Optimization: do not initialize pixel buffer in JpegDecoder

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -936,12 +936,18 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         private Image<TPixel> PostProcessIntoImage<TPixel>()
             where TPixel : struct, IPixel<TPixel>
         {
+            var image = Image.CreateUninitialized<TPixel>(
+                this.configuration,
+                this.ImageWidth,
+                this.ImageHeight,
+                this.MetaData);
+
             using (var postProcessor = new JpegImagePostProcessor(this.configuration, this))
             {
-                var image = new Image<TPixel>(this.configuration, this.ImageWidth, this.ImageHeight, this.MetaData);
                 postProcessor.PostProcess(image.Frames.RootFrame);
-                return image;
             }
+
+            return image;
         }
     }
 }

src/ImageSharp/Image.Decode.cs

@@ -5,6 +5,7 @@ using System.IO;
 using System.Linq;
 using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Memory;
 
@@ -15,6 +16,29 @@ namespace SixLabors.ImageSharp
     /// </content>
     public static partial class Image
     {
+        /// <summary>
+        /// Creates an <see cref="Image{TPixel}"/> instance backed by an uninitialized memory buffer.
+        /// This is an optimized creation method intended to be used by decoders.
+        /// The image might be filled with memory garbage.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel type</typeparam>
+        /// <param name="configuration">The <see cref="Configuration"/></param>
+        /// <param name="width">The width of the image</param>
+        /// <param name="height">The height of the image</param>
+        /// <param name="metadata">The <see cref="ImageMetaData"/></param>
+        /// <returns>The result <see cref="Image{TPixel}"/></returns>
+        internal static Image<TPixel> CreateUninitialized<TPixel>(
+            Configuration configuration,
+            int width,
+            int height,
+            ImageMetaData metadata)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            Buffer2D<TPixel> uninitializedMemoryBuffer =
+                configuration.MemoryAllocator.Allocate2D<TPixel>(width, height);
+            return new Image<TPixel>(configuration, uninitializedMemoryBuffer.MemorySource, width, height, metadata);
+        }
+
         /// <summary>
         /// By reading the header on the provided stream this calculates the images format.
         /// </summary>

tests/ImageSharp.Tests/Image/ImageTests.cs

@@ -2,7 +2,10 @@
 // Licensed under the Apache License, Version 2.0.
 
 using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Tests.Memory;
+
 using Xunit;
 // ReSharper disable InconsistentNaming
 
@@ -46,7 +49,7 @@ namespace SixLabors.ImageSharp.Tests
             }
 
             [Fact]
-            public void Configuration_Width_Height_BackroundColor()
+            public void Configuration_Width_Height_BackgroundColor()
             {
                 Configuration configuration = Configuration.Default.Clone();
                 Rgba32 color = Rgba32.Aquamarine;
@@ -61,6 +64,26 @@ namespace SixLabors.ImageSharp.Tests
                     Assert.Equal(configuration, image.GetConfiguration());
                 }
             }
+
+            [Fact]
+            public void CreateUninitialized()
+            {
+                Configuration configuration = Configuration.Default.Clone();
+
+                byte dirtyValue = 123;
+                configuration.MemoryAllocator = new TestMemoryAllocator(dirtyValue);
+                var metadata = new ImageMetaData();
+
+                using (Image<Gray8> image = Image.CreateUninitialized<Gray8>(configuration, 21, 22, metadata))
+                {
+                    Assert.Equal(21, image.Width);
+                    Assert.Equal(22, image.Height);
+                    Assert.Same(configuration, image.GetConfiguration());
+                    Assert.Same(metadata, image.MetaData);
+
+                    Assert.Equal(dirtyValue, image[5, 5].PackedValue);
+                }
+            }
         }
     }
 }