src/render.rs

@@ -63,7 +63,8 @@ pub fn render_top_down(grid: &HeightGrid, scene: &Scene) -> RgbImage {
     for y in 0..h {
         for x in 0..w {
             let elevation = grid.sample(x, y).unwrap_or(0.0) * vertical_exaggeration;
-            let color = scene_color(scene, elevation);
+            let (nx, nz) = normalized_coords(x, y, w, h);
+            let color = surface_color(scene, elevation, nx, nz);
             img.put_pixel(x, y, Rgb(color));
         }
     }
@@ -154,6 +155,49 @@ fn mix_color(color: [u8; 3], target: [u8; 3], factor: f32) -> [u8; 3] {
     ]
 }
 
+fn normalized_coords(x: u32, y: u32, width: u32, height: u32) -> (f32, f32) {
+    let nx = if width > 1 {
+        x as f32 / (width as f32 - 1.0)
+    } else {
+        0.5
+    };
+    let nz = if height > 1 {
+        y as f32 / (height as f32 - 1.0)
+    } else {
+        0.5
+    };
+    (nx.clamp(0.0, 1.0), nz.clamp(0.0, 1.0))
+}
+
+fn hydrology_overlay_color(scene: &Scene, nx: f32, nz: f32) -> Option<[u8; 3]> {
+    let lake = scene.hydrology.lake_coverage(nx, nz);
+    let river = scene.hydrology.river_coverage(nx, nz);
+    let coverage = lake.max(river);
+    if coverage <= 0.0 {
+        return None;
+    }
+
+    let tint = if lake >= river {
+        mix_color(scene.palette.water, [40, 125, 225], 0.68)
+    } else {
+        mix_color(scene.palette.water, [100, 210, 255], 0.75)
+    };
+    let colored = mix_color(scene.palette.water, tint, coverage);
+    Some(if coverage > 0.72 {
+        mix_color(colored, [245, 250, 255], (coverage - 0.72).clamp(0.0, 0.28) * 3.0)
+    } else {
+        colored
+    })
+}
+
+fn surface_color(scene: &Scene, elevation: f32, nx: f32, nz: f32) -> [u8; 3] {
+    let base = scene_color(scene, elevation);
+    match hydrology_overlay_color(scene, nx, nz) {
+        Some(overlay) => mix_color(base, overlay, 0.9),
+        None => base,
+    }
+}
+
 fn v_add(a: Vec3, b: Vec3) -> Vec3 {
     Vec3::new(a.x + b.x, a.y + b.y, a.z + b.z)
 }