ImageSharp/ImageSharp.Drawing/Processing/EllipticGradientBrush.cs

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

using System;
using System.Numerics;
using SixLabors.ImageSharp.Memory;
using SixLabors.ImageSharp.PixelFormats;

namespace SixLabors.ImageSharp.Drawing.Processing {
  /// <summary>
  /// Provides an implementation of a brush for painting elliptical gradients.
  /// The ellipse is defined by a center point, a point on the longest axis, and the ratio
  /// between the longest and shortest axes.
  /// </summary>
  public sealed class EllipticGradientBrush : GradientBrush
  {
      /// <inheritdoc cref="GradientBrush" />
      /// <param name="center">The center of the elliptical gradient and 0 for the color stops.</param>
      /// <param name="referenceAxisEnd">The end point of the reference axis of the ellipse.</param>
      /// <param name="axisRatio">
      ///   The ratio of the axis widths.
      ///   The second axis is perpendicular to the reference axis and its length is the reference axis length
      ///   multiplied by this factor.
      /// </param>
      /// <param name="repetitionMode">Defines how the colors of the gradients are repeated.</param>
      /// <param name="colorStops">The color stops.</param>
      public EllipticGradientBrush(
          PointF center,
          PointF referenceAxisEnd,
          float axisRatio,
          GradientRepetitionMode repetitionMode,
          params ColorStop[] colorStops)
          : base(repetitionMode, colorStops)
      {
          this.Center = center;
          this.ReferenceAxisEnd = referenceAxisEnd;
          this.AxisRatio = axisRatio;
      }

      /// <summary>
      /// Gets the center of the ellipse.
      /// </summary>
      public PointF Center { get; }

      /// <summary>
      /// Gets the end point of the reference axis.
      /// </summary>
      public PointF ReferenceAxisEnd { get; }

      /// <summary>
      /// Gets the ratio of the secondary axis to the primary axis.
      /// </summary>
      public float AxisRatio { get; }

      /// <inheritdoc/>
      public override Brush Transform(Matrix4x4 matrix)
      {
          PointF tc = PointF.Transform(this.Center, matrix);
          PointF tRef = PointF.Transform(this.ReferenceAxisEnd, matrix);

          // Compute a point on the perpendicular (secondary) axis and transform it.
          float refDx = this.ReferenceAxisEnd.X - this.Center.X;
          float refDy = this.ReferenceAxisEnd.Y - this.Center.Y;
          float refLen = MathF.Sqrt((refDx * refDx) + (refDy * refDy));
          float secondLen = refLen * this.AxisRatio;

          // Perpendicular direction (rotated 90 degrees).
          PointF secondEnd = new(
              this.Center.X + (-refDy / refLen * secondLen),
              this.Center.Y + (refDx / refLen * secondLen));
          PointF tSec = PointF.Transform(secondEnd, matrix);

          // Derive new ratio from transformed lengths.
          float newRefLen = MathF.Sqrt(
              ((tRef.X - tc.X) * (tRef.X - tc.X)) + ((tRef.Y - tc.Y) * (tRef.Y - tc.Y)));
          float newSecLen = MathF.Sqrt(
              ((tSec.X - tc.X) * (tSec.X - tc.X)) + ((tSec.Y - tc.Y) * (tSec.Y - tc.Y)));
          float newRatio = newRefLen > 0f ? newSecLen / newRefLen : this.AxisRatio;

          return new EllipticGradientBrush(tc, tRef, newRatio, this.RepetitionMode, this.ColorStopsArray);
      }

      /// <inheritdoc />
      public override BrushRenderer<TPixel> CreateRenderer<TPixel>(
          Configuration configuration,
          GraphicsOptions options,
          int canvasWidth,
          RectangleF region) =>
          new EllipticGradientBrushRenderer<TPixel>(
              configuration,
              options,
              canvasWidth,
              this,
              this.ColorStopsArray,
              this.RepetitionMode);

      /// <inheritdoc />
      private sealed class EllipticGradientBrushRenderer<TPixel> : GradientBrushRenderer<TPixel>
          where TPixel : unmanaged, IPixel<TPixel>
      {
          private readonly PointF center;

          private readonly float cosRotation;

          private readonly float sinRotation;

          private readonly float referenceRadiusSquared;

          private readonly float secondRadiusSquared;

          /// <summary>
          /// Initializes a new instance of the <see cref="EllipticGradientBrushRenderer{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="brush">The elliptic gradient brush.</param>
          /// <param name="colorStops">Definition of colors.</param>
          /// <param name="repetitionMode">Defines how the gradient colors are repeated.</param>
          public EllipticGradientBrushRenderer(
              Configuration configuration,
              GraphicsOptions options,
              int canvasWidth,
              EllipticGradientBrush brush,
              ColorStop[] colorStops,
              GradientRepetitionMode repetitionMode)
              : base(configuration, options, canvasWidth, colorStops, repetitionMode)
          {
              this.center = brush.Center;

              float refDx = brush.ReferenceAxisEnd.X - brush.Center.X;
              float refDy = brush.ReferenceAxisEnd.Y - brush.Center.Y;
              float rotation = MathF.Atan2(refDy, refDx);
              float referenceRadius = MathF.Sqrt((refDx * refDx) + (refDy * refDy));
              float secondRadius = referenceRadius * brush.AxisRatio;

              this.referenceRadiusSquared = referenceRadius * referenceRadius;
              this.secondRadiusSquared = secondRadius * secondRadius;
              this.sinRotation = MathF.Sin(rotation);
              this.cosRotation = MathF.Cos(rotation);
          }

          /// <inheritdoc />
          protected override float PositionOnGradient(float x, float y)
          {
              float x0 = x - this.center.X;
              float y0 = y - this.center.Y;

              float xR = (x0 * this.cosRotation) - (y0 * this.sinRotation);
              float yR = (x0 * this.sinRotation) + (y0 * this.cosRotation);

              float xSquared = xR * xR;
              float ySquared = yR * yR;

              return MathF.Sqrt((xSquared / this.referenceRadiusSquared) + (ySquared / this.secondRadiusSquared));
          }
      }
  }
}