ImageSharp/ImageSharp.Drawing/OutlinePathExtensions.cs

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

using SixLabors.ImageSharp.Drawing.PolygonGeometry;
using SixLabors.ImageSharp.Drawing.Processing;
using System;

namespace SixLabors.ImageSharp.Drawing {
  /// <summary>
  /// Extensions to <see cref="IPath"/> that allow the generation of outlines.
  /// </summary>
  public static class OutlinePathExtensions
  {
      private static readonly StrokeOptions DefaultOptions = new();

      /// <summary>
      /// Generates an outline of the path.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(this IPath path, float width)
          => GenerateOutline(path, width, DefaultOptions);

      /// <summary>
      /// Generates an outline of the path.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <param name="strokeOptions">The stroke geometry options.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(this IPath path, float width, StrokeOptions strokeOptions)
      {
          if (width <= 0)
          {
              return Path.Empty;
          }

          return StrokedShapeGenerator.GenerateStrokedShapes(path, width, strokeOptions);
      }

      /// <summary>
      /// Generates an outline of the path with alternating on and off segments based on the pattern.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <param name="pattern">The pattern made of multiples of the width.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(this IPath path, float width, ReadOnlySpan<float> pattern)
          => path.GenerateOutline(width, pattern, false);

      /// <summary>
      /// Generates an outline of the path with alternating on and off segments based on the pattern.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <param name="pattern">The pattern made of multiples of the width.</param>
      /// <param name="strokeOptions">The stroke geometry options.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(this IPath path, float width, ReadOnlySpan<float> pattern, StrokeOptions strokeOptions)
          => GenerateOutline(path, width, pattern, false, strokeOptions);

      /// <summary>
      /// Generates an outline of the path with alternating on and off segments based on the pattern.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <param name="pattern">The pattern made of multiples of the width.</param>
      /// <param name="startOff">Whether the first item in the pattern is on or off.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(this IPath path, float width, ReadOnlySpan<float> pattern, bool startOff)
          => GenerateOutline(path, width, pattern, startOff, DefaultOptions);

      /// <summary>
      /// Generates an outline of the path with alternating on and off segments based on the pattern.
      /// </summary>
      /// <param name="path">The path to outline</param>
      /// <param name="width">The outline width.</param>
      /// <param name="pattern">The pattern made of multiples of the width.</param>
      /// <param name="startOff">Whether the first item in the pattern is on or off.</param>
      /// <param name="strokeOptions">The stroke geometry options.</param>
      /// <returns>A new <see cref="IPath"/> representing the outline.</returns>
      public static IPath GenerateOutline(
          this IPath path,
          float width,
          ReadOnlySpan<float> pattern,
          bool startOff,
          StrokeOptions strokeOptions)
      {
          if (width <= 0)
          {
              return Path.Empty;
          }

          if (pattern.Length < 2)
          {
              return path.GenerateOutline(width, strokeOptions);
          }

          IPath dashed = path.GenerateDashes(width, pattern, startOff);

          // GenerateDashes returns the original path when the pattern is degenerate
          // or when segmentation would exceed safety limits; stroke it as solid.
          if (ReferenceEquals(dashed, path))
          {
              return path.GenerateOutline(width, strokeOptions);
          }

          if (dashed == Path.Empty)
          {
              return Path.Empty;
          }

          // Each dash segment is an open sub-path; stroke expansion and boolean merge
          // are handled by the generator.
          return StrokedShapeGenerator.GenerateStrokedShapes(dashed, width, strokeOptions);
      }
  }
}