Fix Grayscale param order

src/ImageSharp/Processing/ColorMatrix/Grayscale.cs

@@ -5,8 +5,6 @@
 
 namespace ImageSharp
 {
-    using System;
-
     using ImageSharp.PixelFormats;
 
     using ImageSharp.Processing;
@@ -17,6 +15,18 @@ namespace ImageSharp
     /// </summary>
     public static partial class ImageExtensions
     {
+        /// <summary>
+        /// Applies <see cref="GrayscaleMode.Bt709"/> Grayscale toning to the image.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            return Grayscale(source, GrayscaleMode.Bt709);
+        }
+
         /// <summary>
         /// Applies Grayscale toning to the image.
         /// </summary>
@@ -24,23 +34,41 @@ namespace ImageSharp
         /// <param name="source">The image this method extends.</param>
         /// <param name="mode">The formula to apply to perform the operation.</param>
         /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source, GrayscaleMode mode = GrayscaleMode.Bt709)
+        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source, GrayscaleMode mode)
             where TPixel : struct, IPixel<TPixel>
         {
-            return Grayscale(source, source.Bounds, mode);
+            return Grayscale(source, mode, source.Bounds);
         }
 
         /// <summary>
-        /// Applies Grayscale toning to the image.
+        /// Applies <see cref="GrayscaleMode.Bt709"/> Grayscale toning to the image.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="rectangle">
         /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
         /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            IImageProcessor<TPixel> processor = new GrayscaleBt709Processor<TPixel>();
+
+            source.ApplyProcessor(processor, rectangle);
+            return source;
+        }
+
+        /// <summary>
+        /// Applies Grayscale toning to the image.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
         /// <param name="mode">The formula to apply to perform the operation.</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
         /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source, Rectangle rectangle, GrayscaleMode mode = GrayscaleMode.Bt709)
+        public static Image<TPixel> Grayscale<TPixel>(this Image<TPixel> source, GrayscaleMode mode, Rectangle rectangle)
             where TPixel : struct, IPixel<TPixel>
         {
             IImageProcessor<TPixel> processor = mode == GrayscaleMode.Bt709

tests/ImageSharp.Tests/Processing/ColorMatrix/GrayscaleTest.cs

@@ -51,7 +51,7 @@ namespace ImageSharp.Tests.Processing.ColorMatrix
             using (var image = new Image<TPixel>(source))
             {
                 var bounds = new Rectangle(image.Width / 4, image.Height / 4, image.Width / 2, image.Height / 2);
-                image.Grayscale(bounds, value)
+                image.Grayscale(value, bounds)
                      .DebugSave(provider, value.ToString());
 
                 // Draw identical shapes over the bounded and compare to ensure changes are constrained.