using System.Collections.Generic;
using UnityEngine;
#if ENABLE_INPUT_SYSTEM
using UnityEngine.InputSystem;
#endif

/// <summary>
/// Live mesh painting in Play mode.
/// Paints a fixed color (brush) onto the mesh's texture with the mouse.
///
/// TWO BACKENDS (switchable via <see cref="useGpu"/>, also live in Play mode):
///
///  * GPU (default): True shader painting into a RenderTexture. The brush is
///    drawn via a vertex/fragment shader - the mesh is unwrapped into the texture
///    layout using its UVs, and per texel the actual 3D position is tested against
///    the brush sphere. Edge padding against seams runs as a GPU blit.
///    Fast, because the GPU rasterizes all texels in parallel.
///
///  * CPU: Rasterization of the hit triangles on the CPU; per texel the 3D
///    position is computed barycentrically and tested against the brush sphere.
///    Edge padding via CPU dilation. Identical visual result, but more expensive.
///
/// BOTH backends paint seamlessly: a 3D sphere around the hit point captures
/// triangles on both sides of a UV seam at once (front/back share the same 3D
/// edge), no matter where they lie in the UV layout.
///
/// Simply attach to the GameObject you want to paint (e.g. paintman_UV/paintman_UV).
/// </summary>
[DisallowMultipleComponent]
public class MeshPainter : MonoBehaviour
{
    [Header("Backend")]
    [Tooltip("ON = GPU painting (shader/RenderTexture, fast). OFF = CPU painting. " +
             "Can be toggled live in Play mode - the painted content is carried over.")]
    public bool useGpu = true;

    [Header("Brush")]
    [Tooltip("Color used for painting.")]
    public Color brushColor = Color.red;

    [Tooltip("Brush radius in texels (visual size). Converted into a 3D radius per hit.")]
    [Range(1, 256)]
    public int brushSize = 16;

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

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

    [Tooltip("Edge padding (texels) against visible UV seams. The color is 'bled' beyond the " +
             "UV island edges so that bilinear sampling there does not pull in the unpainted " +
             "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;

    // --- shared references ---
    private Renderer _renderer;
    private SkinnedMeshRenderer _skinned;
    private MeshCollider _collider;
    private Mesh _bakedMesh;
    private Material _materialInstance;

    // Mesh caches (in the transform's local space = collider space).
    private Vector3[] _verts;
    private int[] _tris;
    private Vector2[] _uvs;
    private Vector3[] _triCenter;   // centroid per triangle (for fast CPU culling)
    private float[] _triRadius;     // circumscribed radius per triangle (for fast CPU culling)
    private float _avgUvToLocal = 1f; // average conversion: UV unit -> local unit

    private bool _hasLastScreen;
    private Vector2 _lastScreen;
    private bool _gpuActive;       // currently configured backend
    private bool _setupValid;

    // --- CPU backend ---
    private Texture2D _paintTexture;
    private Color32[] _pixels;
    private bool[] _painted;                 // true = painted by the brush (persistent dilation source)
    private int[] _fillStamp;                // transient marker for padding texels per dilation call
    private int _dilateStamp;                // incremented per call -> padding from previous frames does not count as a source
    private readonly List<int> _dilateScratch = new List<int>(512);
    private int _dx0, _dy0, _dx1, _dy1;      // dirty rectangle (CPU upload)

    // --- GPU backend ---
    public Shader brushShader;               // optional; otherwise Shader.Find("Hidden/MeshPaintBrush")
    public Shader dilateShader;              // optional; otherwise Shader.Find("Hidden/MeshPaintDilate")
    private Material _brushMat;
    private Material _dilateMat;
    private RenderTexture _displayRT;        // assigned to the material as _BaseMap
    private RenderTexture _tempRT;           // scratch for the dilation blit (ping-pong)
    private bool _drewThisStroke;
    private bool _prevSRGB;

    // Step size (in screen pixels) for interpolation along the mouse movement.
    private const float ScreenStepPixels = 5f;
    // Upper bound of raycasts per frame (safeguard for very fast movements).
    private const int MaxStepsPerFrame = 256;

    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 BrushColorId = Shader.PropertyToID("_BrushColor");
    private static readonly int BrushCenterId = Shader.PropertyToID("_BrushCenter");
    private static readonly int BrushRadiusId = Shader.PropertyToID("_BrushRadius");
    private static readonly int PadId = Shader.PropertyToID("_Pad");

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

        _renderer = GetComponent<Renderer>();
        _skinned = GetComponent<SkinnedMeshRenderer>();
        if (_renderer == null)
        {
            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.
        _materialInstance = _renderer.material; // automatically creates an instance
        // Tint to white so the painted color appears unaltered.
        if (_materialInstance.HasProperty(BaseColorId)) _materialInstance.SetColor(BaseColorId, Color.white);
        else if (_materialInstance.HasProperty(ColorId)) _materialInstance.SetColor(ColorId, Color.white);

        SetupBackend(useGpu, fromColor32: BuildBaseColors());
        _setupValid = true;

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

    // ------------------------------------------------------------------
    //  Setup (shared)
    // ------------------------------------------------------------------

    private void SetupCollider()
    {
        _collider = GetComponent<MeshCollider>();
        if (_collider == null) _collider = gameObject.AddComponent<MeshCollider>();
        _collider.convex = false; // non-convex provides triangleIndex / textureCoord

        _bakedMesh = new Mesh { name = "MeshPainter_Baked" };
        BakeColliderMesh();
    }

    private void BakeColliderMesh()
    {
        if (_skinned != null)
        {
            // Static skinned mesh: bake into local coordinates (scale = 1).
            _skinned.BakeMesh(_bakedMesh);
            _collider.sharedMesh = null;
            _collider.sharedMesh = _bakedMesh;
        }
        else
        {
            var mf = GetComponent<MeshFilter>();
            if (mf != null) _collider.sharedMesh = mf.sharedMesh;
        }
    }

    /// <summary>Reads vertices/triangles/UVs of the collider mesh and computes culling data.</summary>
    private void BuildMeshCaches()
    {
        Mesh m = _collider.sharedMesh;
        if (m == null) return;

        _verts = m.vertices;
        _tris = m.triangles;
        _uvs = m.uv;

        // Fallback: UVs from the original mesh (BakeMesh usually keeps them, but better safe than sorry).
        if ((_uvs == null || _uvs.Length != _verts.Length) && _skinned != null && _skinned.sharedMesh != null)
            _uvs = _skinned.sharedMesh.uv;

        if (_uvs == null || _uvs.Length != _verts.Length) return;

        int triCount = _tris.Length / 3;
        _triCenter = new Vector3[triCount];
        _triRadius = new float[triCount];

        double sumArea3D = 0.0, sumAreaUv = 0.0;

        for (int t = 0; t < triCount; t++)
        {
            int i0 = _tris[t * 3], i1 = _tris[t * 3 + 1], i2 = _tris[t * 3 + 2];
            Vector3 a = _verts[i0], b = _verts[i1], c = _verts[i2];

            Vector3 center = (a + b + c) / 3f;
            _triCenter[t] = center;
            float ra = (a - center).sqrMagnitude;
            float rb = (b - center).sqrMagnitude;
            float rc = (c - center).sqrMagnitude;
            _triRadius[t] = Mathf.Sqrt(Mathf.Max(ra, Mathf.Max(rb, rc)));

            sumArea3D += 0.5 * Vector3.Cross(b - a, c - a).magnitude;
            Vector2 ua = _uvs[i0], ub = _uvs[i1], uc = _uvs[i2];
            sumAreaUv += 0.5 * Mathf.Abs(Cross2D(ub - ua, uc - ua));
        }

        if (sumAreaUv > 1e-9) _avgUvToLocal = (float)System.Math.Sqrt(sumArea3D / sumAreaUv);
    }

    /// <summary>Builds the base-color texels (copy of the existing material texture or baseColor).</summary>
    private Color32[] BuildBaseColors()
    {
        var px = new Color32[textureSize * textureSize];

        Texture sourceTex = null;
        var shared = _renderer.sharedMaterial;
        if (shared != null)
        {
            if (shared.HasProperty(BaseMapId)) sourceTex = shared.GetTexture(BaseMapId);
            if (sourceTex == null && shared.HasProperty(MainTexId)) sourceTex = shared.GetTexture(MainTexId);
        }

        var src = sourceTex as Texture2D;
        if (src != null)
        {
            try
            {
                if (src.width == textureSize && src.height == textureSize && src.isReadable)
                    return src.GetPixels32();
            }
            catch { /* not readable -> fall back to baseColor */ }
        }

        Color32 c = baseColor;
        for (int i = 0; i < px.Length; i++) px[i] = c;
        return px;
    }

    // ------------------------------------------------------------------
    //  Backend selection / switching
    // ------------------------------------------------------------------

    /// <summary>Sets up the desired backend and carries over the initial image content.</summary>
    private void SetupBackend(bool gpu, Color32[] fromColor32)
    {
        if (gpu) SetupGpu(fromColor32);
        else SetupCpu(fromColor32);
        _gpuActive = gpu;
    }

    /// <summary>Switches the backend at runtime and carries over the currently painted content.</summary>
    private void SwitchBackend(bool toGpu)
    {
        // save the current content as Color32
        Color32[] current = (_gpuActive ? ReadGpuPixels() : (Color32[])_pixels.Clone());

        if (_gpuActive) TeardownGpu();
        else TeardownCpu();

        SetupBackend(toGpu, current);
        _hasLastScreen = false;

        Debug.Log($"[MeshPainter] Backend switched -> {(toGpu ? "GPU" : "CPU")} painting.", this);
    }

    // ------------------------------------------------------------------
    //  CPU backend
    // ------------------------------------------------------------------

    private void SetupCpu(Color32[] initial)
    {
        _paintTexture = new Texture2D(textureSize, textureSize, TextureFormat.RGBA32, false, false)
        {
            name = "MeshPainter_Canvas",
            wrapMode = TextureWrapMode.Clamp,
            filterMode = FilterMode.Bilinear
        };

        _pixels = (initial != null && initial.Length == textureSize * textureSize)
            ? initial
            : BuildBaseColors();

        _painted = new bool[textureSize * textureSize];
        _fillStamp = new int[textureSize * textureSize];
        _dilateStamp = 0;

        _paintTexture.SetPixels32(_pixels);
        _paintTexture.Apply(false);

        AssignTextureToMaterial(_paintTexture);
    }

    private void TeardownCpu()
    {
        if (_paintTexture != null) { Destroy(_paintTexture); _paintTexture = null; }
        _pixels = null;
        _painted = null;
        _fillStamp = null;
    }

    // ------------------------------------------------------------------
    //  GPU backend
    // ------------------------------------------------------------------

    private void SetupGpu(Color32[] initial)
    {
        if (brushShader == null) brushShader = Shader.Find("Hidden/MeshPaintBrush");
        if (dilateShader == null) dilateShader = Shader.Find("Hidden/MeshPaintDilate");

        if (brushShader == null || dilateShader == null)
        {
            Debug.LogError("[MeshPainter] GPU shaders not found (Hidden/MeshPaintBrush or " +
                           "Hidden/MeshPaintDilate). Falling back to CPU painting.", this);
            useGpu = false;
            SetupCpu(initial);
            _gpuActive = false;
            return;
        }

        _brushMat = new Material(brushShader) { hideFlags = HideFlags.HideAndDontSave };
        _dilateMat = new Material(dilateShader) { hideFlags = HideFlags.HideAndDontSave };

        _displayRT = CreateRT("MeshPainter_DisplayRT");
        _tempRT = CreateRT("MeshPainter_TempRT");

        // Bring the initial image content into the display RT. Alpha = 0 marks "unpainted"
        // (so the base color is not mistaken for a dilation source).
        var seed = initial ?? BuildBaseColors();
        UploadColorsToRT(seed, _displayRT, paintedAlpha: false);

        AssignTextureToMaterial(_displayRT);
    }

    private void TeardownGpu()
    {
        if (_displayRT != null) { _displayRT.Release(); Destroy(_displayRT); _displayRT = null; }
        if (_tempRT != null) { _tempRT.Release(); Destroy(_tempRT); _tempRT = null; }
        if (_brushMat != null) { Destroy(_brushMat); _brushMat = null; }
        if (_dilateMat != null) { Destroy(_dilateMat); _dilateMat = null; }
    }

    private RenderTexture CreateRT(string rtName)
    {
        var rt = new RenderTexture(textureSize, textureSize, 0, RenderTextureFormat.ARGB32, RenderTextureReadWrite.sRGB)
        {
            name = rtName,
            wrapMode = TextureWrapMode.Clamp,
            filterMode = FilterMode.Bilinear,
            antiAliasing = 1,
            useMipMap = false,
            autoGenerateMips = false
        };
        rt.Create();
        return rt;
    }

    /// <summary>Writes Color32 data into a RenderTexture via a temporary texture.</summary>
    private void UploadColorsToRT(Color32[] colors, RenderTexture rt, bool paintedAlpha)
    {
        var tmp = new Texture2D(textureSize, textureSize, TextureFormat.RGBA32, false, false);
        if (paintedAlpha)
        {
            // mark the entire content as "painted" (alpha = 255) -> no dilation needed.
            for (int i = 0; i < colors.Length; i++) colors[i].a = 255;
        }
        else
        {
            // mark the base color as "unpainted" (alpha = 0).
            for (int i = 0; i < colors.Length; i++) colors[i].a = 0;
        }
        tmp.SetPixels32(colors);
        tmp.Apply(false);
        Graphics.Blit(tmp, rt);
        Destroy(tmp);
    }

    /// <summary>Reads the display RT back as Color32 (for the backend switch).</summary>
    private Color32[] ReadGpuPixels()
    {
        var prev = RenderTexture.active;
        RenderTexture.active = _displayRT;
        var tex = new Texture2D(textureSize, textureSize, TextureFormat.RGBA32, false, false);
        tex.ReadPixels(new Rect(0, 0, textureSize, textureSize), 0, 0, false);
        tex.Apply(false);
        RenderTexture.active = prev;
        var px = tex.GetPixels32();
        Destroy(tex);
        return px;
    }

    // ------------------------------------------------------------------
    //  Material
    // ------------------------------------------------------------------

    private void AssignTextureToMaterial(Texture tex)
    {
        if (_materialInstance == null) return;
        if (_materialInstance.HasProperty(BaseMapId)) _materialInstance.SetTexture(BaseMapId, tex);
        if (_materialInstance.HasProperty(MainTexId)) _materialInstance.SetTexture(MainTexId, tex);
    }

    // ------------------------------------------------------------------
    //  Update / input
    // ------------------------------------------------------------------

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

        // Live backend switching.
        if (useGpu != _gpuActive) SwitchBackend(useGpu);

        if (updateColliderEveryFrame && _skinned != null)
        {
            BakeColliderMesh();
            BuildMeshCaches();
        }

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

        if (!pressed)
        {
            _hasLastScreen = false;
            return;
        }

        if (paintCamera == null) return;

        if (_gpuActive) BeginGpuStroke();
        else { _dx0 = textureSize; _dy0 = textureSize; _dx1 = -1; _dy1 = -1; }

        if (_hasLastScreen)
        {
            // Interpolate along the mouse movement in screen space and re-raycast at
            // each step -> the stroke follows the mesh surface.
            float dist = Vector2.Distance(_lastScreen, screenPos);
            int steps = Mathf.Clamp(Mathf.CeilToInt(dist / ScreenStepPixels), 1, MaxStepsPerFrame);
            for (int s = 1; s <= steps; s++)
                PaintAtScreen(Vector2.Lerp(_lastScreen, screenPos, s / (float)steps));
        }
        else
        {
            PaintAtScreen(screenPos);
        }

        if (_gpuActive) EndGpuStroke();
        else
        {
            if (paddingTexels > 0) DilatePadding(paddingTexels);
            FlushDirty();
        }

        _lastScreen = screenPos;
        _hasLastScreen = true;
    }

    /// <summary>
    /// Raycast at a screen position. If the ray hits the mesh, a 3D sphere is painted
    /// around the hit point (seamless) - via GPU or CPU depending on the backend.
    /// </summary>
    private void PaintAtScreen(Vector2 screenPos)
    {
        Ray ray = paintCamera.ScreenPointToRay(screenPos);
        if (!(Physics.Raycast(ray, out RaycastHit hit, 1000f) && hit.collider == _collider))
            return;
        if (_verts == null || hit.triangleIndex < 0) return;

        // Bring the hit point into local mesh space (= collider/vertex space).
        Vector3 pLocal = transform.InverseTransformPoint(hit.point);

        // Convert the brush radius (texels) into a local 3D radius using the UV
        // density of the hit triangle -> the brush stays visually the same 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);
                // guard against degenerate UVs: keep close to the average.
                scale = Mathf.Clamp(s, _avgUvToLocal * 0.25f, _avgUvToLocal * 4f);
            }
        }

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

        if (_gpuActive) DrawBrushGpu(pLocal, radiusLocal);
        else PaintSphere3D(pLocal, radiusLocal);
    }

    // ------------------------------------------------------------------
    //  GPU painting
    // ------------------------------------------------------------------

    private void BeginGpuStroke()
    {
        _drewThisStroke = false;
        _prevSRGB = GL.sRGBWrite;
        GL.sRGBWrite = true; // correct sRGB encoding when writing into the sRGB RT (linear project)
        Graphics.SetRenderTarget(_displayRT);
        _brushMat.SetColor(BrushColorId, brushColor);
    }

    private void DrawBrushGpu(Vector3 pLocal, float radiusLocal)
    {
        _brushMat.SetVector(BrushCenterId, pLocal);
        _brushMat.SetFloat(BrushRadiusId, radiusLocal);
        if (!_brushMat.SetPass(0)) return;

        int sub = _bakedMesh.subMeshCount;
        for (int i = 0; i < sub; i++)
            Graphics.DrawMeshNow(_bakedMesh, Matrix4x4.identity, i);

        _drewThisStroke = true;
    }

    private void EndGpuStroke()
    {
        Graphics.SetRenderTarget(null);

        // Edge padding against UV seams: blit the display RT through the dilation
        // shader into _tempRT and then swap (the result ends up in _displayRT again).
        if (_drewThisStroke && paddingTexels > 0)
        {
            _dilateMat.SetInt(PadId, paddingTexels);
            Graphics.Blit(_displayRT, _tempRT, _dilateMat, 0);

            var swap = _displayRT; _displayRT = _tempRT; _tempRT = swap;
            AssignTextureToMaterial(_displayRT);
        }

        GL.sRGBWrite = _prevSRGB;
    }

    // ------------------------------------------------------------------
    //  CPU painting (rasterization + dilation)
    // ------------------------------------------------------------------

    /// <summary>
    /// Paints a sphere in local 3D space: rasterizes all triangles in range into
    /// the texture and colors every texel whose 3D position lies inside the sphere.
    /// </summary>
    private void PaintSphere3D(Vector3 pLocal, float radiusLocal)
    {
        Color32 col = brushColor;
        float r2 = radiusLocal * radiusLocal;
        int triCount = _tris.Length / 3;
        float ts = textureSize;

        for (int t = 0; t < triCount; t++)
        {
            // Fast culling: sphere against the triangle's circumscribed circle.
            float reach = radiusLocal + _triRadius[t];
            if ((_triCenter[t] - pLocal).sqrMagnitude > reach * reach) continue;

            int i0 = _tris[t * 3], i1 = _tris[t * 3 + 1], i2 = _tris[t * 3 + 2];
            Vector3 v0 = _verts[i0], v1 = _verts[i1], v2 = _verts[i2];
            Vector2 uv0 = _uvs[i0], uv1 = _uvs[i1], uv2 = _uvs[i2];

            // UV -> pixel.
            float p0x = uv0.x * ts, p0y = uv0.y * ts;
            float p1x = uv1.x * ts, p1y = uv1.y * ts;
            float p2x = uv2.x * ts, p2y = uv2.y * ts;

            // Bounding box in pixel space.
            int minX = Mathf.Clamp(Mathf.FloorToInt(Mathf.Min(p0x, Mathf.Min(p1x, p2x))), 0, textureSize - 1);
            int maxX = Mathf.Clamp(Mathf.CeilToInt(Mathf.Max(p0x, Mathf.Max(p1x, p2x))), 0, textureSize - 1);
            int minY = Mathf.Clamp(Mathf.FloorToInt(Mathf.Min(p0y, Mathf.Min(p1y, p2y))), 0, textureSize - 1);
            int maxY = Mathf.Clamp(Mathf.CeilToInt(Mathf.Max(p0y, Mathf.Max(p1y, p2y))), 0, textureSize - 1);
            if (maxX < minX || maxY < minY) continue;

            // Barycentric denominator.
            float d = (p1y - p2y) * (p0x - p2x) + (p2x - p1x) * (p0y - p2y);
            if (Mathf.Abs(d) < 1e-7f) continue;
            float invD = 1f / d;

            const float eps = -1e-4f; // small tolerance against 1px cracks between triangles

            for (int y = minY; y <= maxY; y++)
            {
                float py = y + 0.5f;
                int rowStart = y * textureSize;
                for (int x = minX; x <= maxX; x++)
                {
                    float px = x + 0.5f;

                    float a = ((p1y - p2y) * (px - p2x) + (p2x - p1x) * (py - p2y)) * invD;
                    if (a < eps) continue;
                    float b = ((p2y - p0y) * (px - p2x) + (p0x - p2x) * (py - p2y)) * invD;
                    if (b < eps) continue;
                    float c = 1f - a - b;
                    if (c < eps) continue;

                    // 3D position of the texel via barycentric interpolation.
                    float wx = a * v0.x + b * v1.x + c * v2.x;
                    float wy = a * v0.y + b * v1.y + c * v2.y;
                    float wz = a * v0.z + b * v1.z + c * v2.z;

                    float ddx = wx - pLocal.x, ddy = wy - pLocal.y, ddz = wz - pLocal.z;
                    if (ddx * ddx + ddy * ddy + ddz * ddz > r2) continue;

                    _pixels[rowStart + x] = col;
                    _painted[rowStart + x] = true;

                    if (x < _dx0) _dx0 = x;
                    if (x > _dx1) _dx1 = x;
                    if (y < _dy0) _dy0 = y;
                    if (y > _dy1) _dy1 = y;
                }
            }
        }
    }

    /// <summary>
    /// Edge padding / texture dilation against visible UV seams (CPU).
    /// Colors a thin border (pad texels) around the freshly painted areas with their
    /// color. The dilation source is exclusively brush-painted texels (_painted).
    /// </summary>
    private void DilatePadding(int pad)
    {
        if (_dx1 < _dx0 || _dy1 < _dy0) return; // nothing painted

        int x0 = Mathf.Max(0, _dx0 - pad);
        int y0 = Mathf.Max(0, _dy0 - pad);
        int x1 = Mathf.Min(textureSize - 1, _dx1 + pad);
        int y1 = Mathf.Min(textureSize - 1, _dy1 + pad);

        _dilateStamp++; // unique per call -> padding from previous frames does not count as a source

        for (int iter = 0; iter < pad; iter++)
        {
            _dilateScratch.Clear();

            for (int y = y0; y <= y1; y++)
            {
                int row = y * textureSize;
                for (int x = x0; x <= x1; x++)
                {
                    int idx = row + x;
                    if (_painted[idx] || _fillStamp[idx] == _dilateStamp) continue;

                    if (TryFindPaddingSource(x, y, out Color32 c))
                    {
                        _pixels[idx] = c;
                        _dilateScratch.Add(idx);

                        if (x < _dx0) _dx0 = x;
                        if (x > _dx1) _dx1 = x;
                        if (y < _dy0) _dy0 = y;
                        if (y > _dy1) _dy1 = y;
                    }
                }
            }

            for (int k = 0; k < _dilateScratch.Count; k++)
                _fillStamp[_dilateScratch[k]] = _dilateStamp;

            if (_dilateScratch.Count == 0) break;
        }
    }

    /// <summary>Searches the 8-neighborhood for a source texel (painted or already padded) and returns its color.</summary>
    private bool TryFindPaddingSource(int x, int y, out Color32 col)
    {
        for (int oy = -1; oy <= 1; oy++)
        {
            int ny = y + oy;
            if (ny < 0 || ny >= textureSize) continue;
            int nrow = ny * textureSize;
            for (int ox = -1; ox <= 1; ox++)
            {
                if (ox == 0 && oy == 0) continue;
                int nx = x + ox;
                if (nx < 0 || nx >= textureSize) continue;
                int nidx = nrow + nx;
                if (_painted[nidx] || _fillStamp[nidx] == _dilateStamp)
                {
                    col = _pixels[nidx];
                    return true;
                }
            }
        }
        col = default;
        return false;
    }

    /// <summary>Uploads only the region changed this frame to the GPU once (CPU backend).</summary>
    private void FlushDirty()
    {
        if (_dx1 < _dx0 || _dy1 < _dy0) return; // nothing painted

        int bw = _dx1 - _dx0 + 1;
        int bh = _dy1 - _dy0 + 1;
        var block = new Color32[bw * bh];
        for (int y = 0; y < bh; y++)
            System.Array.Copy(_pixels, (_dy0 + y) * textureSize + _dx0, block, y * bw, bw);

        _paintTexture.SetPixels32(_dx0, _dy0, bw, bh, block);
        _paintTexture.Apply(false);
    }

    // ------------------------------------------------------------------
    //  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
    }

    private void OnDestroy()
    {
        if (_bakedMesh != null) Destroy(_bakedMesh);
        if (_materialInstance != null) Destroy(_materialInstance);
        TeardownCpu();
        TeardownGpu();
    }
}