Fix #997

src/ImageSharp/ColorSpaces/Conversion/Implementation/Converters/HslAndRgbConverter.cs

@@ -96,7 +96,7 @@ namespace SixLabors.ImageSharp.ColorSpaces.Conversion.Implementation
             }
             else
             {
-                s = chroma / (2F - chroma);
+                s = chroma / (2F - max - min);
             }
 
             return new Hsl(h, s, l);
@@ -157,4 +157,4 @@ namespace SixLabors.ImageSharp.ColorSpaces.Conversion.Implementation
             return value;
         }
     }
-}
+}

tests/ImageSharp.Tests/Colorspaces/Conversion/RgbAndHslConversionTest.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;
@@ -65,6 +65,7 @@ namespace SixLabors.ImageSharp.Tests.Colorspaces.Conversion
         [InlineData(1, 0, 0, 0, 1, .5F)]
         [InlineData(0, 1, 0, 120, 1, .5F)]
         [InlineData(0, 0, 1, 240, 1, .5F)]
+        [InlineData(0.7, 0.8, 0.6, 90, 0.3333, 0.7F)]
         public void Convert_Rgb_To_Hsl(float r, float g, float b, float h, float s, float l)
         {
             // Arrange