ImageSharp/ImageSharp.Drawing/Processing/PathGradientBrush.cs

// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using SixLabors.ImageSharp.Drawing.Helpers;
using SixLabors.ImageSharp.PixelFormats;

namespace SixLabors.ImageSharp.Drawing.Processing {
  /// <summary>
  /// Provides an implementation of a brush for painting gradients between multiple color positions in 2D coordinates.
  /// </summary>
  public sealed class PathGradientBrush : Brush
  {
      private readonly PointF[] points;
      private readonly Color[] colors;
      private readonly Edge[] edges;

      /// <summary>
      /// Initializes a new instance of the <see cref="PathGradientBrush"/> class.
      /// </summary>
      /// <param name="points">Points that constitute a polygon that represents the gradient area.</param>
      /// <param name="colors">Array of colors that correspond to each point in the polygon.</param>
      public PathGradientBrush(PointF[] points, Color[] colors)
      {
          Guard.NotNull(points, nameof(points));
          Guard.MustBeGreaterThanOrEqualTo(points.Length, 3, nameof(points));
          Guard.NotNull(colors, nameof(colors));
          Guard.MustBeGreaterThan(colors.Length, 0, nameof(colors));

          int size = points.Length;

          this.points = [.. points];
          this.colors = [.. colors];
          this.edges = new Edge[this.points.Length];

          for (int i = 0; i < this.points.Length; i++)
          {
              this.edges[i] = new Edge(this.points[i % size], this.points[(i + 1) % size], ColorAt(i), ColorAt(i + 1));
          }

          this.CenterColor = CalculateCenterColor(this.colors);

          Color ColorAt(int index) => this.colors[index % this.colors.Length];
      }

      /// <summary>
      /// Initializes a new instance of the <see cref="PathGradientBrush"/> class.
      /// </summary>
      /// <param name="points">Points that constitute a polygon that represents the gradient area.</param>
      /// <param name="colors">Array of colors that correspond to each point in the polygon.</param>
      /// <param name="centerColor">Color at the center of the gradient area to which the other colors converge.</param>
      public PathGradientBrush(PointF[] points, Color[] colors, Color centerColor)
          : this(points, colors)
      {
          this.CenterColor = centerColor;
          this.HasExplicitCenterColor = true;
      }

      /// <summary>
      /// Gets the polygon points that define the gradient area.
      /// </summary>
      public ReadOnlySpan<PointF> Points => this.points;

      /// <summary>
      /// Gets the colors that are mapped to the polygon points.
      /// </summary>
      public ReadOnlySpan<Color> Colors => this.colors;

      /// <summary>
      /// Gets the color at the center of the gradient area.
      /// </summary>
      public Color CenterColor { get; }

      /// <summary>
      /// Gets a value indicating whether the center color was explicitly supplied.
      /// </summary>
      public bool HasExplicitCenterColor { get; }

      /// <inheritdoc/>
      public override Brush Transform(Matrix4x4 matrix)
      {
          if (matrix.IsIdentity)
          {
              return this;
          }

          PointF[] transformedPoints = new PointF[this.points.Length];
          for (int i = 0; i < transformedPoints.Length; i++)
          {
              transformedPoints[i] = PointF.Transform(this.points[i], matrix);
          }

          return this.HasExplicitCenterColor
              ? new PathGradientBrush(transformedPoints, this.colors, this.CenterColor)
              : new PathGradientBrush(transformedPoints, this.colors);
      }

      /// <inheritdoc />
      public override bool Equals(Brush? other)
      {
          if (other is PathGradientBrush brush)
          {
              return this.CenterColor.Equals(brush.CenterColor)
                  && this.HasExplicitCenterColor.Equals(brush.HasExplicitCenterColor)
                  && this.edges?.SequenceEqual(brush.edges) == true;
          }

          return false;
      }

      /// <inheritdoc/>
      public override int GetHashCode()
          => HashCode.Combine(this.edges, this.CenterColor, this.HasExplicitCenterColor);

      /// <inheritdoc />
      public override BrushRenderer<TPixel> CreateRenderer<TPixel>(
          Configuration configuration,
          GraphicsOptions options,
          int canvasWidth,
          RectangleF region)
          => new PathGradientBrushRenderer<TPixel>(
              configuration,
              options,
              canvasWidth,
              this.edges,
              this.CenterColor,
              this.HasExplicitCenterColor);

      private static Color CalculateCenterColor(Color[] colors)
      {
          Guard.NotNull(colors, nameof(colors));
          Guard.MustBeGreaterThan(colors.Length, 0, nameof(colors));

          return Color.FromScaledVector(colors.Select(c => c.ToScaledVector4()).Aggregate((p1, p2) => p1 + p2) / colors.Length);
      }

      private static float DistanceBetween(Vector2 p1, Vector2 p2) => (p2 - p1).Length();

      private readonly struct Intersection
      {
          public Intersection(PointF point, float distance)
          {
              this.Point = point;
              this.Distance = distance;
          }

          public PointF Point { get; }

          public float Distance { get; }
      }

      /// <summary>
      /// An edge of the polygon that represents the gradient area.
      /// </summary>
      private class Edge : IEquatable<Edge>
      {
          private readonly float length;

          public Edge(Vector2 start, Vector2 end, Color startColor, Color endColor)
          {
              this.Start = start;
              this.End = end;
              this.StartColor = startColor.ToScaledVector4();
              this.EndColor = endColor.ToScaledVector4();

              this.length = DistanceBetween(this.End, this.Start);
          }

          public Vector2 Start { get; }

          public Vector2 End { get; }

          public Vector4 StartColor { get; }

          public Vector4 EndColor { get; }

          public bool Intersect(
              Vector2 start,
              Vector2 end,
              ref Vector2 ip) =>
              PolygonUtilities.LineSegmentToLineSegmentIgnoreCollinear(start, end, this.Start, this.End, ref ip);

          public Vector4 ColorAt(float distance)
          {
              float ratio = this.length > 0 ? distance / this.length : 0;

              return Vector4.Lerp(this.StartColor, this.EndColor, ratio);
          }

          public Vector4 ColorAt(PointF point) => this.ColorAt(DistanceBetween(point, this.Start));

          public bool Equals(Edge? other)
              => other != null &&
                 other.Start == this.Start &&
                 other.End == this.End &&
                 other.StartColor.Equals(this.StartColor) &&
                 other.EndColor.Equals(this.EndColor);

          public override bool Equals(object? obj) => this.Equals(obj as Edge);

          public override int GetHashCode()
              => HashCode.Combine(this.Start, this.End, this.StartColor, this.EndColor);
      }

      /// <summary>
      /// The path gradient brush applicator.
      /// </summary>
      /// <typeparam name="TPixel">The pixel format.</typeparam>
      private sealed class PathGradientBrushRenderer<TPixel> : BrushRenderer<TPixel>
          where TPixel : unmanaged, IPixel<TPixel>
      {
          private readonly Vector2 center;

          private readonly Vector4 centerColor;

          private readonly bool hasSpecialCenterColor;

          private readonly float maxDistance;

          private readonly IList<Edge> edges;

          private readonly TPixel centerPixel;

          private readonly TPixel transparentPixel;

          /// <summary>
          /// Initializes a new instance of the <see cref="PathGradientBrushRenderer{TPixel}"/> class.
          /// </summary>
          /// <param name="configuration">The configuration instance to use when performing operations.</param>
          /// <param name="options">The graphics options.</param>
          /// <param name="canvasWidth">The canvas width for the current render pass.</param>
          /// <param name="edges">Edges of the polygon.</param>
          /// <param name="centerColor">Color at the center of the gradient area to which the other colors converge.</param>
          /// <param name="hasSpecialCenterColor">Whether the center color is different from a smooth gradient between the edges.</param>
          public PathGradientBrushRenderer(
              Configuration configuration,
              GraphicsOptions options,
              int canvasWidth,
              IList<Edge> edges,
              Color centerColor,
              bool hasSpecialCenterColor)
              : base(configuration, options, canvasWidth)
          {
              this.edges = edges;
              Vector2[] points = [.. edges.Select(s => s.Start)];

              this.center = points.Aggregate((p1, p2) => p1 + p2) / edges.Count;
              this.centerColor = centerColor.ToScaledVector4();
              this.hasSpecialCenterColor = hasSpecialCenterColor;
              this.centerPixel = centerColor.ToPixel<TPixel>();
              this.maxDistance = points.Select(p => p - this.center).Max(d => d.Length());
              this.transparentPixel = Color.Transparent.ToPixel<TPixel>();
          }

          internal TPixel this[int x, int y]
          {
              get
              {
                  // Match other gradient brushes by evaluating at pixel centers.
                  Vector2 point = new(x + 0.5F, y + 0.5F);

                  if (point == this.center)
                  {
                      return this.centerPixel;
                  }

                  if (this.edges.Count == 3 && !this.hasSpecialCenterColor)
                  {
                      if (!FindPointOnTriangle(
                          this.edges[0].Start,
                          this.edges[1].Start,
                          this.edges[2].Start,
                          point,
                          out float u,
                          out float v))
                      {
                          return this.transparentPixel;
                      }

                      Vector4 pointColor = ((1 - u - v) * this.edges[0].StartColor)
                          + (u * this.edges[0].EndColor)
                          + (v * this.edges[2].StartColor);

                      return TPixel.FromScaledVector4(pointColor);
                  }

                  Vector2 direction = Vector2.Normalize(point - this.center);
                  Vector2 end = point + (direction * this.maxDistance);

                  (Edge Edge, Vector2 Point)? isc = this.FindIntersection(point, end);

                  if (!isc.HasValue)
                  {
                      return this.transparentPixel;
                  }

                  Vector2 intersection = isc.Value.Point;
                  Vector4 edgeColor = isc.Value.Edge.ColorAt(intersection);

                  float length = DistanceBetween(intersection, this.center);
                  float ratio = length > 0 ? DistanceBetween(intersection, point) / length : 0;

                  Vector4 color = Vector4.Lerp(edgeColor, this.centerColor, ratio);

                  return TPixel.FromScaledVector4(color);
              }
          }

          /// <inheritdoc />
          public override void Apply(
              Span<TPixel> destinationRow,
              ReadOnlySpan<float> scanline,
              int x,
              int y,
              BrushWorkspace<TPixel> workspace)
          {
              Span<float> amounts = workspace.GetAmounts(scanline.Length);
              Span<TPixel> overlays = workspace.GetOverlays(scanline.Length);
              float blendPercentage = this.Options.BlendPercentage;

              // TODO: Remove bounds checks.
              if (blendPercentage < 1)
              {
                  for (int i = 0; i < scanline.Length; i++)
                  {
                      amounts[i] = scanline[i] * blendPercentage;
                      overlays[i] = this[x + i, y];
                  }
              }
              else
              {
                  for (int i = 0; i < scanline.Length; i++)
                  {
                      amounts[i] = scanline[i];
                      overlays[i] = this[x + i, y];
                  }
              }

              this.Blender.Blend(
                  this.Configuration,
                  destinationRow,
                  destinationRow,
                  overlays,
                  amounts,
                  workspace.GetBlendScratch(scanline.Length, 3));
          }

          private (Edge Edge, Vector2 Point)? FindIntersection(
              PointF start,
              PointF end)
          {
              Vector2 ip = default;
              Vector2 closestIntersection = default;
              Edge? closestEdge = null;
              float minDistance = float.MaxValue;
              foreach (Edge edge in this.edges)
              {
                  if (!edge.Intersect(start, end, ref ip))
                  {
                      continue;
                  }

                  float d = Vector2.DistanceSquared(start, ip);
                  if (d < minDistance)
                  {
                      minDistance = d;
                      closestEdge = edge;
                      closestIntersection = ip;
                  }
              }

              return closestEdge != null ? (closestEdge, closestIntersection) : null;
          }

          private static bool FindPointOnTriangle(Vector2 v1, Vector2 v2, Vector2 v3, Vector2 point, out float u, out float v)
          {
              Vector2 e1 = v2 - v1;
              Vector2 e2 = v3 - v2;
              Vector2 e3 = v1 - v3;

              Vector2 pv1 = point - v1;
              Vector2 pv2 = point - v2;
              Vector2 pv3 = point - v3;

              Vector3 d1 = Vector3.Cross(new Vector3(e1.X, e1.Y, 0), new Vector3(pv1.X, pv1.Y, 0));
              Vector3 d2 = Vector3.Cross(new Vector3(e2.X, e2.Y, 0), new Vector3(pv2.X, pv2.Y, 0));
              Vector3 d3 = Vector3.Cross(new Vector3(e3.X, e3.Y, 0), new Vector3(pv3.X, pv3.Y, 0));

              if (Math.Sign(Vector3.Dot(d1, d2)) * Math.Sign(Vector3.Dot(d1, d3)) == -1 || Math.Sign(Vector3.Dot(d1, d2)) * Math.Sign(Vector3.Dot(d2, d3)) == -1)
              {
                  u = 0;
                  v = 0;
                  return false;
              }

              // From Real-Time Collision Detection
              // https://gamedev.stackexchange.com/questions/23743/whats-the-most-efficient-way-to-find-barycentric-coordinates
              float d00 = Vector2.Dot(e1, e1);
              float d01 = -Vector2.Dot(e1, e3);
              float d11 = Vector2.Dot(e3, e3);
              float d20 = Vector2.Dot(pv1, e1);
              float d21 = -Vector2.Dot(pv1, e3);
              float denominator = (d00 * d11) - (d01 * d01);
              u = ((d11 * d20) - (d01 * d21)) / denominator;
              v = ((d00 * d21) - (d01 * d20)) / denominator;
              return true;
          }
      }
  }
}