Lighting: directional sun shadows from the day/night sun angle

DayNight.SunShadow returns a cell offset opposite the sun's east-west position, longest near sunrise/sunset (low sun) and zero at noon/night, tilted slightly south so shadows fall in front of occluders. LightmapBuilder gains a directional shadow pass (MaxShadowCells length cap, SunShadowStrength darkening); LightmapSystem feeds it the current sun shadow each rebuild. Tests cover SunShadow's day-arc behaviour and the builder's directional darkening.

src/MrGameEng.Graphics/Lighting/LightmapBuilder.cs

@@ -1,3 +1,5 @@
+using Microsoft.Xna.Framework;
+
 namespace MrGameEng.Lighting;
 
 /// <summary>One point light projected onto the light grid: a cell position, a radius in cells and an intensity.</summary>
@@ -16,8 +18,10 @@ public static class LightmapBuilder
 
     /// <summary>
     /// Fills <paramref name="light"/> (length <paramref name="width"/>×<paramref name="height"/>) with
-    /// ambient light, shading occluder cells, then adds each point light with grid-traced occlusion.
-    /// Values end clamped to <c>[0, 1]</c>.
+    /// ambient light, shading occluder cells, casts each occluder's directional sun shadow along
+    /// <paramref name="sunShadow"/> (cells), then adds each point light with grid-traced occlusion.
+    /// Values end clamped to <c>[0, 1]</c>. A zero <paramref name="sunShadow"/> or
+    /// <paramref name="sunShadowStrength"/> skips the directional pass (e.g. at night/noon).
     /// </summary>
     public static void Build(
         float[] light,
@@ -25,7 +29,9 @@ public static class LightmapBuilder
         int height,
         float ambient,
         ReadOnlySpan<bool> occluders,
-        IReadOnlyList<LightSample> lights
+        IReadOnlyList<LightSample> lights,
+        Vector2 sunShadow = default,
+        float sunShadowStrength = 0f
     )
     {
         for (var i = 0; i < light.Length; i++)
@@ -33,6 +39,8 @@ public static class LightmapBuilder
             light[i] = occluders[i] ? ambient * OccluderShade : ambient;
         }
 
+        CastSunShadows(light, width, height, occluders, sunShadow, sunShadowStrength);
+
         foreach (var l in lights)
         {
             if (l.Radius <= 0f || l.Intensity <= 0f)
@@ -73,6 +81,62 @@ public static class LightmapBuilder
         }
     }
 
+    // Направленная тень от солнца: каждый окклюдер (гора/зрелая крона) отбрасывает тень вдоль
+    // вектора sunShadow (в клетках). Затемнение гуще у основания и тает к концу тени; сами клетки-
+    // окклюдеры не трогаем (они уже затенены). Окклюдеры разрежены, так что проход дёшев.
+    private static void CastSunShadows(
+        float[] light,
+        int width,
+        int height,
+        ReadOnlySpan<bool> occluders,
+        Vector2 sunShadow,
+        float strength
+    )
+    {
+        if (strength <= 0f)
+        {
+            return;
+        }
+
+        var steps = (int)MathF.Ceiling(sunShadow.Length());
+        if (steps <= 0)
+        {
+            return;
+        }
+
+        var stepX = sunShadow.X / steps;
+        var stepY = sunShadow.Y / steps;
+        for (var oy = 0; oy < height; oy++)
+        {
+            for (var ox = 0; ox < width; ox++)
+            {
+                if (!occluders[oy * width + ox])
+                {
+                    continue;
+                }
+
+                for (var s = 1; s <= steps; s++)
+                {
+                    var cx = ox + (int)MathF.Round(stepX * s);
+                    var cy = oy + (int)MathF.Round(stepY * s);
+                    if (cx < 0 || cx >= width || cy < 0 || cy >= height)
+                    {
+                        break;
+                    }
+
+                    var index = cy * width + cx;
+                    if (occluders[index])
+                    {
+                        continue; // тень проходит над другими окклюдерами — они и так тёмные
+                    }
+
+                    var falloff = 1f - (float)(s - 1) / steps; // гуще у основания тени
+                    light[index] *= 1f - strength * falloff;
+                }
+            }
+        }
+    }
+
     // Есть ли прямая видимость между клетками: проводим линию (Брезенхем) и проверяем
     // промежуточные клетки на окклюдер (концы исключены).
     private static bool Visible(