Fix #1044

Author: JimBobSquarePants

src/ImageSharp.Drawing/Processing/GradientBrush.cs

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-
+using System.Numerics;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Primitives;
 
@@ -120,10 +120,8 @@ protected GradientBrushApplicator(
                         float onLocalGradient = (positionOnCompleteGradient - from.Ratio) / (to.Ratio - from.Ratio);
 
                         // TODO: this should be changeble for different gradienting functions.
-                        // TODO: Why not use Blender property?
-                        return PixelOperations<TPixel>
-                            .Instance.GetPixelBlender(PixelColorBlendingMode.Normal, PixelAlphaCompositionMode.SrcOver)
-                            .Blend(from.Color.ToPixel<TPixel>(), to.Color.ToPixel<TPixel>(), onLocalGradient);
+                        // TODO: Is the comment above still relevent?
+                        return new Color(Vector4.Lerp((Vector4)from.Color, (Vector4)to.Color, onLocalGradient)).ToPixel<TPixel>();
                     }
                 }
             }

src/ImageSharp/ImageSharp.csproj

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
@@ -119,7 +119,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <!--<InternalsVisibleTo Include="SixLabors.ImageSharp.Drawing" />-->
+    <InternalsVisibleTo Include="SixLabors.ImageSharp.Drawing" />
   </ItemGroup>
 
   <ItemGroup>

tests/ImageSharp.Tests/Drawing/FillLinearGradientBrushTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors and contributors.
+// Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -16,6 +16,8 @@ namespace SixLabors.ImageSharp.Tests.Drawing
 {
     using SixLabors.ImageSharp.Advanced;
     using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
+    using SixLabors.Primitives;
+    using SixLabors.Shapes;
 
     [GroupOutput("Drawing/GradientBrushes")]
     public class FillLinearGradientBrushTests
@@ -392,5 +394,43 @@ public void MultiplePointGradients<TPixel>(
                 false,
                 false);
         }
+
+        [Theory]
+        [WithBlankImages(200, 200, PixelTypes.Rgba32)]
+        public void GradientsWithTransparencyOnExistingBackground<TPixel>(TestImageProvider<TPixel> provider)
+                    where TPixel : struct, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                image =>
+                {
+                    image.Mutate(i => i.Fill(Color.Red));
+                    image.Mutate(ApplyGloss);
+
+                });
+
+            void ApplyGloss(IImageProcessingContext ctx)
+            {
+                Size size = ctx.GetCurrentSize();
+                IPathCollection glossPath = BuildGloss(size.Width, size.Height);
+                var graphicsOptions = new GraphicsOptions(true)
+                {
+                    ColorBlendingMode = PixelColorBlendingMode.Normal,
+                    AlphaCompositionMode = PixelAlphaCompositionMode.SrcAtop
+                };
+                var linearGradientBrush = new LinearGradientBrush(new Point(0, 0), new Point(0, size.Height / 2), GradientRepetitionMode.Repeat, new ColorStop(0, Color.White.WithAlpha(0.5f)), new ColorStop(1, Color.White.WithAlpha(0.25f)));
+                ctx.Fill(graphicsOptions, linearGradientBrush, glossPath);
+            }
+
+            IPathCollection BuildGloss(int imageWidth, int imageHeight)
+            {
+                var pathBuilder = new PathBuilder();
+                pathBuilder.AddLine(new PointF(0, 0), new PointF(imageWidth, 0));
+                pathBuilder.AddLine(new PointF(imageWidth, 0), new PointF(imageWidth, imageHeight * 0.4f));
+                pathBuilder.AddBezier(new PointF(imageWidth, imageHeight * 0.4f), new PointF(imageWidth / 2, imageHeight * 0.6f), new PointF(0, imageHeight * 0.4f));
+                pathBuilder.CloseFigure();
+                return new PathCollection(pathBuilder.Build());
+            }
+        }
+
     }
-}
+}