SKShaper: Support SKPaint.TextAlign property (#1910)

Co-authored-by: Tal Keren <tal@keren.dev>
Co-authored-by: Matthew Leibowitz <mattleibow@live.com>

source/SkiaSharp.HarfBuzz/SkiaSharp.HarfBuzz.Shared/CanvasExtensions.cs

@@ -53,8 +53,17 @@ namespace SkiaSharp.HarfBuzz
 			// build
 			using var textBlob = builder.Build();
 
+			// adjust alignment
+			var xOffset = 0f;
+			if (paint.TextAlign != SKTextAlign.Left) {
+				var width = result.Width;
+				if (paint.TextAlign == SKTextAlign.Center)
+					width *= 0.5f;
+				xOffset -= width;
+			}
+
 			// draw the text
-			canvas.DrawText(textBlob, 0, 0, paint);
+			canvas.DrawText(textBlob, xOffset, 0, paint);
 		}
 	}
 }

source/SkiaSharp.HarfBuzz/SkiaSharp.HarfBuzz.Shared/SKShaper.cs

@@ -70,6 +70,7 @@ namespace SkiaSharp.HarfBuzz
 			var points = new SKPoint[len];
 			var clusters = new uint[len];
 			var codepoints = new uint[len];
+			var xOffsetStart = xOffset;
 
 			for (var i = 0; i < len; i++)
 			{
@@ -86,7 +87,9 @@ namespace SkiaSharp.HarfBuzz
 				yOffset += pos[i].YAdvance * textSizeY;
 			}
 
-			return new Result(codepoints, clusters, points);
+			var width = xOffset - xOffsetStart;
+
+			return new Result(codepoints, clusters, points, width);
 		}
 
 		public Result Shape(string text, SKPaint paint) =>
@@ -129,6 +132,7 @@ namespace SkiaSharp.HarfBuzz
 				Codepoints = new uint[0];
 				Clusters = new uint[0];
 				Points = new SKPoint[0];
+				Width = 0f;
 			}
 
 			public Result(uint[] codepoints, uint[] clusters, SKPoint[] points)
@@ -136,13 +140,24 @@ namespace SkiaSharp.HarfBuzz
 				Codepoints = codepoints;
 				Clusters = clusters;
 				Points = points;
+				Width = 0;
 			}
 
-			public uint[] Codepoints { get; private set; }
+			public Result(uint[] codepoints, uint[] clusters, SKPoint[] points, float width)
+			{
+				Codepoints = codepoints;
+				Clusters = clusters;
+				Points = points;
+				Width = width;
+			}
 
-			public uint[] Clusters { get; private set; }
+			public uint[] Codepoints { get; }
 
-			public SKPoint[] Points { get; private set; }
+			public uint[] Clusters { get; }
+
+			public SKPoint[] Points { get; }
+
+			public float Width { get; }
 		}
 	}
 }

tests/Tests/SKShaperTest.cs

@@ -101,5 +101,52 @@ namespace SkiaSharp.HarfBuzz.Tests
 				return new Blob(data, size, MemoryMode.Writeable, () => Marshal.FreeCoTaskMem(data));
 			}
 		}
+
+		[SkippableTheory]
+		[InlineData(SKTextAlign.Left, 300)]
+		[InlineData(SKTextAlign.Center, 162)]
+		[InlineData(SKTextAlign.Right, 23)]
+		public void TextAlignMovesTextPosition(SKTextAlign align, int offset)
+		{
+			var font = Path.Combine(PathToFonts, "segoeui.ttf");
+			using var tf = SKTypeface.FromFile(font);
+
+			using var bitmap = new SKBitmap(600, 200);
+			using var canvas = new SKCanvas(bitmap);
+
+			canvas.Clear(SKColors.White);
+
+			using var paint = new SKPaint();
+			paint.Typeface = tf;
+			paint.IsAntialias = true;
+			paint.TextSize = 64;
+			paint.Color = SKColors.Black;
+			paint.TextAlign = align;
+
+			canvas.DrawShapedText("SkiaSharp", 300, 100, paint);
+
+			AssertTextAlign(bitmap, offset, 0);
+		}
+
+		private static void AssertTextAlign(SKBitmap bitmap, int x, int y)
+		{
+			// [S]kia[S]har[p]
+
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 6, y + 66));
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 28, y + 87));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 28, y + 66));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 6, y + 87));
+
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 120, y + 66));
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 142, y + 87));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 142, y + 66));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 120, y + 87));
+
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 246, y + 70));
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 246, y + 113));
+			Assert.Equal(SKColors.Black, bitmap.GetPixel(x + 271, y + 83));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 258, y + 83));
+			Assert.Equal(SKColors.White, bitmap.GetPixel(x + 258, y + 113));
+		}
 	}
 }