Introduce a non-generic Image base class

ImageSharp.sln.DotSettings

@@ -388,4 +388,5 @@
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002ECSharpPlaceAttributeOnSameLineMigration/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002EMigrateBlankLinesAroundFieldToBlankLinesAroundProperty/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/Environment/SettingsMigration/IsMigratorApplied/=JetBrains_002EReSharper_002EPsi_002ECSharp_002ECodeStyle_002ESettingsUpgrade_002EMigrateThisQualifierSettings/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=Quantizer/@EntryIndexedValue">True</s:Boolean>
 </wpf:ResourceDictionary>

src/ImageSharp.Drawing/ImageSharp.Drawing.csproj

@@ -10,7 +10,9 @@
 
     <VersionPrefix Condition="$(packageversion) != ''">$(packageversion)</VersionPrefix>
     <VersionPrefix Condition="$(packageversion) == ''">0.0.1</VersionPrefix>
+
     <TargetFrameworks>netcoreapp2.1;netstandard1.3;netstandard2.0</TargetFrameworks>
+
     <LangVersion>7.3</LangVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>

src/ImageSharp/Advanced/AdvancedImageExtensions.cs

@@ -18,11 +18,9 @@ public static class AdvancedImageExtensions
         /// <summary>
         /// Gets the configuration for the image.
         /// </summary>
-        /// <typeparam name="TPixel">The Pixel format.</typeparam>
         /// <param name="source">The source image.</param>
         /// <returns>Returns the configuration.</returns>
-        public static Configuration GetConfiguration<TPixel>(this Image<TPixel> source)
-            where TPixel : struct, IPixel<TPixel>
+        public static Configuration GetConfiguration(this Image source)
             => GetConfiguration((IConfigurable)source);
 
         /// <summary>

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -30,6 +30,9 @@ public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
             return new BmpDecoderCore(configuration, this).Decode<TPixel>(stream);
         }
 
+        /// <inheritdoc />
+        public Image Decode(Configuration configuration, Stream stream) => this.Decode<Rgba32>(configuration, stream);
+
         /// <inheritdoc/>
         public IImageInfo Identify(Configuration configuration, Stream stream)
         {

src/ImageSharp/Formats/Bmp/ImageExtensions.cs

@@ -2,38 +2,35 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Formats.Bmp;
-using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp
 {
     /// <summary>
-    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// Extension methods for the <see cref="Image"/> type.
     /// </summary>
     public static partial class ImageExtensions
     {
         /// <summary>
         /// Saves the image to the given stream with the bmp format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsBmp<TPixel>(this Image<TPixel> source, Stream stream)
-            where TPixel : struct, IPixel<TPixel>
-                        => source.SaveAsBmp(stream, null);
+        public static void SaveAsBmp(this Image source, Stream stream) => source.SaveAsBmp(stream, null);
 
         /// <summary>
         /// Saves the image to the given stream with the bmp format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <param name="encoder">The encoder to save the image with.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsBmp<TPixel>(this Image<TPixel> source, Stream stream, BmpEncoder encoder)
-            where TPixel : struct, IPixel<TPixel>
-                        => source.Save(stream, encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(BmpFormat.Instance));
+        public static void SaveAsBmp(this Image source, Stream stream, BmpEncoder encoder) =>
+            source.Save(
+                stream,
+                encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(BmpFormat.Instance));
     }
-}
+}

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -44,5 +44,8 @@ public IImageInfo Identify(Configuration configuration, Stream stream)
             var decoder = new GifDecoderCore(configuration, this);
             return decoder.Identify(stream);
         }
+
+        /// <inheritdoc />
+        public Image Decode(Configuration configuration, Stream stream) => this.Decode<Rgba32>(configuration, stream);
     }
 }

src/ImageSharp/Formats/Gif/ImageExtensions.cs

@@ -2,38 +2,35 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Formats.Gif;
-using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp
 {
     /// <summary>
-    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// Extension methods for the <see cref="Image"/> type.
     /// </summary>
     public static partial class ImageExtensions
     {
         /// <summary>
         /// Saves the image to the given stream in the gif format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsGif<TPixel>(this Image<TPixel> source, Stream stream)
-            where TPixel : struct, IPixel<TPixel>
-             => source.SaveAsGif(stream, null);
+        public static void SaveAsGif(this Image source, Stream stream) => source.SaveAsGif(stream, null);
 
         /// <summary>
         /// Saves the image to the given stream in the gif format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <param name="encoder">The options for the encoder.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsGif<TPixel>(this Image<TPixel> source, Stream stream, GifEncoder encoder)
-            where TPixel : struct, IPixel<TPixel>
-            => source.Save(stream, encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(GifFormat.Instance));
+        public static void SaveAsGif(this Image source, Stream stream, GifEncoder encoder) =>
+            source.Save(
+                stream,
+                encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(GifFormat.Instance));
     }
-}
+}

src/ImageSharp/Formats/IImageDecoder.cs

@@ -12,13 +12,22 @@ namespace SixLabors.ImageSharp.Formats
     public interface IImageDecoder
     {
         /// <summary>
-        /// Decodes the image from the specified stream to the <see cref="ImageFrame{TPixel}"/>.
+        /// Decodes the image from the specified stream to an <see cref="Image{TPixel}"/> of a specific pixel type.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="configuration">The configuration for the image.</param>
         /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
-        /// <returns>The decoded image</returns>
+        /// <returns>The decoded image of a given pixel type.</returns>
         Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
             where TPixel : struct, IPixel<TPixel>;
+
+        /// <summary>
+        /// Decodes the image from the specified stream to an <see cref="Image"/>.
+        /// The decoder is free to choose the pixel type.
+        /// </summary>
+        /// <param name="configuration">The configuration for the image.</param>
+        /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
+        /// <returns>The decoded image of a pixel type chosen by the decoder.</returns>
+        Image Decode(Configuration configuration, Stream stream);
     }
 }

src/ImageSharp/Formats/Jpeg/ImageExtensions.cs

@@ -2,38 +2,35 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Formats.Jpeg;
-using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp
 {
     /// <summary>
-    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// Extension methods for the <see cref="Image"/> type.
     /// </summary>
     public static partial class ImageExtensions
     {
         /// <summary>
         /// Saves the image to the given stream with the jpeg format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsJpeg<TPixel>(this Image<TPixel> source, Stream stream)
-            where TPixel : struct, IPixel<TPixel>
-         => SaveAsJpeg(source, stream, null);
+        public static void SaveAsJpeg(this Image source, Stream stream) => SaveAsJpeg(source, stream, null);
 
         /// <summary>
         /// Saves the image to the given stream with the jpeg format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <param name="encoder">The options for the encoder.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsJpeg<TPixel>(this Image<TPixel> source, Stream stream, JpegEncoder encoder)
-            where TPixel : struct, IPixel<TPixel>
-            => source.Save(stream, encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(JpegFormat.Instance));
+        public static void SaveAsJpeg(this Image source, Stream stream, JpegEncoder encoder) =>
+            source.Save(
+                stream,
+                encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(JpegFormat.Instance));
     }
-}
+}

src/ImageSharp/Formats/Jpeg/JpegDecoder.cs

@@ -38,5 +38,8 @@ public IImageInfo Identify(Configuration configuration, Stream stream)
                 return decoder.Identify(stream);
             }
         }
+
+        /// <inheritdoc />
+        public Image Decode(Configuration configuration, Stream stream) => this.Decode<Rgba32>(configuration, stream);
     }
 }

src/ImageSharp/Formats/PixelTypeInfo.cs

@@ -1,6 +1,10 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System.Runtime.CompilerServices;
+
+using SixLabors.ImageSharp.PixelFormats;
+
 namespace SixLabors.ImageSharp.Formats
 {
     /// <summary>
@@ -21,5 +25,9 @@ internal PixelTypeInfo(int bitsPerPixel)
         /// Gets color depth, in number of bits per pixel.
         /// </summary>
         public int BitsPerPixel { get; }
+
+        internal static PixelTypeInfo Create<TPixel>()
+            where TPixel : struct, IPixel<TPixel> =>
+            new PixelTypeInfo(Unsafe.SizeOf<TPixel>() * 8);
     }
 }

src/ImageSharp/Formats/Png/ImageExtensions.cs

@@ -2,39 +2,35 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+
 using SixLabors.ImageSharp.Advanced;
-using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Png;
-using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp
 {
     /// <summary>
-    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// Extension methods for the <see cref="Image"/> type.
     /// </summary>
     public static partial class ImageExtensions
     {
         /// <summary>
         /// Saves the image to the given stream with the png format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsPng<TPixel>(this Image<TPixel> source, Stream stream)
-            where TPixel : struct, IPixel<TPixel>
-                => SaveAsPng(source, stream, null);
+        public static void SaveAsPng(this Image source, Stream stream) => SaveAsPng(source, stream, null);
 
         /// <summary>
         /// Saves the image to the given stream with the png format.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="stream">The stream to save the image to.</param>
         /// <param name="encoder">The options for the encoder.</param>
         /// <exception cref="System.ArgumentNullException">Thrown if the stream is null.</exception>
-        public static void SaveAsPng<TPixel>(this Image<TPixel> source, Stream stream, PngEncoder encoder)
-            where TPixel : struct, IPixel<TPixel>
-            => source.Save(stream, encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(PngFormat.Instance));
+        public static void SaveAsPng(this Image source, Stream stream, PngEncoder encoder) =>
+            source.Save(
+                stream,
+                encoder ?? source.GetConfiguration().ImageFormatsManager.FindEncoder(PngFormat.Instance));
     }
 }

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -59,5 +59,8 @@ public IImageInfo Identify(Configuration configuration, Stream stream)
             var decoder = new PngDecoderCore(configuration, this);
             return decoder.Identify(stream);
         }
+
+        /// <inheritdoc />
+        public Image Decode(Configuration configuration, Stream stream) => this.Decode<Rgba32>(configuration, stream);
     }
 }

src/ImageSharp/IImageVisitor.cs

@@ -0,0 +1,22 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// A visitor to implement double-dispatch pattern in order to apply pixel-specific operations
+    /// on non-generic <see cref="Image"/> instances. The operation is dispatched by <see cref="Image.AcceptVisitor"/>.
+    /// </summary>
+    internal interface IImageVisitor
+    {
+        /// <summary>
+        /// Provides a pixel-specific implementation for a given operation.
+        /// </summary>
+        /// <param name="image">The image.</param>
+        /// <typeparam name="TPixel">The pixel type.</typeparam>
+        void Visit<TPixel>(Image<TPixel> image)
+            where TPixel : struct, IPixel<TPixel>;
+    }
+}

src/ImageSharp/Image.Decode.cs

@@ -14,15 +14,15 @@ namespace SixLabors.ImageSharp
     /// <content>
     /// Adds static methods allowing the decoding of new images.
     /// </content>
-    public static partial class Image
+    public abstract 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="configuration">The <see cref="ImageSharp.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>
@@ -103,6 +103,18 @@ private static (Image<TPixel> img, IImageFormat format) Decode<TPixel>(Stream st
             return (img, format);
         }
 
+        private static (Image img, IImageFormat format) Decode(Stream stream, Configuration config)
+        {
+            IImageDecoder decoder = DiscoverDecoder(stream, config, out IImageFormat format);
+            if (decoder is null)
+            {
+                return (null, null);
+            }
+
+            Image img = decoder.Decode(config, stream);
+            return (img, format);
+        }
+
         /// <summary>
         /// Reads the raw image information from the specified stream.
         /// </summary>