Fix CSS linear gradient stroke offsets

tests/Svg.Skia.UnitTests/Issue441Tests.cs

@@ -0,0 +1,135 @@
+using System.Linq;
+using ShimSkiaSharp;
+using ShimSkiaSharp.Editing;
+using SkiaSharp;
+using Xunit;
+using ShimPaintStyle = ShimSkiaSharp.SKPaintStyle;
+using ShimPoint = ShimSkiaSharp.SKPoint;
+using SkiaAlphaType = SkiaSharp.SKAlphaType;
+using SkiaBitmap = SkiaSharp.SKBitmap;
+using SkiaColor = SkiaSharp.SKColor;
+using SkiaColorSpace = SkiaSharp.SKColorSpace;
+using SkiaColorType = SkiaSharp.SKColorType;
+using SkiaPicture = SkiaSharp.SKPicture;
+
+namespace Svg.Skia.UnitTests;
+
+public class Issue441Tests
+{
+    [Fact]
+    public void CssLinearGradientStroke_ResolvesStylesheetPaintServerAndRendersStroke()
+    {
+        using var svg = new SKSvg();
+        svg.FromSvg(Issue441Svg);
+
+        Assert.NotNull(svg.Model);
+        Assert.NotNull(svg.Picture);
+
+        var gradientStroke = svg.Model!
+            .FindCommands<DrawPathCanvasCommand>()
+            .Single(command => command.Paint?.Shader is LinearGradientShader);
+        AssertIssue441GradientStroke(gradientStroke);
+
+        var retainedModel = svg.CreateRetainedSceneGraphModel();
+        Assert.NotNull(retainedModel);
+
+        var retainedGradientStroke = retainedModel!
+            .FindCommands<DrawPathCanvasCommand>()
+            .Single(command => command.Paint?.Shader is LinearGradientShader);
+        AssertIssue441GradientStroke(retainedGradientStroke);
+
+        using var bitmap = RenderBitmap(svg.Picture!, 4f);
+        AssertContainsYellowPixel(bitmap, 16, 16, 24, 28);
+    }
+
+    private static void AssertIssue441GradientStroke(DrawPathCanvasCommand gradientStroke)
+    {
+        var shader = Assert.IsType<LinearGradientShader>(gradientStroke.Paint!.Shader);
+        Assert.Equal(ShimPaintStyle.Stroke, gradientStroke.Paint.Style);
+        Assert.Equal(3f, gradientStroke.Paint.StrokeWidth);
+        Assert.Equal(new ShimPoint(6.5f, 5f), shader.Start);
+        Assert.Equal(new ShimPoint(6.5f, 17f), shader.End);
+        Assert.NotNull(shader.Colors);
+        Assert.NotNull(shader.ColorPos);
+        Assert.Equal(6, shader.Colors!.Length);
+        Assert.Equal(shader.Colors.Length, shader.ColorPos!.Length);
+        AssertGradientStops(shader.ColorPos);
+        AssertYellow(shader.Colors[0]);
+    }
+
+    private static void AssertGradientStops(float[] colorPos)
+    {
+        float[] expected = [0.16f, 0.33f, 0.72f, 0.86f, 0.9f, 1f];
+
+        Assert.Equal(expected.Length, colorPos.Length);
+        for (var i = 0; i < expected.Length; i++)
+        {
+            Assert.Equal(expected[i], colorPos[i], precision: 3);
+        }
+    }
+
+    private static void AssertYellow(ShimSkiaSharp.SKColorF color)
+    {
+        Assert.True(color.Red > 0.9f, $"Expected yellow red channel but got {color}.");
+        Assert.True(color.Green > 0.9f, $"Expected yellow green channel but got {color}.");
+        Assert.True(color.Blue < 0.2f, $"Expected yellow blue channel but got {color}.");
+    }
+
+    private static void AssertContainsYellowPixel(SkiaBitmap bitmap, int left, int top, int width, int height)
+    {
+        for (var y = top; y < top + height; y++)
+        {
+            for (var x = left; x < left + width; x++)
+            {
+                var color = bitmap.GetPixel(x, y);
+                if (IsYellowPixel(color))
+                {
+                    return;
+                }
+            }
+        }
+
+        Assert.Fail("Expected the rendered CSS gradient stroke to contain a visible yellow pixel.");
+    }
+
+    private static bool IsYellowPixel(SkiaColor color)
+    {
+        return color.Alpha > 180
+            && color.Red > 180
+            && color.Green > 170
+            && color.Blue < 100;
+    }
+
+    private static SkiaBitmap RenderBitmap(SkiaPicture picture, float scale)
+    {
+        var bitmap = picture.ToBitmap(
+            SKColors.Transparent,
+            scale,
+            scale,
+            SkiaColorType.Rgba8888,
+            SkiaAlphaType.Unpremul,
+            SkiaColorSpace.CreateSrgb());
+
+        return Assert.IsType<SkiaBitmap>(bitmap);
+    }
+
+    private const string Issue441Svg = """
+        <?xml version="1.0" encoding="UTF-8"?>
+        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 24">
+          <defs>
+            <style>
+              .cls-3{fill:none;stroke:url(#linear-gradient);stroke-width:3px;stroke-miterlimit:10;}
+            </style>
+            <linearGradient id="linear-gradient" x1="6.5" y1="5" x2="6.5" y2="17" gradientUnits="userSpaceOnUse">
+              <stop offset=".16" stop-color="#ffff29"/>
+              <stop offset=".33" stop-color="#cfce26"/>
+              <stop offset=".72" stop-color="#585421"/>
+              <stop offset=".86" stop-color="#2a2620"/>
+              <stop offset=".9" stop-color="#1e1b17"/>
+              <stop offset="1" stop-color="#000"/>
+            </linearGradient>
+          </defs>
+          <line class="cls-3" x1="6.5" y1="5" x2="6.5" y2="17"/>
+        </svg>
+        """;
+}