feat(mods): adjust computed world fog color

Co-Authored-By: Codex <[email protected]>

crates/horizon-client/src/game/native_hooks.rs

@@ -1331,6 +1331,17 @@ impl Game<'_> {
     pub(super) fn native_zoom(&self, zoom: &mut Option<NativeZoom>) {
         let _ = zoom;
     }
+    /// Computed world fog RGB, after ordinary effects and before upload. Title panoramas skip it.
+    /// Only finite RGB components in 0..=1 are accepted; alpha and distances stay unchanged.
+    pub(super) fn native_compute_fog_color(
+        &self,
+        partial: f32,
+        camera: [f64; 3],
+        fog_type: horizon_client::fog::FogType,
+        rgb: &mut [f32; 3],
+    ) {
+        let _ = (partial, camera, fog_type, rgb);
+    }
     /// The camera shake native mods ask for in this frame, `partial` ticks
     /// past the last tick (0 to 1): add your pitch, yaw and roll in degrees
     /// and your offset in blocks, along the camera's right, up and back, to

crates/horizon-client/src/game/render_tick.rs

@@ -140,8 +140,27 @@ impl Game<'_> {
         if !self.settings.horizon.fog {
             horizon_client::fog::clear_distance_fog(&mut fog, &input);
         }
+        if self.title.is_none() {
+            fog.color =
+                self.native_fog_color_now(self.clock.fraction() as f32, at, fog_type, fog.color);
+        }
         fog
     }
+    fn native_fog_color_now(
+        &self,
+        partial: f32,
+        camera: [f64; 3],
+        fog_type: FogType,
+        color: [f32; 4],
+    ) -> [f32; 4] {
+        let mut rgb = [color[0], color[1], color[2]];
+        self.native_compute_fog_color(partial, camera, fog_type, &mut rgb);
+        if rgb.iter().all(|v| v.is_finite() && (0.0..=1.0).contains(v)) {
+            [rgb[0], rgb[1], rgb[2], color[3]]
+        } else {
+            color
+        }
+    }
     /// `FogRenderer.computeFogColor`'s night vision brightenFactor:
     /// `GameRenderer.nightVisionScale`, but not while Darkness is on.
     fn fog_night_vision(&self) -> f32 {

tests/test_compute_fog_color_hook.py

@@ -0,0 +1,51 @@
+"""Run the actual fog color dispatch with a fake mod, without a GPU/window."""
+from pathlib import Path
+import shutil
+import subprocess
+import tempfile
+import unittest
+
+from test_world_input_capture import method
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+class ComputeFogColorHookTests(unittest.TestCase):
+    @unittest.skipUnless(shutil.which("rustc"), "requires Rust")
+    def test_rgb_override_fallback_and_alpha(self):
+        source = (ROOT / "crates/horizon-client/src/game/render_tick.rs").read_text(encoding="utf-8")
+        actual = method(source, "native_fog_color_now")
+        frame = method(source.replace("pub(super) fn", "fn"), "frame_fog")
+        self.assertIn("if self.title.is_none() { fog.color = self.native_fog_color_now", " ".join(frame.split()))
+        fixture = r"""
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum FogType { Atmospheric, Water, Lava, PowderSnow }
+struct Game { replacement: Option<[f32; 3]> }
+impl Game {
+    fn native_compute_fog_color(&self, partial: f32, camera: [f64; 3],
+                               _kind: FogType, rgb: &mut [f32; 3]) {
+        assert_eq!((partial, camera, *rgb), (0.25, [1.0, 2.0, 3.0], [0.2, 0.4, 0.6]));
+        if let Some(value) = self.replacement { *rgb = value; }
+    }
+    METHOD
+}
+#[test]
+fn all_environments_valid_invalid_and_unchanged() {
+    let original = [0.2, 0.4, 0.6, 0.75];
+    for kind in [FogType::Atmospheric, FogType::Water, FogType::Lava, FogType::PowderSnow] {
+        for (replacement, expected) in [(None, original),
+            (Some([0.0, 1.0, 0.5]), [0.0, 1.0, 0.5, 0.75]),
+            (Some([f32::NAN, 0.0, 0.0]), original),
+            (Some([0.0, f32::INFINITY, 0.0]), original),
+            (Some([0.0, 0.0, -0.1]), original),
+            (Some([1.1, 0.0, 0.0]), original)] {
+            assert_eq!(Game { replacement }.native_fog_color_now(0.25, [1.0, 2.0, 3.0], kind, original), expected);
+        }
+    }
+}
+""".replace("METHOD", actual)
+        with tempfile.TemporaryDirectory() as folder:
+            src, exe = Path(folder) / "fog.rs", Path(folder) / "fog.exe"
+            src.write_text(fixture, encoding="utf-8")
+            subprocess.run(["rustc", "--edition=2024", "--test", str(src), "-o", str(exe)], check=True)
+            subprocess.run([str(exe)], check=True)

tools/native-hook-fragments/compute_fog_color.json

@@ -0,0 +1,12 @@
+{
+  "hooks": [
+    {
+      "name": "compute_fog_color",
+      "target": "horizon_client::game::native_hooks::Game<'_>::native_compute_fog_color",
+      "neoforge": "ViewportEvent.ComputeFogColor",
+      "side": "client"
+    }
+  ],
+  "rust_summary": "a computed fog color hook (`native_compute_fog_color`).",
+  "doc_summary": "`native_compute_fog_color(partial, camera, fog_type, rgb)` adjusts the final world fog RGB after normal biome, weather and effect calculation and before GPU upload. Camera is its world position; fog_type distinguishes atmosphere, water, lava and powder snow. Only finite components in 0..=1 are accepted; invalid requests retain the original RGB. Alpha and all distance fields are unchanged. Title panoramas skip this hook. Bundles need \"hooks\": @API:compute_fog_color@."
+}