using System;
using UnityEngine;

public partial class MeshPainter
{
    // --- Pipette (eyedropper) ---
    //
    // A sampling MODE rather than a paint channel: while active (Shift or Ctrl held, or the HUD toggle), a
    // left-click raycasts onto ANY surface (not just the paint mesh) and reads every channel value at
    // the hit point (albedo, metallic, smoothness, occlusion, emission) into the brush properties -
    // with every opacity/alpha forced to 1 so re-painting reproduces the picked look at full strength.
    //
    // On a FOREIGN object the pick is a HYBRID: the data channels (metallic/smoothness/occlusion/
    // emission) come texel-exact from the material (raw, UNLIT), while albedo is taken from the
    // rendered, fully-lit on-screen pixel ("what you see" - honours baked lightmaps/ambient/SSAO).
    // This combines the PBR fidelity of the material read with the lit look of the screen. When no
    // texel-exact material read is possible (no readable UV / headless) it falls back to a pure screen
    // pick (lit albedo, data channels at defaults). Our own paint mesh and terrain are read texel-exact.

    [Header("Brush - pipette (eyedropper)")]
    [Tooltip("Pipette mode: a click samples ALL channel values at the hit point on ANY surface " +
             "(albedo, metallic, smoothness, occlusion, emission) and adopts them into the brush " +
             "(all alphas set to 1) instead of painting. On a foreign object the data channels are read " +
             "texel-exact from the material while albedo comes from the lit on-screen pixel. Held button picks once.")]
    public bool pipette = false;

    /// <summary>
    /// Raised after a successful pick, once the brush has adopted the sampled values and all four
    /// paint-channel flags have been enabled. The in-game HUD listens to re-sync its widgets.
    /// </summary>
    public event Action Picked;

    private bool _pickHeld; // edge tracking so a held button picks once, not every frame

    // A terrain hit higher than this above the heightfield is a tree (trees share the terrain
    // collider but stand proud of the ground); on the surface it is bare ground.
    private const float TerrainFoliageHeightEpsilon = 0.3f;

    /// <summary>
    /// Raycasts at a screen position against ANY collider, samples every channel value at the hit
    /// (texel-exact where possible) and copies it into the brush, forcing all opacities/alphas to
    /// 1 and enabling every paint channel. Reads regardless of which channels are currently toggled.
    /// Pick paths, by what was hit:
    ///   - our paint mesh        -> texel-exact from the painted maps (full PBR);
    ///   - terrain ground        -> blend of the terrain layers by their splat weights (full PBR);
    ///   - terrain tree / detail -> the rendered on-screen colour (no per-instance renderer/UV);
    ///   - any foreign object     -> hybrid: data channels texel-exact from the material, albedo from
    ///                               the rendered, fully-lit on-screen pixel (baked lightmaps/ambient/SSAO);
    ///   - anything else / sky    -> the rendered on-screen colour.
    /// </summary>
    private void PickAtScreen(Vector2 screenPos)
    {
        bool ok;
        if (TryRaycastAny(screenPos, out RaycastHit hit))
        {
            if (hit.collider == _collider)
                ok = PickFromPaintTextures(hit);
            else
            {
                Terrain terrain = hit.collider is TerrainCollider ? hit.collider.GetComponent<Terrain>() : null;
                if (terrain)
                    ok = PickFromTerrain(terrain, hit, screenPos);
                else
                    ok = PickFromForeignHybrid(hit, screenPos);
            }
        }
        else
        {
            // Ray left the scene entirely (e.g. foliage silhouetted against the sky): the
            // on-screen pixel is the only thing to sample.
            ok = PickFromScreen(screenPos);
        }
        if (!ok) return;

        FinalizePick();
    }

    /// <summary>
    /// Common tail of every successful pick: forces all opacities/alphas to 1, enables every paint
    /// channel (a whole-material pick implies repainting all of it), leaves pipette mode (one-shot)
    /// and notifies listeners (the in-game HUD) to re-sync their widgets.
    /// </summary>
    private void FinalizePick()
    {
        metallicSmoothnessOpacity = 1f;
        occlusionOpacity = 1f;

        paintAlbedo = true;
        paintMetallicSmoothness = true;
        paintOcclusion = true;
        paintEmission = true;

        pipette = false;

        Picked?.Invoke();
    }

    /// <summary>
    /// Picks from our own painted maps at the hit texel (bit-exact, backend-agnostic).
    /// </summary>
    private bool PickFromPaintTextures(RaycastHit hit)
    {
        if (_verts == null || _uvs == null || _tris == null || hit.triangleIndex < 0) return false;

        int t = hit.triangleIndex;
        if (t * 3 + 2 >= _tris.Length) return false;
        int i0 = _tris[t * 3], i1 = _tris[t * 3 + 1], i2 = _tris[t * 3 + 2];

        // UV at the hit via barycentric interpolation of the cached UVs - same UV space the
        // brush paints into, so the sampled texel is exactly the one that would be painted.
        Vector3 bc = hit.barycentricCoordinate;
        Vector2 uv = _uvs[i0] * bc.x + _uvs[i1] * bc.y + _uvs[i2] * bc.z;

        int px = Mathf.Clamp(Mathf.FloorToInt(uv.x * textureSize), 0, textureSize - 1);
        int py = Mathf.Clamp(Mathf.FloorToInt(uv.y * textureSize), 0, textureSize - 1);
        int idx = py * textureSize + px;

        // ReadCanonical gives material layout for either backend:
        //   albedo   -> sRGB RGB
        //   MSO      -> R = metallic, G = occlusion, A = smoothness (linear data)
        //   emission -> sRGB RGB
        Color32[] alb = ReadCanonical(MAP_ALBEDO);
        Color32[] mso = ReadCanonical(MAP_MSO);
        Color32[] em = ReadCanonical(MAP_EM);
        if (idx < 0 || idx >= alb.Length || idx >= mso.Length || idx >= em.Length) return false;

        Color32 a = alb[idx];
        Color32 m = mso[idx];
        Color32 e = em[idx];

        // sRGB bytes map straight back into the gamma-space brush colours (round-trips exactly).
        brushColor = new Color(a.r / 255f, a.g / 255f, a.b / 255f, 1f);
        metallic = m.r / 255f;
        occlusion = m.g / 255f;
        smoothness = m.a / 255f;
        emissionColor = new Color(e.r / 255f, e.g / 255f, e.b / 255f, 1f);
        return true;
    }

    /// <summary>
    /// Hybrid foreign-object pick: the data channels (metallic/smoothness/occlusion/emission) come
    /// texel-exact from the material via <see cref="PickFromForeignMaterial"/> (raw, unlit), while
    /// albedo is overwritten with the rendered, fully-lit on-screen pixel ("what you see" - honours
    /// baked lightmaps/ambient/SSAO). When no texel-exact material read is possible (no readable UV /
    /// headless) it falls back to a pure screen pick (lit albedo, data channels at defaults).
    /// </summary>
    private bool PickFromForeignHybrid(RaycastHit hit, Vector2 screenPos)
    {
        if (PickFromForeignMaterial(hit))
        {
            // Keep the material's texel-exact PBR; replace only albedo with the lit on-screen colour.
            // No camera (headless) -> keep the material's raw albedo rather than failing the pick.
            if (TrySampleScreenColor(screenPos, out Color lit))
                brushColor = new Color(Mathf.Clamp01(lit.r), Mathf.Clamp01(lit.g), Mathf.Clamp01(lit.b), 1f);
            return true;
        }

        // No texel-exact material read possible (no renderer/UV, or non-readable mesh behind a convex
        // collider): fall back to the lit screen pixel alone (data channels reset to defaults).
        return PickFromScreen(screenPos);
    }

    /// <summary>
    /// Picks from a foreign object's material at the hit UV: texel-exact texture samples
    /// (where a UV at the hit can be resolved and the map exists) combined with the material
    /// scalars per the URP/Lit conventions, with scalar fallbacks everywhere else.
    /// </summary>
    private bool PickFromForeignMaterial(RaycastHit hit)
    {
        Collider col = hit.collider;
        if (!col) return false;
        Renderer rend = col.GetComponent<Renderer>();
        if (!rend) rend = col.GetComponentInParent<Renderer>();
        if (!rend) return false;
        Material mat = rend.sharedMaterial;
        if (!mat) return false;

        // A UV at the hit is required to sample any material map texel-exact. hit.textureCoord is
        // only populated for a NON-convex MeshCollider; convex MeshColliders and primitive colliders
        // (Box/Sphere/Capsule/Terrain) return Vector2.zero. In that case recover the UV from the
        // renderer's own mesh (ray vs. triangles), which works for any solid collider whose mesh is
        // readable.
        Vector2 uv;
        bool hasUv;
        if (col is MeshCollider mc && !mc.convex && hit.triangleIndex >= 0)
        {
            uv = hit.textureCoord;
            hasUv = true;
        }
        else
        {
            hasUv = TryRecoverMeshUv(rend, hit, out uv);
        }

        // A base map exists but no UV could be resolved (e.g. a non-readable mesh behind a convex
        // collider): a texel-exact read is impossible, so defer to the rendered on-screen pixel,
        // which at least reflects the texture - rather than silently picking the flat _BaseColor.
        if (!hasUv && GetTexProp(mat, "_BaseMap", "_MainTex")) return false;

        // --- Albedo: BaseMap (sRGB) * BaseColor ---
        Color baseCol = GetColorProp(mat, "_BaseColor", "_Color", Color.white);
        Texture baseTex = GetTexProp(mat, "_BaseMap", "_MainTex");
        Color albedo = baseCol;
        if (hasUv && baseTex)
            albedo = SampleTexel(baseTex, ApplyST(mat, "_BaseMap", "_MainTex", uv), true) * baseCol;
        brushColor = new Color(Mathf.Clamp01(albedo.r), Mathf.Clamp01(albedo.g), Mathf.Clamp01(albedo.b), 1f);

        // --- Metallic / Smoothness: MetallicGloss map (linear) ---
        // URP/Lit: metallic = map.r ; smoothness = map.a * _Smoothness (or _Metallic/_Glossiness scalars).
        float metallicVal = GetFloatProp(mat, "_Metallic", null, 0f);
        float smoothVal = GetFloatProp(mat, "_Smoothness", "_Glossiness", 0.5f);
        Texture mgTex = GetTexProp(mat, "_MetallicGlossMap", null);
        if (hasUv && mgTex)
        {
            Color mg = SampleTexel(mgTex, ApplyST(mat, "_MetallicGlossMap", null, uv), false);
            metallicVal = mg.r;
            smoothVal = mg.a * smoothVal;
        }
        metallic = Mathf.Clamp01(metallicVal);
        smoothness = Mathf.Clamp01(smoothVal);

        // --- Occlusion: OcclusionMap.g (linear), lerp(1, g, strength) ---
        float occStrength = GetFloatProp(mat, "_OcclusionStrength", null, 1f);
        float occ = 1f;
        Texture occTex = GetTexProp(mat, "_OcclusionMap", null);
        if (hasUv && occTex)
            occ = Mathf.Lerp(1f, SampleTexel(occTex, ApplyST(mat, "_OcclusionMap", null, uv), false).g, occStrength);
        occlusion = Mathf.Clamp01(occ);

        // --- Emission: EmissionMap (sRGB) * EmissionColor ---
        Color emColor = GetColorProp(mat, "_EmissionColor", null, Color.black);
        Texture emTex = GetTexProp(mat, "_EmissionMap", null);
        Color emCol = emColor;
        if (hasUv && emTex)
            emCol = SampleTexel(emTex, ApplyST(mat, "_EmissionMap", null, uv), true) * emColor;
        emissionColor = new Color(Mathf.Clamp01(emCol.r), Mathf.Clamp01(emCol.g), Mathf.Clamp01(emCol.b), 1f);

        return true;
    }

    /// <summary>
    /// Picks at a terrain hit. The TerrainCollider also carries the painted TREES (which sit above
    /// the heightfield and expose no per-instance renderer/UV here), and grass/mushroom DETAILS have
    /// no collider at all:
    ///   - hit above the surface  -> a tree: adopt the rendered on-screen colour;
    ///   - hit on a cell that has grass/mushroom detail -> adopt the on-screen colour;
    ///   - otherwise (bare ground) -> blend the terrain layers by their splat weights (full PBR).
    /// </summary>
    private bool PickFromTerrain(Terrain terrain, RaycastHit hit, Vector2 screenPos)
    {
        TerrainData td = terrain.terrainData;
        if (!td) return false;
        Vector3 tpos = terrain.transform.position;
        Vector3 size = td.size;
        if (size.x <= 1e-4f || size.z <= 1e-4f) return false;

        // Trees share the terrain collider but stand proud of the heightfield.
        float surfaceY = terrain.SampleHeight(hit.point) + tpos.y;
        if (hit.point.y > surfaceY + TerrainFoliageHeightEpsilon)
            return PickFromScreen(screenPos);

        Vector3 local = hit.point - tpos;
        float nx = Mathf.Clamp01(local.x / size.x);
        float nz = Mathf.Clamp01(local.z / size.z);

        // Grass/mushroom detail rendered on this ground cell -> what's visible there is the
        // foliage (which has no collider of its own), so trust the screen pixel.
        if (TerrainHasDetail(td, nx, nz))
            return PickFromScreen(screenPos);

        return PickFromTerrainLayers(td, local, nx, nz);
    }

    /// <summary>
    /// Blends the terrain's layers at a ground point by their splat (alphamap) weights: the weighted
    /// diffuse colour becomes albedo and the weighted per-layer metallic/smoothness scalars feed
    /// those channels; occlusion is 1 and emission black. Terrain has no Renderer/material, so this
    /// is the only way to read its surface.
    /// </summary>
    private bool PickFromTerrainLayers(TerrainData td, Vector3 local, float nx, float nz)
    {
        int aw = td.alphamapWidth, ah = td.alphamapHeight;
        int mapX = Mathf.Clamp(Mathf.FloorToInt(nx * aw), 0, aw - 1);
        int mapZ = Mathf.Clamp(Mathf.FloorToInt(nz * ah), 0, ah - 1);
        float[,,] weights = td.GetAlphamaps(mapX, mapZ, 1, 1);

        TerrainLayer[] layers = td.terrainLayers;
        int count = Mathf.Min(layers.Length, weights.GetLength(2));
        Color albedo = Color.black;
        float metallicVal = 0f, smoothVal = 0f, weightSum = 0f;
        for (int i = 0; i < count; i++)
        {
            TerrainLayer L = layers[i];
            if (L == null) continue;
            float w = weights[0, 0, i];
            if (w <= 0f) continue;

            Color col = Color.gray;
            if (L.diffuseTexture)
            {
                // Terrain tiles each layer across the world by tileSize (metres); sample that texel.
                Vector2 uv = new Vector2(
                    (local.x - L.tileOffset.x) / Mathf.Max(1e-4f, L.tileSize.x),
                    (local.z - L.tileOffset.y) / Mathf.Max(1e-4f, L.tileSize.y));
                col = SampleTexel(L.diffuseTexture, uv, true);
            }
            albedo += col * w;
            metallicVal += L.metallic * w;
            smoothVal += L.smoothness * w;
            weightSum += w;
        }
        if (weightSum <= 1e-4f) return false;
        float inv = 1f / weightSum; // alphamap weights normally sum to 1, but guard anyway

        brushColor = new Color(Mathf.Clamp01(albedo.r * inv), Mathf.Clamp01(albedo.g * inv), Mathf.Clamp01(albedo.b * inv), 1f);
        metallic = Mathf.Clamp01(metallicVal * inv);
        smoothness = Mathf.Clamp01(smoothVal * inv);
        occlusion = 1f;
        emissionColor = new Color(0f, 0f, 0f, 1f);
        return true;
    }

    /// <summary>
    /// True when any grass/mushroom detail layer has coverage at the given normalized terrain
    /// position - i.e. foliage (which carries no collider) is rendered on that ground cell.
    /// </summary>
    private static bool TerrainHasDetail(TerrainData td, float nx, float nz)
    {
        int dw = td.detailWidth, dh = td.detailHeight;
        if (dw <= 0 || dh <= 0) return false;
        int dx = Mathf.Clamp(Mathf.FloorToInt(nx * dw), 0, dw - 1);
        int dy = Mathf.Clamp(Mathf.FloorToInt(nz * dh), 0, dh - 1);
        int layerCount = td.detailPrototypes.Length;
        for (int i = 0; i < layerCount; i++)
        {
            int[,] d = td.GetDetailLayer(dx, dy, 1, 1, i);
            if (d[0, 0] > 0) return true;
        }
        return false;
    }

    /// <summary>
    /// Fallback colour pick: reads the rendered pixel under the cursor (the paint camera re-rendered
    /// to a temporary target, so it works on either render pipeline). Used for surfaces that can't be
    /// sampled texel-exact - terrain trees and grass/mushroom details (no per-instance renderer/UV)
    /// and anything hit without a readable material, plus rays that miss all geometry. Only albedo is
    /// meaningful from a screen pixel; the data channels default to matte/unoccluded/no-emission.
    /// </summary>
    private bool PickFromScreen(Vector2 screenPos)
    {
        if (!TrySampleScreenColor(screenPos, out Color c)) return false;

        brushColor = new Color(Mathf.Clamp01(c.r), Mathf.Clamp01(c.g), Mathf.Clamp01(c.b), 1f);
        metallic = 0f;
        smoothness = 0f;
        occlusion = 1f;
        emissionColor = new Color(0f, 0f, 0f, 1f);
        return true;
    }

    /// <summary>
    /// Reads the rendered pixel under the cursor (the paint camera re-rendered to a temporary target,
    /// so it works on either render pipeline) without touching any brush channel. Returns the fully-lit
    /// on-screen colour - what is visible, including baked lightmaps/ambient/SSAO. False when no camera
    /// is bound (headless). Shared by <see cref="PickFromScreen"/> and the hybrid albedo override.
    /// </summary>
    private bool TrySampleScreenColor(Vector2 screenPos, out Color color)
    {
        color = default;
        Camera cam = paintCamera;
        if (!cam) return false;
        int w = Mathf.Max(1, cam.pixelWidth), h = Mathf.Max(1, cam.pixelHeight);

        RenderTexture rt = RenderTexture.GetTemporary(w, h, 24, RenderTextureFormat.ARGB32, RenderTextureReadWrite.sRGB);
        RenderTexture prevTarget = cam.targetTexture;
        RenderTexture prevActive = RenderTexture.active;
        cam.targetTexture = rt;
        cam.Render();
        RenderTexture.active = rt;

        var tmp = new Texture2D(1, 1, TextureFormat.RGBA32, false, false);
        int px = Mathf.Clamp(Mathf.RoundToInt(screenPos.x), 0, w - 1);
        int py = Mathf.Clamp(Mathf.RoundToInt(screenPos.y), 0, h - 1);
        tmp.ReadPixels(new Rect(px, py, 1, 1), 0, 0, false);
        tmp.Apply(false);
        color = tmp.GetPixel(0, 0);

        cam.targetTexture = prevTarget;
        RenderTexture.active = prevActive;
        RenderTexture.ReleaseTemporary(rt);
        if (Application.isPlaying) Destroy(tmp); else DestroyImmediate(tmp);
        return true;
    }

    // --- Material read helpers (name-tolerant: URP and legacy Standard) ---

    private static Color GetColorProp(Material m, string nameA, string nameB, Color fallback)
    {
        if (m.HasProperty(nameA)) return m.GetColor(nameA);
        if (nameB != null && m.HasProperty(nameB)) return m.GetColor(nameB);
        return fallback;
    }

    private static float GetFloatProp(Material m, string nameA, string nameB, float fallback)
    {
        if (m.HasProperty(nameA)) return m.GetFloat(nameA);
        if (nameB != null && m.HasProperty(nameB)) return m.GetFloat(nameB);
        return fallback;
    }

    private static Texture GetTexProp(Material m, string nameA, string nameB)
    {
        if (m.HasProperty(nameA)) { Texture t = m.GetTexture(nameA); if (t) return t; }
        if (nameB != null && m.HasProperty(nameB)) { Texture t = m.GetTexture(nameB); if (t) return t; }
        return null;
    }

    private static Vector2 ApplyST(Material m, string nameA, string nameB, Vector2 uv)
    {
        string prop = m.HasProperty(nameA) ? nameA : (nameB != null && m.HasProperty(nameB) ? nameB : null);
        if (prop == null) return uv;
        Vector2 s = m.GetTextureScale(prop);
        Vector2 o = m.GetTextureOffset(prop);
        return new Vector2(uv.x * s.x + o.x, uv.y * s.y + o.y);
    }

    /// <summary>
    /// Samples a single texel from a texture at a UV (texel-exact). Uses GetPixelBilinear when the
    /// texture is CPU-readable; otherwise blits it to a temporary RenderTexture and reads back the
    /// one texel, so it works regardless of the texture's "Read/Write" import flag. <paramref
    /// name="srgb"/> selects the colour space so the returned value matches the readable path
    /// (gamma for colour maps, linear for data maps).
    /// </summary>
    private static Color SampleTexel(Texture tex, Vector2 uv, bool srgb)
    {
        Texture2D t2d = tex as Texture2D;
        if (t2d != null && t2d.isReadable)
            return t2d.GetPixelBilinear(uv.x, uv.y);

        int w = Mathf.Max(1, tex.width), h = Mathf.Max(1, tex.height);
        var rw = srgb ? RenderTextureReadWrite.sRGB : RenderTextureReadWrite.Linear;
        RenderTexture rt = RenderTexture.GetTemporary(w, h, 0, RenderTextureFormat.ARGB32, rw);
        RenderTexture prev = RenderTexture.active;
        Graphics.Blit(tex, rt);
        RenderTexture.active = rt;

        var tmp = new Texture2D(1, 1, TextureFormat.RGBA32, false, !srgb);
        int px = Mathf.Clamp(Mathf.FloorToInt(Mathf.Repeat(uv.x, 1f) * w), 0, w - 1);
        int py = Mathf.Clamp(Mathf.FloorToInt(Mathf.Repeat(uv.y, 1f) * h), 0, h - 1);
        tmp.ReadPixels(new Rect(px, py, 1, 1), 0, 0, false);
        tmp.Apply(false);
        Color c = tmp.GetPixel(0, 0);

        RenderTexture.active = prev;
        RenderTexture.ReleaseTemporary(rt);
        if (Application.isPlaying) Destroy(tmp); else DestroyImmediate(tmp);
        return c;
    }

    /// <summary>
    /// Recovers the UV at a hit point by intersecting the camera ray with the renderer's own mesh
    /// (Moller-Trumbore against each triangle, nearest hit wins). Needed when the collider that was
    /// hit cannot supply hit.textureCoord - a convex MeshCollider or any primitive collider. Requires
    /// a MeshFilter whose mesh is readable and UV-mapped; returns false otherwise so the caller can
    /// fall back. A one-shot click cost, so the O(triangles) sweep is acceptable.
    /// </summary>
    private bool TryRecoverMeshUv(Renderer rend, RaycastHit hit, out Vector2 uv)
    {
        uv = default;
        if (!rend) return false;
        MeshFilter mf = rend.GetComponent<MeshFilter>();
        Mesh mesh = mf ? mf.sharedMesh : null;
        if (!mesh || !mesh.isReadable) return false;

        Vector2[] uvs = mesh.uv;
        if (uvs == null || uvs.Length == 0) return false;
        Vector3[] verts = mesh.vertices;
        int[] tris = mesh.triangles;

        // Ray from the paint camera through the surface point (its UV is what we want). Fall back to
        // shooting along the inverted hit normal when no camera is bound.
        Vector3 ro, rd;
        if (paintCamera)
        {
            ro = paintCamera.transform.position;
            rd = (hit.point - ro).normalized;
        }
        else
        {
            rd = -hit.normal;
            ro = hit.point - rd * 0.01f;
        }

        Matrix4x4 l2w = rend.transform.localToWorldMatrix;
        float bestT = float.MaxValue;
        Vector3 bestBary = default;
        int bestTri = -1;
        for (int i = 0; i + 2 < tris.Length; i += 3)
        {
            Vector3 a = l2w.MultiplyPoint3x4(verts[tris[i]]);
            Vector3 b = l2w.MultiplyPoint3x4(verts[tris[i + 1]]);
            Vector3 c = l2w.MultiplyPoint3x4(verts[tris[i + 2]]);
            if (RayTriangle(ro, rd, a, b, c, out float t, out float u, out float v) && t < bestT)
            {
                bestT = t;
                bestBary = new Vector3(1f - u - v, u, v);
                bestTri = i;
            }
        }
        if (bestTri < 0) return false;

        uv = uvs[tris[bestTri]] * bestBary.x + uvs[tris[bestTri + 1]] * bestBary.y + uvs[tris[bestTri + 2]] * bestBary.z;
        return true;
    }

    /// <summary>
    /// Moller-Trumbore ray/triangle intersection. On a hit, <paramref name="t"/> is the ray distance
    /// and (<paramref name="u"/>, <paramref name="v"/>) are the barycentric weights of vertices b and
    /// c (vertex a's weight is 1 - u - v). Small epsilons admit edge hits.
    /// </summary>
    private static bool RayTriangle(Vector3 ro, Vector3 rd, Vector3 a, Vector3 b, Vector3 c,
        out float t, out float u, out float v)
    {
        t = u = v = 0f;
        const float eps = 1e-7f;
        Vector3 e1 = b - a, e2 = c - a;
        Vector3 p = Vector3.Cross(rd, e2);
        float det = Vector3.Dot(e1, p);
        if (det > -eps && det < eps) return false; // ray parallel to the triangle
        float inv = 1f / det;
        Vector3 tvec = ro - a;
        u = Vector3.Dot(tvec, p) * inv;
        if (u < -1e-4f || u > 1f + 1e-4f) return false;
        Vector3 q = Vector3.Cross(tvec, e1);
        v = Vector3.Dot(rd, q) * inv;
        if (v < -1e-4f || u + v > 1f + 1e-4f) return false;
        t = Vector3.Dot(e2, q) * inv;
        return t > eps;
    }
}