mrgameeng/src/MrGameEng.Graphics/Lighting/LightmapBuilder.cs

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>
public readonly record struct LightSample(int X, int Y, float Radius, float Intensity);

/// <summary>
/// Builds a per-cell light grid (CPU, GPU-free, testable): an ambient base dimmed under occluders,
/// plus point lights that attenuate with distance and are blocked by occluders between the source
/// and the cell (grid-traced shadows). Pure data — a <see cref="Lightmap"/> turns the grid into a
/// texture and the simulation samples it for local light.
/// </summary>
public static class LightmapBuilder
{
    /// <summary>How much of the ambient light reaches a cell that is itself an occluder (canopy/shade).</summary>
    public const float OccluderShade = 0.35f;

    /// <summary>
    /// Fills <paramref name="light"/> (length <paramref name="width"/>×<paramref name="height"/>) with
    /// 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,
        int width,
        int height,
        float ambient,
        ReadOnlySpan<bool> occluders,
        IReadOnlyList<LightSample> lights,
        Vector2 sunShadow = default,
        float sunShadowStrength = 0f
    )
    {
        for (var i = 0; i < light.Length; i++)
        {
            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)
            {
                continue;
            }

            var r = (int)MathF.Ceiling(l.Radius);
            var minX = Math.Max(0, l.X - r);
            var maxX = Math.Min(width - 1, l.X + r);
            var minY = Math.Max(0, l.Y - r);
            var maxY = Math.Min(height - 1, l.Y + r);
            for (var y = minY; y <= maxY; y++)
            {
                for (var x = minX; x <= maxX; x++)
                {
                    var dx = x - l.X;
                    var dy = y - l.Y;
                    var d = MathF.Sqrt(dx * dx + dy * dy);
                    if (d > l.Radius)
                    {
                        continue;
                    }

                    if (!Visible(l.X, l.Y, x, y, occluders, width))
                    {
                        continue; // в тени за препятствием
                    }

                    light[y * width + x] += l.Intensity * (1f - d / l.Radius);
                }
            }
        }

        for (var i = 0; i < light.Length; i++)
        {
            light[i] = Math.Clamp(light[i], 0f, 1f);
        }
    }

    // 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(
        int x0,
        int y0,
        int x1,
        int y1,
        ReadOnlySpan<bool> occluders,
        int width
    )
    {
        var dx = Math.Abs(x1 - x0);
        var dy = Math.Abs(y1 - y0);
        var sx = x0 < x1 ? 1 : -1;
        var sy = y0 < y1 ? 1 : -1;
        var err = dx - dy;
        var x = x0;
        var y = y0;
        while (true)
        {
            if (x == x1 && y == y1)
            {
                return true;
            }

            if ((x != x0 || y != y0) && occluders[y * width + x])
            {
                return false;
            }

            var e2 = 2 * err;
            if (e2 > -dy)
            {
                err -= dy;
                x += sx;
            }

            if (e2 < dx)
            {
                err += dx;
                y += sy;
            }
        }
    }
}