chameleon/Assets/Scripts/MeshPainter.cs

using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
#if ENABLE_INPUT_SYSTEM
using UnityEngine.InputSystem;
#endif
using Random = UnityEngine.Random; // System (for Action) also defines Random -> keep the UnityEngine one

/// <summary>
/// Live mesh painting in Play mode: paints the brush onto the mesh textures with the
/// mouse. Three paint maps are driven at once (each can be toggled): albedo color
/// (_BaseMap), an HDRP-style mask map (R = metallic, G = occlusion, A = smoothness; one
/// texture bound to BOTH _MetallicGlossMap and _OcclusionMap) and emission (_EmissionMap).
/// Metallic/smoothness and occlusion share that mask map but stay independently paintable.
/// Two backends (switchable live via <see cref="useGpu"/>):
/// GPU shader painting into RenderTextures (fast, default) and CPU rasterization
/// (slower, identical result). Both paint seamlessly across UV seams because the brush
/// is evaluated in 3D local space around the hit point, capturing triangles on both
/// sides of a seam at once.
///
/// The brush is a square stamp in the surface tangent plane whose shape is given by a
/// grayscale mask texture (the mask value = per-texel opacity). A list of masks is
/// assignable in the Inspector; if it is empty a procedural circle mask is used, which
/// reproduces the classic round brush. Attach to the GameObject you want to paint.
///
/// A non-destructive brush preview (size + mask) is shown at the hover point without
/// painting (<see cref="previewMode"/>): either a flat decal quad or a real on-surface
/// overlay that wraps the mesh exactly like the paint would land.
/// </summary>
[DisallowMultipleComponent]
public partial class MeshPainter : MonoBehaviour
{
    /// <summary>How the square stamp is oriented around the surface normal.</summary>
    public enum BrushAlignment
    {
        /// <summary>Stamp "up"/"right" follow the camera (projected onto the surface) - natural decal feel.</summary>
        Camera,
        /// <summary>Stamp "up" = world up (projected onto the surface) - stable, does not rotate with the camera.</summary>
        World
    }

    /// <summary>How the brush preview at the hover point is rendered.</summary>
    public enum BrushPreviewMode
    {
        /// <summary>No preview.</summary>
        Off,
        /// <summary>A flat quad in the tangent plane (cheap, does not follow curvature).</summary>
        FlatDecal,
        /// <summary>Real on-surface overlay: wraps the mesh exactly like the paint would land.</summary>
        OnSurface
    }

    /// <summary>When false, mouse painting is ignored. Driven by the GameModeManager (true only in
    /// PaintMode on the local owner). Defaults OFF so a painter that is not (yet) managed by a mode -
    /// the pre-spawn window, a remote replica, or any binding gap - never paints from local input.</summary>
    public bool PaintingEnabled { get; set; } = false;

    [Header("Backend")]
    [Tooltip("ON = GPU painting (fast), OFF = CPU painting. Can be toggled live in Play mode.")]
    public bool useGpu = true;

    [Header("Brush - channels")]
    [Tooltip("Paint the albedo (base color) map.")]
    public bool paintAlbedo = true;

    [Tooltip("Paint metallic + smoothness into the mask map (R = metallic, A = smoothness). " +
             "Shares one texture with occlusion but is toggled independently.")]
    public bool paintMetallicSmoothness = false;

    [Tooltip("Paint ambient occlusion into the mask map (G channel, also bound to _OcclusionMap). " +
             "Shares one texture with metallic/smoothness but is toggled independently.")]
    public bool paintOcclusion = false;

    [Tooltip("Paint the emission map (_EmissionMap).")]
    public bool paintEmission = false;

    [Header("Brush - albedo")]
    [Tooltip("Color used for painting. The alpha channel is the global opacity multiplier on " +
             "top of the mask (alpha 1 = mask opacity as-is, lower = fainter).")]
    public Color brushColor = Color.red;

    [Header("Brush - metallic / smoothness / occlusion")]
    [Tooltip("Metallic value painted into the mask map (0 = dielectric, 1 = metal).")]
    [Range(0f, 1f)]
    public float metallic = 0f;

    [Tooltip("Smoothness value painted into the mask map (0 = rough, 1 = glossy).")]
    [Range(0f, 1f)]
    public float smoothness = 0.5f;

    [Tooltip("Paint opacity for metallic + smoothness on top of the mask (1 = mask opacity " +
             "as-is, lower = blends the metallic/smoothness target in more faintly, 0 = no-op). " +
             "The albedo equivalent of brushColor.a, but for the metallic/smoothness channels.")]
    [Range(0f, 1f)]
    public float metallicSmoothnessOpacity = 1f;

    [Tooltip("Ambient occlusion painted into the mask map (1 = fully lit / no darkening, " +
             "0 = fully occluded / dark). NOTE: URP only darkens INDIRECT light (ambient + " +
             "reflections), so the effect is subtle under strong direct lighting. 1 = no-op.")]
    [Range(0f, 1f)]
    public float occlusion = 0.5f;

    [Tooltip("Paint opacity for occlusion on top of the mask (1 = mask opacity as-is, lower = " +
             "blends the occlusion target in more faintly, 0 = no-op). The albedo equivalent of " +
             "brushColor.a, but for the occlusion channel.")]
    [Range(0f, 1f)]
    public float occlusionOpacity = 1f;

    [Header("Brush - emission")]
    [Tooltip("Emission color painted into the map. Alpha = global opacity multiplier on top of the mask.")]
    public Color emissionColor = Color.black;

    [Tooltip("Global emission intensity multiplier on the painted emission (drives the material's _EmissionColor). " +
             "Above 1 makes the emission HDR/bloom-bright.")]
    public float emissionIntensity = 1f;

    [Header("Brush - shape")]
    [Tooltip("Brush half-extent in texels (= half the square side). Converted into a 3D size per hit.")]
    [Range(1, 256)]
    public int brushSize = 16;

    [Tooltip("Texels added/removed per mouse-wheel notch while Shift is held (paint mode brush resize).")]
    public int brushSizeScrollStep = 4;

    /// <summary>
    /// Raised after Shift+wheel changes <see cref="brushSize"/> so the in-game HUD can re-sync its
    /// brush-size slider (the slider then re-clamps the value to its own configured range).
    /// </summary>
    public event Action BrushSizeChanged;

    [Tooltip("Brush masks: the mask value (luminance x alpha) is the per-texel opacity " +
             "(gray = 50%). Empty = built-in circle (= classic round brush). Import grayscale " +
             "masks as Non-Color so midtones map 1:1; alpha-based masks (e.g. soft particles) work too.")]
    public List<Texture2D> brushMasks = new List<Texture2D>();

    [Tooltip("Index of the active mask in the list above. Out of range = built-in circle.")]
    public int activeBrushIndex = 0;

    [Tooltip("How the square stamp is oriented around the surface normal (only matters for non-symmetric masks).")]
    public BrushAlignment brushAlignment = BrushAlignment.Camera;

    [Tooltip("Extra rotation of the stamp around the surface normal, in degrees.")]
    [Range(-180f, 180f)]
    public float brushRotation = 0f;

    [Tooltip("ON = paint once on mouse-down, then only while the mouse moves (a held but " +
             "stationary button no longer keeps re-stamping the same spot). OFF = continuous painting while held.")]
    public bool paintOnMoveOnly = false;

    [Tooltip("Seconds between paint stamps while the button is held. 0 = paint every frame as " +
             "an interpolated, continuous stroke (as before). Above 0 = deposit one stamp at the " +
             "cursor every this many seconds (timed dabs, no movement interpolation - pairs well " +
             "with the spray can).")]
    [Min(0f)]
    public float paintInterval = 0f;

    [Header("Preview")]
    [Tooltip("Brush preview at the hover point (shows size + mask without painting). " +
             "Flat Decal = a flat quad in the tangent plane (cheap). On Surface = a real " +
             "overlay that wraps the mesh exactly like the paint would land. Off = no preview.")]
    public BrushPreviewMode previewMode = BrushPreviewMode.OnSurface;

    [Tooltip("Opacity of the preview overlay (the mask still modulates it per texel). " +
             "Can exceed 1 to make the preview show up more strongly.")]
    [Range(0f, 4f)]
    public float previewOpacity = 0.5f;

    [Header("Canvas")]
    [Tooltip("Resolution of the paint textures (square).")]
    public int textureSize = 1024;

    [Tooltip("Base color of the albedo texture at startup (if none exists).")]
    public Color baseColor = Color.white;

    [Tooltip("Edge padding (texels) bled beyond UV island edges so bilinear sampling does not pull in the base color. 0 = off.")]
    [Range(0, 16)]
    public int paddingTexels = 4;

    [Header("Setup")]
    [Tooltip("Camera for the mouse ray. Empty = Camera.main.")]
    public Camera paintCamera;

    [Tooltip("Only for animated meshes: re-bake the collider every frame (expensive!).")]
    public bool updateColliderEveryFrame = false;

    // --- paint maps ---
    private const int MAP_ALBEDO = 0;
    // Mask map: R = metallic, G = occlusion, A = smoothness. One texture bound to BOTH
    // _MetallicGlossMap (reads R/A) and _OcclusionMap (reads G).
    private const int MAP_MSO = 1;
    private const int MAP_EM = 2;        // emission (RGB)
    private const int MAP_COUNT = 3;

    /// <summary>CPU backend per map: a Texture2D mirror + rasterization/dilation state.</summary>
    private class CpuMap
    {
        public Texture2D tex;
        public Color32[] pixels;
        public bool[] painted;          // true = painted by the brush (dilation source)
        public int[] fillStamp;         // per-call marker for padding texels
        public int dilateStamp;         // bumped per call so old padding is not a source
        public int dx0, dy0, dx1, dy1;  // dirty rectangle (CPU upload)
    }

    // --- shared references ---
    private Renderer _renderer;
    private SkinnedMeshRenderer _skinned;
    private MeshCollider _collider;
    private Mesh _bakedMesh;
    // When present, this always-on baker owns the collider bake (also outside paint mode); we
    // stop baking and only borrow its mesh. Null = no baker -> we keep our own legacy bake.
    private SkinnedColliderBaker _baker;
    private Material _materialInstance;

    // Mesh caches (in local/collider space).
    private Vector3[] _verts;
    private int[] _tris;
    private Vector2[] _uvs;
    private Vector3[] _triCenter;   // centroid per triangle (CPU culling)
    private float[] _triRadius;     // circumscribed radius per triangle (CPU culling)
    private float _avgUvToLocal = 1f; // average UV unit -> local unit
    private bool[] _islandMask;       // texel lies inside a UV island: dilation source / overwrite guard (built once)

    private bool _hasLastScreen;
    private Vector2 _lastScreen;
    private float _intervalTimer;     // accumulates while held; gates timed-dab stamping (paintInterval > 0)
    private bool _gpuActive;
    private bool _setupValid;

    // Seed colors per map in canonical (material) layout, built once at Start:
    //   albedo   -> RGB color
    //   MSO      -> R = metallic, G = occlusion (1 = unoccluded), A = smoothness (mask map)
    //   emission -> RGB emission
    private Color32[][] _seedCanonical;

    // --- backends ---
    private CpuMap[] _cpu;
    private readonly List<int> _dilateScratch = new List<int>(512);
    private readonly RaycastHit[] _rayHits = new RaycastHit[32];

    // --- preview overlay ---
    public Shader previewShader;             // optional; else Shader.Find("Hidden/MeshPaintPreview")
    private Material _previewMat;
    private Mesh _previewQuad;               // unit quad in the tangent plane (flat decal mode)

    // --- brush mask (shared) ---
    private Texture2D _proceduralMask;       // built-in circle, used when the list is empty/invalid
    private float[] _proceduralOpacity;      // cached opacity (luma*alpha) of the procedural mask (CPU)
    private int _procW, _procH;
    private Texture2D _resolvedMask;          // currently active mask (user or procedural)
    private float[] _maskOpacity;             // opacity (luma*alpha) of the resolved mask (CPU sampling)
    private int _maskW, _maskH;

    private const float ScreenStepPixels = 5f;   // interpolation step along mouse movement
    private const int MaxStepsPerFrame = 256;     // raycast safeguard for fast movement
    private const float MoveOnlyMinPixels = 0.75f; // move-only mode: min mouse travel before re-stamping
    private const float SquareReach = 1.7321f;    // sqrt(3): max 3D distance from center to a painted box texel (CPU culling)

    private static readonly int BaseMapId = Shader.PropertyToID("_BaseMap");
    private static readonly int MainTexId = Shader.PropertyToID("_MainTex");
    private static readonly int BaseColorId = Shader.PropertyToID("_BaseColor");
    private static readonly int ColorId = Shader.PropertyToID("_Color");
    private static readonly int MetallicGlossMapId = Shader.PropertyToID("_MetallicGlossMap");
    private static readonly int MetallicId = Shader.PropertyToID("_Metallic");
    private static readonly int SmoothnessId = Shader.PropertyToID("_Smoothness");
    private static readonly int EmissionMapId = Shader.PropertyToID("_EmissionMap");
    private static readonly int EmissionColorId = Shader.PropertyToID("_EmissionColor");
    private static readonly int OcclusionMapId = Shader.PropertyToID("_OcclusionMap");
    private static readonly int OcclusionStrengthId = Shader.PropertyToID("_OcclusionStrength");
    private static readonly int BrushColorId = Shader.PropertyToID("_BrushColor");
    private static readonly int ColorWriteMaskId = Shader.PropertyToID("_ColorWriteMask");
    private static readonly int BrushCenterId = Shader.PropertyToID("_BrushCenter");
    private static readonly int BrushRadiusId = Shader.PropertyToID("_BrushRadius");
    private static readonly int BrushUId = Shader.PropertyToID("_BrushU");
    private static readonly int BrushVId = Shader.PropertyToID("_BrushV");
    private static readonly int BrushNormalId = Shader.PropertyToID("_BrushNormal");
    private static readonly int MaskTexId = Shader.PropertyToID("_MaskTex");
    private static readonly int PadId = Shader.PropertyToID("_Pad");
    private static readonly int PreviewOpacityId = Shader.PropertyToID("_PreviewOpacity");
    private static readonly int PreviewModeId = Shader.PropertyToID("_PreviewMode");

    private void Start()
    {
        if (!paintCamera) paintCamera = Camera.main;

        _renderer = GetComponent<Renderer>();
        _skinned = GetComponent<SkinnedMeshRenderer>();
        if (!_renderer)
        {
            Debug.LogError("[MeshPainter] No Renderer found on this GameObject.", this);
            enabled = false;
            return;
        }

        textureSize = Mathf.Clamp(textureSize, 32, 4096);

        SetupCollider();
        BuildMeshCaches();

        if (_uvs == null || _uvs.Length == 0)
        {
            Debug.LogError("[MeshPainter] Mesh has no UVs - painting not possible.", this);
            enabled = false;
            return;
        }

        // Own material instance so the shared asset is not modified; tint base color to
        // white so the painted color appears unaltered.
        _materialInstance = _renderer.material;
        if (_materialInstance.HasProperty(BaseColorId)) _materialInstance.SetColor(BaseColorId, Color.white);
        else if (_materialInstance.HasProperty(ColorId)) _materialInstance.SetColor(ColorId, Color.white);

        BuildSeedColors();
        // Always start fresh from the material seed. Saved paintings are loaded explicitly from the
        // HUD library (MeshPainter.Persistence), never auto-loaded on start.
        SetupBackend(useGpu, _seedCanonical);
        ConfigureMaterialPbr();
        InitUndoHistory(); // baseline = material seed, captured before any stroke

        EnsureProceduralMask();
        ResolveActiveMask();
        SetupPreview();

        _setupValid = true;

        Debug.Log($"[MeshPainter] Ready ({(_gpuActive ? "GPU" : "CPU")} painting, seamless). " +
                  "Hold the left mouse button and drag over the mesh.", this);
    }

    // --- Brush mask ---

    /// <summary>Builds the built-in circle mask (white inscribed circle on black), used when no mask is assigned.</summary>
    private void EnsureProceduralMask()
    {
        if (_proceduralMask) return;

        const int s = 256;
        _proceduralMask = new Texture2D(s, s, TextureFormat.RGBA32, false, true) // linear: raw mask values
        {
            name = "MeshPainter_CircleMask",
            wrapMode = TextureWrapMode.Clamp,
            filterMode = FilterMode.Bilinear,
            hideFlags = HideFlags.HideAndDontSave
        };

        var px = new Color32[s * s];
        _proceduralOpacity = new float[s * s];
        float half = (s - 1) * 0.5f;
        for (int y = 0; y < s; y++)
        {
            for (int x = 0; x < s; x++)
            {
                float nx = (x - half) / half;
                float ny = (y - half) / half;
                bool inside = nx * nx + ny * ny <= 1f;
                byte v = inside ? (byte)255 : (byte)0;
                int idx = y * s + x;
                px[idx] = new Color32(v, v, v, 255);
                _proceduralOpacity[idx] = inside ? 1f : 0f;
            }
        }
        _proceduralMask.SetPixels32(px);
        _proceduralMask.Apply(false);
        _procW = s;
        _procH = s;
    }

    /// <summary>Picks the active mask (list entry or procedural circle) and refreshes the CPU opacity cache on change.</summary>
    private void ResolveActiveMask()
    {
        Texture2D desired = null;
        if (brushMasks != null && activeBrushIndex >= 0 && activeBrushIndex < brushMasks.Count)
            desired = brushMasks[activeBrushIndex];
        if (!desired) desired = _proceduralMask;

        if (desired == _resolvedMask) return;
        _resolvedMask = desired;

        if (_resolvedMask == _proceduralMask)
        {
            _maskOpacity = _proceduralOpacity;
            _maskW = _procW;
            _maskH = _procH;
            return;
        }

        // User mask: cache opacity (luma*alpha) for the CPU backend so it matches the GPU.
        // Non-readable textures are read back via the GPU (so even built-ins like
        // Default-Particle work on the CPU path, not just the GPU one).
        var px = ReadMaskPixels(_resolvedMask, out int w, out int h);
        _maskW = w;
        _maskH = h;
        _maskOpacity = new float[px.Length];
        for (int i = 0; i < px.Length; i++)
        {
            Color32 p = px[i];
            float luma = (0.299f * p.r + 0.587f * p.g + 0.114f * p.b) / 255f;
            _maskOpacity[i] = luma * (p.a / 255f);
        }
    }

    /// <summary>
    /// Reads a mask texture's pixels. Readable textures are read directly (correct
    /// orientation, matches GPU tex2D); non-readable ones are blitted to a temp RT and
    /// read back, so any texture (incl. built-ins without Read/Write) works on the CPU.
    /// </summary>
    private static Color32[] ReadMaskPixels(Texture2D tex, out int w, out int h)
    {
        w = tex.width;
        h = tex.height;
        if (tex.isReadable) return tex.GetPixels32();

        var tmp = RenderTexture.GetTemporary(w, h, 0, RenderTextureFormat.ARGB32, RenderTextureReadWrite.Linear);
        var prev = RenderTexture.active;
        Graphics.Blit(tex, tmp);
        RenderTexture.active = tmp;
        var readTex = new Texture2D(w, h, TextureFormat.RGBA32, false, true);
        readTex.ReadPixels(new Rect(0, 0, w, h), 0, 0, false);
        readTex.Apply(false);
        RenderTexture.active = prev;
        RenderTexture.ReleaseTemporary(tmp);
        var px = readTex.GetPixels32();
        Destroy(readTex);
        return px;
    }

    /// <summary>Bilinear sample of the active stamp mask opacity at uv in [0,1]. Returns 1 (full) if no mask.</summary>
    private float SampleMaskOpacity(float u, float v)
    {
        if (_stampOpacity == null || _stampW <= 0 || _stampH <= 0) return 1f;

        float fx = Mathf.Clamp01(u) * (_stampW - 1);
        float fy = Mathf.Clamp01(v) * (_stampH - 1);
        int x0 = Mathf.FloorToInt(fx);
        int y0 = Mathf.FloorToInt(fy);
        int x1 = Mathf.Min(x0 + 1, _stampW - 1);
        int y1 = Mathf.Min(y0 + 1, _stampH - 1);
        float tx = fx - x0;
        float ty = fy - y0;

        float l00 = _stampOpacity[y0 * _stampW + x0];
        float l10 = _stampOpacity[y0 * _stampW + x1];
        float l01 = _stampOpacity[y1 * _stampW + x0];
        float l11 = _stampOpacity[y1 * _stampW + x1];

        float a = Mathf.Lerp(l00, l10, tx);
        float b = Mathf.Lerp(l01, l11, tx);
        return Mathf.Lerp(a, b, ty);
    }

    /// <summary>
    /// Builds an orthonormal tangent frame (u = right, v = up) for the stamp at a hit
    /// with the given local-space surface normal, honoring the alignment mode and the
    /// manual rotation. u/v span the tangent plane; the mask is sampled along them.
    /// </summary>
    private void ComputeBrushFrame(Vector3 nLocal, out Vector3 u, out Vector3 v)
    {
        nLocal = nLocal.normalized;

        if (brushAlignment == BrushAlignment.Camera && paintCamera)
        {
            // Stamp horizontal follows the camera: u = camera right projected onto the surface.
            Vector3 camRight = transform.InverseTransformDirection(paintCamera.transform.right);
            u = ProjectOntoPlane(camRight, nLocal);
            if (u.sqrMagnitude < 1e-8f)
            {
                Vector3 camUp = transform.InverseTransformDirection(paintCamera.transform.up);
                u = ProjectOntoPlane(camUp, nLocal);
                if (u.sqrMagnitude < 1e-8f) u = AnyTangent(nLocal);
            }
            u.Normalize();
            // up = right x n, so the frame reads correctly when viewed from the front
            // (normal toward the camera): u x v = -n. Using n x u would flip v -> upside down.
            v = Vector3.Cross(u, nLocal).normalized;
        }
        else
        {
            // Stamp vertical = world up projected onto the surface.
            Vector3 worldUp = transform.InverseTransformDirection(Vector3.up);
            v = ProjectOntoPlane(worldUp, nLocal);
            if (v.sqrMagnitude < 1e-8f) v = AnyTangent(nLocal);
            v.Normalize();
            // right = n x up (front-readable: u x v = -n). Using up x n would mirror u.
            u = Vector3.Cross(nLocal, v).normalized;
        }

        if (Mathf.Abs(brushRotation) > 1e-4f)
        {
            Quaternion q = Quaternion.AngleAxis(brushRotation, nLocal);
            u = q * u;
            v = q * v;
        }
    }

    private static Vector3 ProjectOntoPlane(Vector3 dir, Vector3 normal)
        => dir - normal * Vector3.Dot(dir, normal);

    private static Vector3 AnyTangent(Vector3 n)
    {
        Vector3 t = Vector3.Cross(n, Vector3.right);
        if (t.sqrMagnitude < 1e-6f) t = Vector3.Cross(n, Vector3.up);
        return t.normalized;
    }

    // --- Brush preview ---

    /// <summary>Creates the preview material and the unit quad used by the flat-decal mode.</summary>
    private void SetupPreview()
    {
        if (!previewShader) previewShader = Shader.Find("Hidden/MeshPaintPreview");
        if (!previewShader)
        {
            Debug.LogWarning("[MeshPainter] Preview shader (Hidden/MeshPaintPreview) not found - brush preview disabled.", this);
            return;
        }
        _previewMat = new Material(previewShader) { hideFlags = HideFlags.HideAndDontSave };
        _previewQuad = BuildPreviewQuad();
    }

    /// <summary>Unit quad in its XY plane ([-1,1], normal +Z, uv 0..1) - oriented per hit in the flat-decal mode.</summary>
    private static Mesh BuildPreviewQuad()
    {
        var q = new Mesh { name = "MeshPainter_PreviewQuad", hideFlags = HideFlags.HideAndDontSave };
        q.vertices = new[]
        {
            new Vector3(-1f, -1f, 0f), new Vector3(1f, -1f, 0f),
            new Vector3(1f, 1f, 0f), new Vector3(-1f, 1f, 0f)
        };
        q.uv = new[]
        {
            new Vector2(0f, 0f), new Vector2(1f, 0f),
            new Vector2(1f, 1f), new Vector2(0f, 1f)
        };
        q.triangles = new[] { 0, 1, 2, 0, 2, 3 };
        q.RecalculateBounds();
        return q;
    }

    /// <summary>
    /// Draws the brush preview at the hover point for this frame (no painting). On-surface
    /// mode re-draws the baked mesh and clips it to the brush box in the shader; flat-decal
    /// mode draws the oriented quad. Both sample the active mask (size + shape preview).
    /// </summary>
    private void UpdatePreview(Vector2 screenPos)
    {
        if (!TryComputeBrushPlacement(screenPos, out Vector3 pLocal, out float radiusLocal,
                                      out Vector3 u, out Vector3 v, out Vector3 n))
            return;

        _previewMat.SetColor(BrushColorId, brushColor);
        // Spray paints scattered dots, but the preview shows the round footprint (the area
        // the dots land in), so the hover ring stays a stable, readable size indicator.
        _previewMat.SetTexture(MaskTexId, spray ? _proceduralMask : _resolvedMask);
        _previewMat.SetFloat(PreviewOpacityId, Mathf.Max(0f, previewOpacity) * Mathf.Clamp01(brushColor.a));

        int layer = gameObject.layer;

        if (previewMode == BrushPreviewMode.OnSurface)
        {
            _previewMat.SetFloat(PreviewModeId, 0f);
            _previewMat.SetVector(BrushCenterId, pLocal);
            _previewMat.SetFloat(BrushRadiusId, radiusLocal);
            _previewMat.SetVector(BrushUId, u);
            _previewMat.SetVector(BrushVId, v);
            _previewMat.SetVector(BrushNormalId, n);

            // Draw the collider mesh again (object transform): the shader clips it to the
            // brush box, so only the footprint shows, wrapped onto the real surface. Using
            // the collider mesh keeps it in the same local space as the raycast hit and
            // valid for both skinned (baked) and static meshes.
            Mesh previewMesh = _collider ? _collider.sharedMesh : null;
            if (!previewMesh) return;
            Matrix4x4 m = transform.localToWorldMatrix;
            int sub = previewMesh.subMeshCount;
            for (int i = 0; i < sub; i++)
                Graphics.DrawMesh(previewMesh, m, _previewMat, layer, paintCamera, i);
        }
        else // FlatDecal
        {
            _previewMat.SetFloat(PreviewModeId, 1f);

            // Quad frame in local space: x -> u*r, y -> v*r, lifted slightly along n so
            // it floats on the surface; uv 0..1 matches the on-surface mask mapping.
            Vector3 pos = pLocal + n * (radiusLocal * 0.02f);
            var frame = new Matrix4x4();
            frame.SetColumn(0, new Vector4(u.x * radiusLocal, u.y * radiusLocal, u.z * radiusLocal, 0f));
            frame.SetColumn(1, new Vector4(v.x * radiusLocal, v.y * radiusLocal, v.z * radiusLocal, 0f));
            frame.SetColumn(2, new Vector4(n.x, n.y, n.z, 0f));
            frame.SetColumn(3, new Vector4(pos.x, pos.y, pos.z, 1f));

            Graphics.DrawMesh(_previewQuad, transform.localToWorldMatrix * frame, _previewMat, layer, paintCamera, 0);
        }
    }

    // --- Update / input ---

    private void Update()
    {
        if (!_setupValid) return;

        if (useGpu != _gpuActive) SwitchBackend(useGpu);

        RefreshEmissionColor(); // live-tunable emission intensity

        // Painting disabled by the game mode (e.g. player-control mode): do nothing.
        if (!PaintingEnabled)
        {
            EndStrokeIfActive(); // finalize an in-progress stroke if the mode was left mid-paint
            _hasLastScreen = false;
            return;
        }

        // Undo / redo (Ctrl+Z / Ctrl+Y): only here, since PaintingEnabled is true only on the local
        // owner in paint mode. Commit any in-progress stroke first so it becomes its own history step.
        GetUndoRedoInput(out bool wantUndo, out bool wantRedo);
        if (wantUndo || wantRedo)
        {
            EndStrokeIfActive();
            if (wantUndo) Undo();
            else Redo();
            return;
        }

        // Modifier keys: Shift or Ctrl = pipette, Shift/Ctrl + wheel resizes the brush. Runs before the
        // pipette/paint branches below so this frame's pipette state is current.
        HandleBrushModifiers();

        if (updateColliderEveryFrame && _skinned)
        {
            BakeColliderMesh();
            BuildMeshCaches();
        }

        ResolveActiveMask();

        if (!TryGetMouse(out bool pressed, out Vector2 screenPos))
            return;

        // Don't paint through the in-game UI: clicking a HUD slider/toggle must not
        // also stamp the mesh behind it.
        if (pressed && IsPointerOverUi())
            pressed = false;

        // Brush preview (size + mask) at the hover point - independent of painting AND of the
        // pipette, so you always see the brush footprint: before pressing the button, while the
        // momentary pipette holds Shift/Ctrl, and live as Shift/Ctrl+wheel resizes the brush.
        if (previewMode != BrushPreviewMode.Off && _previewMat && paintCamera)
            UpdatePreview(screenPos);

        // Pipette (eyedropper): a fresh click samples ALL channel values at the hit point and adopts
        // them into the brush instead of painting. It takes over the click entirely (no painting),
        // but the brush preview above stays visible.
        if (pipette)
        {
            EndStrokeIfActive(); // switching to the eyedropper ends any active stroke
            if (pressed)
            {
                if (!_pickHeld) PickAtScreen(screenPos);
                _pickHeld = true;
            }
            else _pickHeld = false;
            _hasLastScreen = false; // keep the paint stroke state clean for when pipette turns off
            return;
        }
        _pickHeld = false;

        if (!pressed)
        {
            EndStrokeIfActive(); // mouse released -> commit the stroke as one undo step
            _hasLastScreen = false;
            return;
        }

        if (!paintCamera) return;

        bool anyChannel = AnyChannelActive(); // includes occlusion (matches IsChannelActive / PaintStamp3D)
        bool firstPress = !_hasLastScreen;
        float moveDist = firstPress ? 0f : Vector2.Distance(_lastScreen, screenPos);

        bool stamp;
        if (paintInterval > 0f)
        {
            // Timed-dab mode: one stamp on press, then one at the cursor every paintInterval
            // seconds while held (discrete dabs, no movement interpolation).
            if (firstPress)
            {
                _intervalTimer = 0f;
                stamp = anyChannel;
            }
            else
            {
                _intervalTimer += Time.deltaTime;
                stamp = anyChannel && _intervalTimer >= paintInterval;
                if (stamp) _intervalTimer = 0f;
            }
        }
        else
        {
            // Continuous mode: stamp every frame as an interpolated stroke. Move-only optionally
            // gates it - a held but stationary button no longer re-stamps the same spot.
            stamp = anyChannel && (firstPress || !paintOnMoveOnly || moveDist > MoveOnlyMinPixels);
        }

        if (stamp)
        {
            if (_gpuActive) BeginGpuStroke();
            else ResetCpuDirty();

            if (!firstPress && paintInterval <= 0f)
            {
                // Interpolate along the mouse movement and re-raycast at each step so the
                // stroke follows the mesh surface.
                int steps = Mathf.Clamp(Mathf.CeilToInt(moveDist / ScreenStepPixels), 1, MaxStepsPerFrame);
                for (int s = 1; s <= steps; s++)
                    PaintAtScreen(Vector2.Lerp(_lastScreen, screenPos, s / (float)steps));
            }
            else
            {
                // First stamp of a stroke, or a timed dab: a single discrete stamp at the cursor.
                PaintAtScreen(screenPos);
            }

            if (_gpuActive) EndGpuStroke();
            else EndCpuStroke();
        }

        _lastScreen = screenPos;
        _hasLastScreen = true;
    }

    /// <summary>Raycasts at a screen position and stamps the masked square brush onto the mesh hit point.</summary>
    private void PaintAtScreen(Vector2 screenPos)
    {
        if (!TryComputeBrushPlacement(screenPos, out Vector3 pLocal, out float radiusLocal,
                                      out Vector3 u, out Vector3 v, out Vector3 nLocal))
            return;

        // Pick what this stamp paints with: a fresh random spray pattern, or the resolved
        // standard mask. Both backends read it through the _stamp* fields. When this stamp is
        // being replicated (networked owner), the spray is seeded so remotes scatter identically.
        if (spray)
        {
            if (WantsStampCapture) { _lastSpraySeed = Random.Range(1, int.MaxValue); BuildSprayStampMaskSeeded(_lastSpraySeed); }
            else BuildSprayStampMask();
        }
        else UseResolvedStampMask();

        if (_gpuActive) DrawBrushGpu(pLocal, radiusLocal, u, v, nLocal);
        else PaintStamp3D(pLocal, radiusLocal, u, v, nLocal);

        EmitStamp(pLocal, radiusLocal, u, v, nLocal);
        MarkStrokePainted(); // a stamp landed -> this stroke is undoable
    }

    /// <summary>
    /// Raycasts at a screen position and returns the nearest hit that belongs to THIS paint
    /// mesh. A plain raycast would return only the closest collider, so anything in front of
    /// the mesh (the player capsule, props, the camera-collision proxy) could block it; we
    /// scan all hits and keep the nearest on our own collider. Shared by painting, preview
    /// and the pipette.
    /// </summary>
    private bool TryRaycastPaintMesh(Vector2 screenPos, out RaycastHit hit)
    {
        hit = default;
        if (!paintCamera) return false;

        Ray ray = paintCamera.ScreenPointToRay(screenPos);
        int hitCount = Physics.RaycastNonAlloc(ray, _rayHits, 1000f, ~0, QueryTriggerInteraction.Ignore);
        float nearest = float.MaxValue;
        bool found = false;
        for (int i = 0; i < hitCount; i++)
        {
            if (_rayHits[i].collider != _collider) continue;
            if (_rayHits[i].distance < nearest)
            {
                nearest = _rayHits[i].distance;
                hit = _rayHits[i];
                found = true;
            }
        }
        return found;
    }

    /// <summary>
    /// Raycasts at a screen position and returns the nearest hit on ANY collider (not just the
    /// paint mesh). Used by the pipette, which can sample foreign objects' materials too.
    /// </summary>
    private bool TryRaycastAny(Vector2 screenPos, out RaycastHit hit)
    {
        hit = default;
        if (!paintCamera) return false;
        Ray ray = paintCamera.ScreenPointToRay(screenPos);
        return Physics.Raycast(ray, out hit, 1000f, ~0, QueryTriggerInteraction.Ignore);
    }

    /// <summary>
    /// Raycasts at a screen position against the paint mesh and computes the brush
    /// placement: hit point (local space), local 3D radius (constant visual size via UV
    /// density) and the orthonormal tangent frame (u/v/n). Shared by painting and preview.
    /// </summary>
    private bool TryComputeBrushPlacement(Vector2 screenPos, out Vector3 pLocal, out float radiusLocal,
                                          out Vector3 u, out Vector3 v, out Vector3 nLocal)
    {
        pLocal = default; radiusLocal = 0f; u = default; v = default; nLocal = default;

        if (!TryRaycastPaintMesh(screenPos, out RaycastHit hit)) return false;
        if (_verts == null || hit.triangleIndex < 0) return false;

        pLocal = transform.InverseTransformPoint(hit.point);

        // Convert the brush radius (texels) into a local 3D radius using the hit
        // triangle's UV density so the brush keeps a constant visual size.
        float scale = _avgUvToLocal;
        int t = hit.triangleIndex;
        if (t * 3 + 2 < _tris.Length)
        {
            int i0 = _tris[t * 3], i1 = _tris[t * 3 + 1], i2 = _tris[t * 3 + 2];
            float area3D = 0.5f * Vector3.Cross(_verts[i1] - _verts[i0], _verts[i2] - _verts[i0]).magnitude;
            float areaUv = 0.5f * Mathf.Abs(Cross2D(_uvs[i1] - _uvs[i0], _uvs[i2] - _uvs[i0]));
            if (areaUv > 1e-9f)
            {
                float s = Mathf.Sqrt(area3D / areaUv);
                scale = Mathf.Clamp(s, _avgUvToLocal * 0.25f, _avgUvToLocal * 4f); // guard degenerate UVs
            }
        }

        radiusLocal = (brushSize / (float)textureSize) * scale;
        if (radiusLocal <= 0f) return false;

        // Tangent-plane frame for the square stamp / mask projection.
        nLocal = transform.InverseTransformDirection(hit.normal);
        ComputeBrushFrame(nLocal, out u, out v);
        nLocal = nLocal.normalized;
        return true;
    }

    // --- Channels (shared by both backends) ---

    /// <summary>True if at least one paint channel is enabled (used to gate the per-frame stroke).</summary>
    private bool AnyChannelActive()
    {
        for (int mi = 0; mi < MAP_COUNT; mi++)
            if (IsChannelActive(mi)) return true;
        return false;
    }

    private bool IsChannelActive(int mi)
    {
        switch (mi)
        {
            case MAP_ALBEDO: return paintAlbedo;
            case MAP_MSO: return paintMetallicSmoothness || paintOcclusion;
            default: return paintEmission;
        }
    }

    // --- Helpers ---

    private static float Cross2D(Vector2 a, Vector2 b) => a.x * b.y - a.y * b.x;

    private bool TryGetMouse(out bool pressed, out Vector2 screenPos)
    {
        pressed = false;
        screenPos = Vector2.zero;
#if ENABLE_INPUT_SYSTEM
        var mouse = Mouse.current;
        if (mouse != null)
        {
            pressed = mouse.leftButton.isPressed;
            screenPos = mouse.position.ReadValue();
            return true;
        }
#endif
#if ENABLE_LEGACY_INPUT_MANAGER
        pressed = Input.GetMouseButton(0);
        screenPos = Input.mousePosition;
        return true;
#else
        return false;
#endif
    }

    // --- Shift/Ctrl modifiers (paint mode) ---

    private bool _pipetteModHeld;   // edge tracking for the momentary pipette (Shift or Ctrl)
    private bool _pipetteBeforeMod; // pipette toggle state to restore when the modifier is released

    /// <summary>
    /// While Shift or Ctrl is held the brush enters the pipette (eyedropper), restoring the toggle's
    /// prior state on release. Shift or Ctrl + the mouse wheel resizes the brush (the orbit camera
    /// ignores the wheel while either is held, so it does not also zoom). Called every paint-mode frame.
    /// </summary>
    private void HandleBrushModifiers()
    {
        // Momentary pipette: arm on the Shift/Ctrl press (remembering the toggle's state), force it on
        // while held (re-arms after a one-shot pick), and restore the prior state on release.
        bool shift = ShiftHeld();
        bool ctrl = CtrlHeld();
        bool mod = shift || ctrl;
        if (mod && !_pipetteModHeld) _pipetteBeforeMod = pipette;
        if (mod) pipette = true;
        else if (_pipetteModHeld) pipette = _pipetteBeforeMod;
        _pipetteModHeld = mod;

        // Shift or Ctrl + wheel resizes the brush (the orbit camera ignores the wheel while either is
        // held, so it does not also zoom).
        if (!mod) return;

        float scroll = ReadScroll();
        if (Mathf.Approximately(scroll, 0f)) return;

        int step = Mathf.Max(1, brushSizeScrollStep);
        int next = Mathf.Clamp(brushSize + (int)Mathf.Sign(scroll) * step, 1, 256);
        if (next == brushSize) return;

        brushSize = next;
        BrushSizeChanged?.Invoke();
    }

    private static bool ShiftHeld()
    {
#if ENABLE_INPUT_SYSTEM
        var kb = Keyboard.current;
        if (kb != null) return kb.leftShiftKey.isPressed || kb.rightShiftKey.isPressed;
#endif
#if ENABLE_LEGACY_INPUT_MANAGER
        return Input.GetKey(KeyCode.LeftShift) || Input.GetKey(KeyCode.RightShift);
#else
        return false;
#endif
    }

    private static bool CtrlHeld()
    {
#if ENABLE_INPUT_SYSTEM
        var kb = Keyboard.current;
        if (kb != null) return kb.leftCtrlKey.isPressed || kb.rightCtrlKey.isPressed;
#endif
#if ENABLE_LEGACY_INPUT_MANAGER
        return Input.GetKey(KeyCode.LeftControl) || Input.GetKey(KeyCode.RightControl);
#else
        return false;
#endif
    }

    private static float ReadScroll()
    {
#if ENABLE_INPUT_SYSTEM
        if (Mouse.current != null) return Mouse.current.scroll.ReadValue().y;
#endif
#if ENABLE_LEGACY_INPUT_MANAGER
        return Input.GetAxis("Mouse ScrollWheel");
#else
        return 0f;
#endif
    }

    /// <summary>True if the pointer is currently over an interactive UI element (raycast target).</summary>
    private static bool IsPointerOverUi()
    {
        var es = EventSystem.current;
        return es && es.IsPointerOverGameObject();
    }

    private void OnDestroy()
    {
        if (_bakedMesh && !_baker) Destroy(_bakedMesh); // _baker owns its mesh; only free our own
        if (_materialInstance) Destroy(_materialInstance);
        if (_proceduralMask) Destroy(_proceduralMask);
        if (_previewMat) Destroy(_previewMat);
        if (_previewQuad) Destroy(_previewQuad);
        TeardownSpray();
        TeardownCpu();
        TeardownGpu();
    }
}