Basic metadata-only image parsing API

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
-using System.Collections.Generic;
 using System.IO;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -23,7 +21,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
     /// Formats will be supported in a later releases. We advise always
     /// to use only 24 Bit Windows bitmaps.
     /// </remarks>
-    public sealed class BmpDecoder : IImageDecoder, IBmpDecoderOptions
+    public sealed class BmpDecoder : IImageDecoder, IBmpDecoderOptions, IImageInfoDetector
     {
         /// <inheritdoc/>
         public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
@@ -34,5 +32,13 @@ public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
 
             return new BmpDecoderCore(configuration, this).Decode<TPixel>(stream);
         }
+
+        /// <inheritdoc/>
+        public IImage Identify(Configuration configuration, Stream stream)
+        {
+            Guard.NotNull(stream, "stream");
+
+            return new BmpDecoderCore(configuration, this).Identify(stream);
+        }
     }
 }

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Bmp
@@ -94,62 +95,9 @@ public BmpDecoderCore(Configuration configuration, IBmpDecoderOptions options)
         public Image<TPixel> Decode<TPixel>(Stream stream)
             where TPixel : struct, IPixel<TPixel>
         {
-            this.currentStream = stream;
-
             try
             {
-                this.ReadFileHeader();
-                this.ReadInfoHeader();
-
-                // see http://www.drdobbs.com/architecture-and-design/the-bmp-file-format-part-1/184409517
-                // If the height is negative, then this is a Windows bitmap whose origin
-                // is the upper-left corner and not the lower-left.The inverted flag
-                // indicates a lower-left origin.Our code will be outputting an
-                // upper-left origin pixel array.
-                bool inverted = false;
-                if (this.infoHeader.Height < 0)
-                {
-                    inverted = true;
-                    this.infoHeader.Height = -this.infoHeader.Height;
-                }
-
-                int colorMapSize = -1;
-
-                if (this.infoHeader.ClrUsed == 0)
-                {
-                    if (this.infoHeader.BitsPerPixel == 1 ||
-                        this.infoHeader.BitsPerPixel == 4 ||
-                        this.infoHeader.BitsPerPixel == 8)
-                    {
-                        colorMapSize = (int)Math.Pow(2, this.infoHeader.BitsPerPixel) * 4;
-                    }
-                }
-                else
-                {
-                    colorMapSize = this.infoHeader.ClrUsed * 4;
-                }
-
-                byte[] palette = null;
-
-                if (colorMapSize > 0)
-                {
-                    // 256 * 4
-                    if (colorMapSize > 1024)
-                    {
-                        throw new ImageFormatException($"Invalid bmp colormap size '{colorMapSize}'");
-                    }
-
-                    palette = new byte[colorMapSize];
-
-                    this.currentStream.Read(palette, 0, colorMapSize);
-                }
-
-                if (this.infoHeader.Width > int.MaxValue || this.infoHeader.Height > int.MaxValue)
-                {
-                    throw new ArgumentOutOfRangeException(
-                        $"The input bitmap '{this.infoHeader.Width}x{this.infoHeader.Height}' is "
-                        + $"bigger then the max allowed size '{int.MaxValue}x{int.MaxValue}'");
-                }
+                this.ReadImageHeaders(stream, out bool inverted, out byte[] palette);
 
                 var image = new Image<TPixel>(this.configuration, this.infoHeader.Width, this.infoHeader.Height);
                 using (PixelAccessor<TPixel> pixels = image.Lock())
@@ -192,6 +140,16 @@ public Image<TPixel> Decode<TPixel>(Stream stream)
             }
         }
 
+        /// <summary>
+        /// Reads the image base information from the specified stream.
+        /// </summary>
+        /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
+        public IImage Identify(Stream stream)
+        {
+            this.ReadImageHeaders(stream, out _, out _);
+            return new ImageInfo(new PixelTypeInfo(this.infoHeader.BitsPerPixel), this.infoHeader.Width, this.infoHeader.Height, new ImageMetaData());
+        }
+
         /// <summary>
         /// Returns the y- value based on the given height.
         /// </summary>
@@ -624,5 +582,73 @@ private void ReadFileHeader()
                 Offset = BitConverter.ToInt32(data, 10)
             };
         }
+
+        /// <summary>
+        /// Reads the <see cref="BmpFileHeader"/> and <see cref="BmpInfoHeader"/> from the stream and sets the corresponding fields.
+        /// </summary>
+        private void ReadImageHeaders(Stream stream, out bool inverted, out byte[] palette)
+        {
+            this.currentStream = stream;
+
+            try
+            {
+                this.ReadFileHeader();
+                this.ReadInfoHeader();
+
+                // see http://www.drdobbs.com/architecture-and-design/the-bmp-file-format-part-1/184409517
+                // If the height is negative, then this is a Windows bitmap whose origin
+                // is the upper-left corner and not the lower-left.The inverted flag
+                // indicates a lower-left origin.Our code will be outputting an
+                // upper-left origin pixel array.
+                inverted = false;
+                if (this.infoHeader.Height < 0)
+                {
+                    inverted = true;
+                    this.infoHeader.Height = -this.infoHeader.Height;
+                }
+
+                int colorMapSize = -1;
+
+                if (this.infoHeader.ClrUsed == 0)
+                {
+                    if (this.infoHeader.BitsPerPixel == 1 ||
+                        this.infoHeader.BitsPerPixel == 4 ||
+                        this.infoHeader.BitsPerPixel == 8)
+                    {
+                        colorMapSize = (int)Math.Pow(2, this.infoHeader.BitsPerPixel) * 4;
+                    }
+                }
+                else
+                {
+                    colorMapSize = this.infoHeader.ClrUsed * 4;
+                }
+
+                palette = null;
+
+                if (colorMapSize > 0)
+                {
+                    // 256 * 4
+                    if (colorMapSize > 1024)
+                    {
+                        throw new ImageFormatException($"Invalid bmp colormap size '{colorMapSize}'");
+                    }
+
+                    palette = new byte[colorMapSize];
+
+                    this.currentStream.Read(palette, 0, colorMapSize);
+                }
+
+                if (this.infoHeader.Width > int.MaxValue || this.infoHeader.Height > int.MaxValue)
+                {
+                    throw new ArgumentOutOfRangeException(
+                        $"The input bitmap '{this.infoHeader.Width}x{this.infoHeader.Height}' is "
+                        + $"bigger then the max allowed size '{int.MaxValue}x{int.MaxValue}'");
+                }
+            }
+            catch (IndexOutOfRangeException e)
+            {
+                throw new ImageFormatException("Bitmap does not have a valid format.", e);
+            }
+        }
     }
 }

src/ImageSharp/Formats/Gif/GifDecoder.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
-using System.Collections.Generic;
 using System.IO;
 using System.Text;
 using SixLabors.ImageSharp.PixelFormats;
@@ -12,7 +10,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
     /// <summary>
     /// Decoder for generating an image out of a gif encoded stream.
     /// </summary>
-    public sealed class GifDecoder : IImageDecoder, IGifDecoderOptions
+    public sealed class GifDecoder : IImageDecoder, IGifDecoderOptions, IImageInfoDetector
     {
         /// <summary>
         /// Gets or sets a value indicating whether the metadata should be ignored when the image is being decoded.
@@ -33,8 +31,17 @@ public sealed class GifDecoder : IImageDecoder, IGifDecoderOptions
         public Image<TPixel> Decode<TPixel>(Configuration configuration, Stream stream)
             where TPixel : struct, IPixel<TPixel>
         {
-            var decoder = new GifDecoderCore<TPixel>(configuration, this);
-            return decoder.Decode(stream);
+            var decoder = new GifDecoderCore(configuration, this);
+            return decoder.Decode<TPixel>(stream);
+        }
+
+        /// <inheritdoc/>
+        public IImage Identify(Configuration configuration, Stream stream)
+        {
+            Guard.NotNull(stream, "stream");
+
+            var decoder = new GifDecoderCore(configuration, this);
+            return decoder.Identify(stream);
         }
     }
 }