ImageSharp/SixLabors.Fonts/PlaceholderGlyphMetrics.cs

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

using System;
using System.Numerics;
using SixLabors.Fonts.Rendering;
using SixLabors.Fonts.Unicode;

namespace SixLabors.Fonts {
  /// <summary>
  /// Represents synthetic glyph metrics for an atomic inline placeholder.
  /// </summary>
  internal sealed class PlaceholderGlyphMetrics : FontGlyphMetrics
  {
      private readonly TextPlaceholder placeholder;
      private readonly float pointSize;
      private readonly float dpi;

      /// <summary>
      /// Initializes a new instance of the <see cref="PlaceholderGlyphMetrics"/> class.
      /// </summary>
      /// <param name="font">The font metrics used for shared line metrics and decoration settings.</param>
      /// <param name="placeholder">The placeholder dimensions and alignment settings.</param>
      /// <param name="pointSize">The point size used for layout.</param>
      /// <param name="dpi">The resolution used to convert placeholder pixels into layout units.</param>
      /// <param name="textRun">The text run this placeholder belongs to.</param>
      internal PlaceholderGlyphMetrics(
          StreamFontMetrics font,
          TextPlaceholder placeholder,
          float pointSize,
          float dpi,
          TextRun textRun)
          : base(
              font,
              0,
              CodePoint.ObjectReplacementChar,
              GetBounds(placeholder, pointSize, dpi, font),
              ToGlyphUnits(placeholder.Width, pointSize, dpi, font.ScaleFactor),
              ToGlyphUnits(placeholder.Height, pointSize, dpi, font.ScaleFactor),
              0,
              0,
              font.UnitsPerEm,
              Vector2.Zero,
              new Vector2(font.ScaleFactor),
              textRun,
              GlyphType.Placeholder)
      {
          this.placeholder = placeholder;
          this.pointSize = pointSize;
          this.dpi = dpi;
      }

      /// <inheritdoc/>
      internal override FontGlyphMetrics CloneForRendering(TextRun textRun)
          => new PlaceholderGlyphMetrics(
              this.FontMetrics,
              this.placeholder,
              this.pointSize,
              this.dpi,
              textRun);

      /// <inheritdoc/>
      internal override void RenderTo(
          IGlyphRenderer renderer,
          int graphemeIndex,
          Vector2 glyphOrigin,
          Vector2 decorationOrigin,
          GlyphLayoutMode mode,
          TextOptions options)
      {
          // Placeholders reserve layout space only; the caller owns the object rendering.
      }

      /// <summary>
      /// Converts the placeholder box into glyph bounds in the same synthetic font-unit space as its advances.
      /// </summary>
      /// <param name="placeholder">The placeholder dimensions and baseline offset.</param>
      /// <param name="pointSize">The point size used for layout.</param>
      /// <param name="dpi">The resolution used to convert placeholder pixels into layout units.</param>
      /// <param name="font">The font metrics used by glyph layout.</param>
      /// <returns>The placeholder bounds expressed in synthetic font units.</returns>
      private static Bounds GetBounds(TextPlaceholder placeholder, float pointSize, float dpi, StreamFontMetrics font)
      {
          float scaleFactor = font.ScaleFactor;
          float width = ToGlyphUnitsFloat(placeholder.Width, pointSize, dpi, scaleFactor);
          float height = ToGlyphUnitsFloat(placeholder.Height, pointSize, dpi, scaleFactor);
          float baselineOffset = ToGlyphUnitsFloat(placeholder.BaselineOffset, pointSize, dpi, scaleFactor);
          float lineHeight = font.UnitsPerEm;
          float metricsDelta = (font.HorizontalMetrics.LineHeight - lineHeight) * .5F;
          float ascender = font.HorizontalMetrics.Ascender - metricsDelta;
          float descender = Math.Abs(font.HorizontalMetrics.Descender) - metricsDelta;
          float coreHeight = ascender + descender + (2 * metricsDelta);
          float extra = lineHeight - coreHeight;

          // Top/middle/bottom align against the surrounding run font's normal
          // line box, expressed relative to the text baseline in Y-up font units.
          float lineTop = ascender + metricsDelta + (extra * .5F);
          float lineBottom = lineTop - lineHeight;
          float top = baselineOffset;
          float bottom = baselineOffset - height;

          switch (placeholder.Alignment)
          {
              case TextPlaceholderAlignment.AboveBaseline:
                  top = height;
                  bottom = 0;
                  break;

              case TextPlaceholderAlignment.BelowBaseline:
                  top = 0;
                  bottom = -height;
                  break;

              case TextPlaceholderAlignment.Top:
                  top = lineTop;
                  bottom = top - height;
                  break;

              case TextPlaceholderAlignment.Bottom:
                  // Top, middle, and bottom align against the full line-height
                  // box, not just the ascender/descender band.
                  bottom = lineBottom;
                  top = bottom + height;
                  break;

              case TextPlaceholderAlignment.Middle:
                  float center = (lineTop + lineBottom) * .5F;
                  top = center + (height * .5F);
                  bottom = center - (height * .5F);
                  break;

              default:
                  top = baselineOffset;
                  bottom = baselineOffset - height;
                  break;
          }

          // Placeholder bounds are authored in device pixels and converted into
          // synthetic font units so the normal glyph scaling path maps them back
          // to device-space size while preserving the requested baseline alignment.
          return new Bounds(0, top, width, bottom);
      }

      /// <summary>
      /// Converts a placeholder pixel measurement into glyph units for the current layout scale.
      /// </summary>
      /// <param name="pixels">The placeholder measurement in pixels.</param>
      /// <param name="pointSize">The point size used for layout.</param>
      /// <param name="dpi">The resolution used to convert placeholder pixels into layout units.</param>
      /// <param name="scaleFactor">The font scale factor used by glyph layout.</param>
      /// <returns>The measurement expressed in synthetic font units.</returns>
      private static ushort ToGlyphUnits(float pixels, float pointSize, float dpi, float scaleFactor)
          => (ushort)MathF.Round(ToGlyphUnitsFloat(pixels, pointSize, dpi, scaleFactor));

      /// <summary>
      /// Converts a placeholder pixel measurement into fractional glyph units for bounds placement.
      /// </summary>
      /// <param name="pixels">The placeholder measurement in pixels.</param>
      /// <param name="pointSize">The point size used for layout.</param>
      /// <param name="dpi">The resolution used to convert placeholder pixels into layout units.</param>
      /// <param name="scaleFactor">The font scale factor used by glyph layout.</param>
      /// <returns>The measurement expressed in synthetic font units.</returns>
      private static float ToGlyphUnitsFloat(float pixels, float pointSize, float dpi, float scaleFactor)
          => pixels * scaleFactor / (pointSize * dpi);
  }
}