniforms {
+            fog_start: 0.0,
+            fog_end: f32::from_bits(0x4423_89e6),
+            ..Default::default()
+        };
+        assert!(frame.is_finite());
+        let bytes = frame.to_ne_bytes();
+        let read = |offset| f32::from_ne_bytes(bytes[offset..offset + 4].try_into().unwrap());
+        assert_eq!(read(344), 0.0);
+        assert_eq!(read(348).to_bits(), 0x48d0_f1ca);
+        let visibility = 1.0 - (f32::from_bits(0x4770_7b04) - read(344)) / (read(348) - read(344));
+        assert!((visibility - f32::from_bits(0x3f5b_2b94)).abs() < 1.0e-6);
+        assert!(VulkanFrameUniforms::default().is_finite());
+        assert!(!VulkanFrameUniforms {
+            fog_start: 0.0,
+            fog_end: f32::MAX,
+            ..frame
+        }
+        .is_finite());
+    }
+
     #[test]
     fn static_camera_composes_recovered_d3d7_view_before_projection() {
         let transform = RawCameraTransform {
diff --git a/apps/fparkan-game/src/main.rs b/apps/fparkan-game/src/main.rs
index 30380ed..279fa2c 100644
--- a/apps/fparkan-game/src/main.rs
+++ b/apps/fparkan-game/src/main.rs
@@ -9454,6 +9454,7 @@ impl StaticVulkanApp {
             })
             .count();
         eprintln!("render_coverage control_op1_jitter_unsupported={randomized_lights} (using authored color and radius; native RNG sampling pending)");
+        eprintln!("render_coverage fog_pass_eligibility_approx={} (global world fog and horizon RGB; native shade-state/color verification pending)", self.world_range_indices.len());
         for &range_index in &self.world_range_indices {
             let range = self
                 .mesh
@@ -14499,7 +14500,7 @@ mod tests {
     }
 
     #[test]
-    fn captured_native_camera_uses_view_dependent_sky_fog_color() {
+    fn captured_native_camera_selects_view_dependent_screen_horizon_color() {
         use fparkan_fx::atmosphere::{PackedColor, SkySample, SKY_COLOR_COUNT};
 
         // Compact camera fields from the native 1493.337-second AutoDemo capture.
@@ -14521,12 +14522,12 @@ mod tests {
             values: [0.0; 2],
             packed: [PackedColor(0); 2],
         };
-        let fog_color =
+        let horizon_color =
             fparkan_fx::sky::screen_gradient_frame(&sample, camera.native_sky_heading(), [0.0; 3])
                 .color;
 
         assert_eq!(capture.atmosphere_seconds, Some(1493.337));
-        assert_eq!(fog_color, PackedColor(0xFF3E_3E64));
+        assert_eq!(horizon_color, PackedColor(0xFF3E_3E64));
     }
 
     #[test]
diff --git a/docs/appendices/knowledge-boundaries.md b/docs/appendices/knowledge-boundaries.md
index 6fbedf4..7f049fc 100644
--- a/docs/appendices/knowledge-boundaries.md
+++ b/docs/appendices/knowledge-boundaries.md
@@ -10,6 +10,10 @@ authoring API без динамического подтверждения.
 Не доказаны символами точные имена renderer vtable slots, полный набор CShade
 state transitions и окончательный порядок части transparent/FX/shadow subpasses.
 
+Дистанционная кривая тумана по квадратам границ подтверждена в живом CShade.
+Выбор проходов по native shade-state и GPU fog color пока заменён общим туманом мира с цветом горизонта;
+это приближение считается как `render_coverage fog_pass_eligibility_approx`.
+
 После ограничения цели рельефом интерактивная камера проверяет движение
 против каждого отрисовываемого корневого строения класса `FORT` и его
 отрисовываемых компонентов. Для этого пути используются текущая Node38-поза
diff --git a/docs/reference/render-frame.md b/docs/reference/render-frame.md
index 3244d1b..b2a6a93 100644
--- a/docs/reference/render-frame.md
+++ b/docs/reference/render-frame.md
@@ -71,6 +71,26 @@ RVA `0x1B0F0`, заменяет diffuse alpha пакета без type 16 с bat
 путь и ещё не декодирован. Это ограничивает приёмку на частично отсечённых
 порталах и не позволяет считать интерьер принятой эталонной сценой.
 
+## Дистанционная кривая тумана
+
+Программный `CShade::ShadeIndexedStrided` в GOG Terrain.dll сравнивает квадрат
+дистанции с квадратами ближней и дальней границ. По RVA `0x2EEED–0x2EF45`
+видимость равна `1 - (distance² - near²) / (far² - near²)` между границами,
+единице перед ближней и нулю за дальней. Такой же закон использует отдельный
+путь цвета пакета `0x2F230`. Линейная интерполяция по самой дистанции давала
+слишком плотный туман в середине диапазона.
+
+В работающем оригинале границы 0 и `0x442389E6`, квадрат дистанции
+`0x47707B04` дали видимость `0x3F5B2B94`. Rust-тест проверяет передаваемые GPU
+квадраты границ по этому наблюдению. GLSL вычисляет квадрат расстояния вершины,
+после чего растеризатор интерполирует коэффициент по треугольнику.
+`native_fog.effective.sink_rgb` совпадает с `sky.packed[1]`, нижней границей
+освещения CShade. Это не подтверждает значение GPU fog color. Приложение пока
+сохраняет цвет экранного горизонта; выбор цвета и проходов требует отдельной
+проверки GPU-состояния.
+Выбор проходов по native shade-state пока приближён общим туманом мира и
+считается как `render_coverage fog_pass_eligibility_approx`.
+
 ## Радиус Control Op1
 
 GOG `Effect.dll` (SHA-256
diff --git a/docs/tomes/05-render.md b/docs/tomes/05-render.md
index bd23875..72b0685 100644
--- a/docs/tomes/05-render.md
+++ b/docs/tomes/05-render.md
@@ -845,6 +845,9 @@ projected shadows. Радиус источника масштабируется
 синий свет завода почти незаметным. Неположительный итог заменяется на 0,01.
 Случайные добавки к цвету и радиусу пока явно считаются в
 `render_coverage control_op1_jitter_unsupported`.
+Туман также зависит от расстояния нелинейно: между ближней и дальней границами
+движок интерполирует по квадрату дистанции. При границах 0 и 700 точка в 350
+получает четверть тумана, а не половину. Это сохраняет контраст среднего плана.
 Effect renderer подавляет header bit `0x800` только при
 render selector 0. Native AniMesh component path захвачен с selector 1, а в
 его 53-entry manager присутствуют FX entries с header `0xD00`; preview