fixes on code review

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -39,14 +39,14 @@ namespace ImageSharp.Formats
         }
 
         /// <inheritdoc/>
-        public int DetectPixelSize(Configuration configuration, Stream stream)
+        public PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream)
         {
             Guard.NotNull(stream, "stream");
 
             byte[] buffer = new byte[2];
             stream.Skip(28);
             stream.Read(buffer, 0, 2);
-            return BitConverter.ToInt16(buffer, 0);
+            return new PixelTypeInfo(BitConverter.ToInt16(buffer, 0));
         }
     }
 }

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -35,14 +35,15 @@ namespace ImageSharp.Formats
         }
 
         /// <inheritdoc/>
-        public int DetectPixelSize(Configuration configuration, Stream stream)
+        public PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream)
         {
             Guard.NotNull(stream, "stream");
 
             byte[] buffer = new byte[1];
             stream.Skip(10); // Skip the identifier and size
             stream.Read(buffer, 0, 1); // Skip the identifier and size
-            return (buffer[0] & 0x07) + 1;  // The lowest 3 bits represent the bit depth minus 1
+            int bitsPerPixel = (buffer[0] & 0x07) + 1;  // The lowest 3 bits represent the bit depth minus 1
+            return new PixelTypeInfo(bitsPerPixel);
         }
     }
 }

src/ImageSharp/Formats/IImageDecoder.cs

@@ -31,7 +31,7 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="configuration">The configuration for the image.</param>
         /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
-        /// <returns>The color depth, in number of bits per pixel</returns>
-        int DetectPixelSize(Configuration configuration, Stream stream);
+        /// <returns>The <see cref="PixelTypeInfo"/> object</returns>
+        PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream);
     }
 }

src/ImageSharp/Formats/Jpeg/JpegDecoder.cs

@@ -34,13 +34,13 @@ namespace ImageSharp.Formats
         }
 
         /// <inheritdoc/>
-        public int DetectPixelSize(Configuration configuration, Stream stream)
+        public PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream)
         {
             Guard.NotNull(stream, "stream");
 
             using (JpegDecoderCore decoder = new JpegDecoderCore(configuration, this))
             {
-                return decoder.DetectPixelSize(stream);
+                return new PixelTypeInfo(decoder.DetectPixelSize(stream));
             }
         }
     }

src/ImageSharp/Formats/PixelTypeInfo.cs

@@ -0,0 +1,22 @@
+namespace ImageSharp.Formats
+{
+    /// <summary>
+    /// Stores information about pixel
+    /// </summary>
+    public class PixelTypeInfo
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PixelTypeInfo"/> class.
+        /// </summary>
+        /// <param name="bitsPerPixel">color depth, in number of bits per pixel</param>
+        internal PixelTypeInfo(int bitsPerPixel)
+        {
+            this.BitsPerPixel = bitsPerPixel;
+        }
+
+        /// <summary>
+        /// Gets color depth, in number of bits per pixel
+        /// </summary>
+        public int BitsPerPixel { get; }
+    }
+}

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -63,10 +63,10 @@ namespace ImageSharp.Formats
         /// <param name="configuration">The configuration for the image.</param>
         /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
         /// <returns>The color depth, in number of bits per pixel</returns>
-        public int DetectPixelSize(Configuration configuration, Stream stream)
+        public PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream)
         {
             var decoder = new PngDecoderCore(configuration, this);
-            return decoder.DetectPixelSize(stream);
+            return new PixelTypeInfo(decoder.DetectPixelSize(stream));
         }
     }
 }

src/ImageSharp/Image/Image.Decode.cs

@@ -88,12 +88,12 @@ namespace ImageSharp
         /// <param name="stream">The stream.</param>
         /// <param name="config">the configuration.</param>
         /// <returns>
-        /// The color depth, in number of bits per pixel or null if suitable decoder not found.
+        /// The <see cref="PixelTypeInfo"/> or null if suitable decoder not found.
         /// </returns>
-        private static int? InternalDetectPixelSize(Stream stream, Configuration config)
+        private static PixelTypeInfo InternalDetectPixelType(Stream stream, Configuration config)
         {
             IImageDecoder decoder = DiscoverDecoder(stream, config, out IImageFormat _);
-            return decoder?.DetectPixelSize(config, stream);
+            return decoder?.DetectPixelType(config, stream);
         }
     }
 }

src/ImageSharp/Image/Image.FromStream.cs

@@ -46,10 +46,12 @@ namespace ImageSharp
         /// <exception cref="NotSupportedException">
         /// Thrown if the stream is not readable nor seekable.
         /// </exception>
-        /// <returns>The color depth, in number of bits per pixel or null if suitable decoder not found</returns>
-        public static int? DetectPixelSize(Stream stream)
+        /// <returns>
+        /// The <see cref="PixelTypeInfo"/> or null if suitable decoder not found.
+        /// </returns>
+        public static PixelTypeInfo DetectPixelType(Stream stream)
         {
-            return DetectPixelSize(null, stream);
+            return DetectPixelType(null, stream);
         }
 
         /// <summary>
@@ -60,10 +62,12 @@ namespace ImageSharp
         /// <exception cref="NotSupportedException">
         /// Thrown if the stream is not readable nor seekable.
         /// </exception>
-        /// <returns>The color depth, in number of bits per pixel or null if suitable decoder not found</returns>
-        public static int? DetectPixelSize(Configuration config, Stream stream)
+        /// <returns>
+        /// The <see cref="PixelTypeInfo"/> or null if suitable decoder not found.
+        /// </returns>
+        public static PixelTypeInfo DetectPixelType(Configuration config, Stream stream)
         {
-            return WithSeekableStream(stream, s => InternalDetectPixelSize(s, config ?? Configuration.Default));
+            return WithSeekableStream(stream, s => InternalDetectPixelType(s, config ?? Configuration.Default));
         }
 
         /// <summary>

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -22,7 +22,7 @@ namespace ImageSharp.Tests
             TestFile testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
-                Assert.Equal(expectedPixelSize, Image.DetectPixelSize(stream));
+                Assert.Equal(expectedPixelSize, Image.DetectPixelType(stream)?.BitsPerPixel);
             }
         }
     }

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -92,7 +92,7 @@ namespace ImageSharp.Tests
             TestFile testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
-                Assert.Equal(expectedPixelSize, Image.DetectPixelSize(stream));
+                Assert.Equal(expectedPixelSize, Image.DetectPixelType(stream)?.BitsPerPixel);
             }
         }
     }

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs

@@ -165,7 +165,7 @@ namespace ImageSharp.Tests
             TestFile testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
-                Assert.Equal(expectedPixelSize, Image.DetectPixelSize(stream));
+                Assert.Equal(expectedPixelSize, Image.DetectPixelType(stream)?.BitsPerPixel);
             }
         }
     }

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -97,7 +97,7 @@ namespace ImageSharp.Tests
             TestFile testFile = TestFile.Create(imagePath);
             using (var stream = new MemoryStream(testFile.Bytes, false))
             {
-                Assert.Equal(expectedPixelSize, Image.DetectPixelSize(stream));
+                Assert.Equal(expectedPixelSize, Image.DetectPixelType(stream)?.BitsPerPixel);
             }
         }
 

tests/ImageSharp.Tests/TestFormat.cs

@@ -204,7 +204,7 @@ namespace ImageSharp.Tests
                 return this.testFormat.Sample<TPixel>();
             }
 
-            public int DetectPixelSize(Configuration configuration, Stream stream)
+            public PixelTypeInfo DetectPixelType(Configuration configuration, Stream stream)
             {
                 throw new NotImplementedException();
             }