add save pixel data extension method

src/ImageSharp/Image/ImageExtensions.cs

@@ -4,7 +4,9 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Runtime.CompilerServices;
 using System.Text;
+using SixLabors.ImageSharp.Advanced.Unsafe;
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Primitives;
@@ -129,6 +131,22 @@ namespace SixLabors.ImageSharp
             source.Save(stream, encoder);
         }
 
+        /// <summary>
+        /// Saves the raw image to the given bytes.
+        /// </summary>
+        /// <typeparam name="TPixel">The Pixel format.</typeparam>
+        /// <param name="source">The source image</param>
+        /// <param name="buffer">The buffer to save the raw pixel data to.</param>
+        /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
+        public static void SavePixelData<TPixel>(this Image<TPixel> source, Span<byte> buffer)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            Span<byte> byteBuffer = source.GetPixelSpan().AsBytes();
+            Guard.MustBeGreaterThanOrEqualTo(buffer.Length, byteBuffer.Length, nameof(buffer));
+
+            byteBuffer.CopyTo(buffer);
+        }
+
         /// <summary>
         /// Returns a Base64 encoded string from the given image.
         /// </summary>

tests/ImageSharp.Tests/Image/ImageLoadTests.cs

@@ -318,6 +318,7 @@ namespace SixLabors.ImageSharp.Tests
             }
         }
 
+
         [Fact]
         public void LoadsImageWithoutThrowingCrcException()
         {

tests/ImageSharp.Tests/Image/ImageSaveTests.cs

@@ -47,6 +47,93 @@ namespace SixLabors.ImageSharp.Tests
             this.Image = new Image<Rgba32>(config, 1, 1);
         }
 
+        [Fact]
+        public void SavePixelData_Rgba32()
+        {
+            using (var img = new Image<Rgba32>(2, 2))
+            {
+                img[0, 0] = Rgba32.White;
+                img[1, 0] = Rgba32.Black;
+
+                img[0, 1] = Rgba32.Red;
+                img[1, 1] = Rgba32.Blue;
+                var buffer = new byte[2 * 2 * 4]; // width * height * bytes per pixel
+                img.SavePixelData(buffer);
+
+                Assert.Equal(255, buffer[0]); // 0, 0, R
+                Assert.Equal(255, buffer[1]); // 0, 0, G
+                Assert.Equal(255, buffer[2]); // 0, 0, B
+                Assert.Equal(255, buffer[3]); // 0, 0, A
+
+                Assert.Equal(0, buffer[4]); // 1, 0, R
+                Assert.Equal(0, buffer[5]); // 1, 0, G
+                Assert.Equal(0, buffer[6]); // 1, 0, B
+                Assert.Equal(255, buffer[7]); // 1, 0, A
+
+                Assert.Equal(255, buffer[8]); // 0, 1, R
+                Assert.Equal(0, buffer[9]); // 0, 1, G
+                Assert.Equal(0, buffer[10]); // 0, 1, B
+                Assert.Equal(255, buffer[11]); // 0, 1, A
+
+                Assert.Equal(0, buffer[12]); // 1, 1, R
+                Assert.Equal(0, buffer[13]); // 1, 1, G
+                Assert.Equal(255, buffer[14]); // 1, 1, B
+                Assert.Equal(255, buffer[15]); // 1, 1, A
+            }
+        }
+
+
+        [Fact]
+        public void SavePixelData_Bgr24()
+        {
+            using (var img = new Image<Bgr24>(2, 2))
+            {
+                img[0, 0] = NamedColors<Bgr24>.White;
+                img[1, 0] = NamedColors<Bgr24>.Black;
+
+                img[0, 1] = NamedColors<Bgr24>.Red;
+                img[1, 1] = NamedColors<Bgr24>.Blue;
+
+                var buffer = new byte[2 * 2 * 3]; // width * height * bytes per pixel
+                img.SavePixelData(buffer);
+
+                Assert.Equal(255, buffer[0]); // 0, 0, B
+                Assert.Equal(255, buffer[1]); // 0, 0, G
+                Assert.Equal(255, buffer[2]); // 0, 0, R
+
+                Assert.Equal(0, buffer[3]); // 1, 0, B
+                Assert.Equal(0, buffer[4]); // 1, 0, G
+                Assert.Equal(0, buffer[5]); // 1, 0, R
+
+                Assert.Equal(0, buffer[6]); // 0, 1, B
+                Assert.Equal(0, buffer[7]); // 0, 1,   G
+                Assert.Equal(255, buffer[8]); // 0, 1,  R
+
+                Assert.Equal(255, buffer[9]); // 1, 1,  B
+                Assert.Equal(0, buffer[10]); // 1, 1,  G
+                Assert.Equal(0, buffer[11]); // 1, 1, R
+            }
+        }
+
+        [Fact]
+        public void SavePixelData_Rgba32_Buffer_must_be_bigger()
+        {
+            using (var img = new Image<Rgba32>(2, 2))
+            {
+                img[0, 0] = Rgba32.White;
+                img[1, 0] = Rgba32.Black;
+
+                img[0, 1] = Rgba32.Red;
+                img[1, 1] = Rgba32.Blue;
+                var buffer = new byte[2 * 2]; // width * height * bytes per pixel
+
+                Assert.Throws<ArgumentOutOfRangeException>(() =>
+                {
+                    img.SavePixelData(buffer);
+                });
+            }
+        }
+
         [Fact]
         public void SavePath()
         {