Added NegateGrayscale to MagickImage.

src/Magick.NET.Core/IMagickImage.cs

@@ -2398,6 +2398,19 @@ public partial interface IMagickImage : IDisposable
         /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
         void Negate(Channels channels);
 
+        /// <summary>
+        /// Negate the grayscale colors in image.
+        /// </summary>
+        /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
+        void NegateGrayscale();
+
+        /// <summary>
+        /// Negate the grayscale colors in image for the specified channel.
+        /// </summary>
+        /// <param name="channels">The channel(s) that should be negated.</param>
+        /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
+        void NegateGrayscale(Channels channels);
+
         /// <summary>
         /// Normalize image (increase contrast by normalizing the pixel values to span the full range
         /// of color values).

src/Magick.NET/MagickImage.cs

@@ -4521,6 +4521,21 @@ public void Negate(bool onlyGrayscale, Channels channels)
         public void Negate(Channels channels)
             => Negate(false, channels);
 
+        /// <summary>
+        /// Negate the grayscale colors in image.
+        /// </summary>
+        /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
+        public void NegateGrayscale()
+            => NegateGrayscale(ImageMagick.Channels.Composite);
+
+        /// <summary>
+        /// Negate the grayscale colors in image for the specified channel.
+        /// </summary>
+        /// <param name="channels">The channel(s) that should be negated.</param>
+        /// <exception cref="MagickException">Thrown when an error is raised by ImageMagick.</exception>
+        public void NegateGrayscale(Channels channels)
+            => _nativeInstance.Negate(true, channels);
+
         /// <summary>
         /// Normalize image (increase contrast by normalizing the pixel values to span the full range
         /// of color values).

tests/Magick.NET.Tests/MagickImageTests/TheNegateGrayscaleMethod.cs

@@ -0,0 +1,52 @@
+// Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
+// Licensed under the Apache License, Version 2.0.
+
+using ImageMagick;
+using Xunit;
+
+namespace Magick.NET.Tests
+{
+    public partial class MagickImageTests
+    {
+        public class TheNegateGrayscaleMethod
+        {
+            [Fact]
+            public void ShouldOnlyNegateGrayscale()
+            {
+                using (var image = new MagickImage("xc:white", 2, 1))
+                {
+                    using (var pixels = image.GetPixels())
+                    {
+                        var pixel = pixels.GetPixel(1, 0);
+                        pixel.SetChannel(1, 0);
+                        pixel.SetChannel(2, 0);
+                    }
+
+                    image.NegateGrayscale();
+
+                    ColorAssert.Equal(MagickColors.Black, image, 0, 0);
+                    ColorAssert.Equal(MagickColors.Red, image, 1, 0);
+                }
+            }
+
+            [Fact]
+            public void ShouldNegateTheSpecifedChannels()
+            {
+                using (var image = new MagickImage("xc:white", 2, 1))
+                {
+                    using (var pixels = image.GetPixels())
+                    {
+                        var pixel = pixels.GetPixel(1, 0);
+                        pixel.SetChannel(1, 0);
+                        pixel.SetChannel(2, 0);
+                    }
+
+                    image.NegateGrayscale(Channels.Green);
+
+                    ColorAssert.Equal(MagickColors.Fuchsia, image, 0, 0);
+                    ColorAssert.Equal(MagickColors.Red, image, 1, 0);
+                }
+            }
+        }
+    }
+}

tests/Magick.NET.Tests/MagickImageTests/TheNegateMethod.cs

@@ -10,25 +10,25 @@ public partial class MagickImageTests
     {
         public class TheNegateMethod
         {
-            public class WithBoolean
+            [Fact]
+            public void ShouldNegateTheImage()
             {
-                [Fact]
-                public void ShouldOnlyNegateGrayscaleWhenSetToTrue()
+                using (var image = new MagickImage("xc:white", 1, 1))
                 {
-                    using (var image = new MagickImage("xc:white", 2, 1))
-                    {
-                        using (var pixels = image.GetPixels())
-                        {
-                            var pixel = pixels.GetPixel(1, 0);
-                            pixel.SetChannel(1, 0);
-                            pixel.SetChannel(2, 0);
-                        }
+                    image.Negate();
 
-                        image.Negate(true);
+                    ColorAssert.Equal(MagickColors.Black, image, 0, 0);
+                }
+            }
+
+            [Fact]
+            public void ShouldNegateTheSpecifedChannels()
+            {
+                using (var image = new MagickImage("xc:white", 1, 1))
+                {
+                    image.Negate(Channels.Red);
 
-                        ColorAssert.Equal(MagickColors.Black, image, 0, 0);
-                        ColorAssert.Equal(MagickColors.Red, image, 1, 0);
-                    }
+                    ColorAssert.Equal(MagickColors.Aqua, image, 0, 0);
                 }
             }
         }