c# - 在不读取整个文件的情况下获取图像尺寸

Question

有没有一种获取图像尺寸(jpg、png、...)的廉价方法？最好，我想只使用标准类库来实现这一点(由于托管限制)。我知道读取图像头并自己解析它应该相对容易，但看起来像这样的东西应该已经存在了。此外，我已经验证了以下代码片段读取了整个图像(我不想要):

using System;
using System.Drawing;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Image img = new Bitmap("test.png");
            System.Console.WriteLine(img.Width + " x " + img.Height);
        }
    }
}

Answer 1

与往常一样，您最好的选择是找到一个经过良好测试的库。但是，您说这很难，所以这里有一些大部分未经测试的狡猾代码应该适用于相当多的情况:

using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.Linq;

namespace ImageDimensions
{
    public static class ImageHelper
    {
        const string errorMessage = "Could not recognize image format.";

        private static Dictionary<byte[], Func<BinaryReader, Size>> imageFormatDecoders = new Dictionary<byte[], Func<BinaryReader, Size>>()
        {
            { new byte[]{ 0x42, 0x4D }, DecodeBitmap},
            { new byte[]{ 0x47, 0x49, 0x46, 0x38, 0x37, 0x61 }, DecodeGif },
            { new byte[]{ 0x47, 0x49, 0x46, 0x38, 0x39, 0x61 }, DecodeGif },
            { new byte[]{ 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, DecodePng },
            { new byte[]{ 0xff, 0xd8 }, DecodeJfif },
        };

        /// <summary>
        /// Gets the dimensions of an image.
        /// </summary>
        /// <param name="path">The path of the image to get the dimensions of.</param>
        /// <returns>The dimensions of the specified image.</returns>
        /// <exception cref="ArgumentException">The image was of an unrecognized format.</exception>
        public static Size GetDimensions(string path)
        {
            using (BinaryReader binaryReader = new BinaryReader(File.OpenRead(path)))
            {
                try
                {
                    return GetDimensions(binaryReader);
                }
                catch (ArgumentException e)
                {
                    if (e.Message.StartsWith(errorMessage))
                    {
                        throw new ArgumentException(errorMessage, "path", e);
                    }
                    else
                    {
                        throw e;
                    }
                }
            }
        }

        /// <summary>
        /// Gets the dimensions of an image.
        /// </summary>
        /// <param name="path">The path of the image to get the dimensions of.</param>
        /// <returns>The dimensions of the specified image.</returns>
        /// <exception cref="ArgumentException">The image was of an unrecognized format.</exception>    
        public static Size GetDimensions(BinaryReader binaryReader)
        {
            int maxMagicBytesLength = imageFormatDecoders.Keys.OrderByDescending(x => x.Length).First().Length;

            byte[] magicBytes = new byte[maxMagicBytesLength];

            for (int i = 0; i < maxMagicBytesLength; i += 1)
            {
                magicBytes[i] = binaryReader.ReadByte();

                foreach(var kvPair in imageFormatDecoders)
                {
                    if (magicBytes.StartsWith(kvPair.Key))
                    {
                        return kvPair.Value(binaryReader);
                    }
                }
            }

            throw new ArgumentException(errorMessage, "binaryReader");
        }

        private static bool StartsWith(this byte[] thisBytes, byte[] thatBytes)
        {
            for(int i = 0; i < thatBytes.Length; i+= 1)
            {
                if (thisBytes[i] != thatBytes[i])
                {
                    return false;
                }
            }
            return true;
        }

        private static short ReadLittleEndianInt16(this BinaryReader binaryReader)
        {
            byte[] bytes = new byte[sizeof(short)];
            for (int i = 0; i < sizeof(short); i += 1)
            {
                bytes[sizeof(short) - 1 - i] = binaryReader.ReadByte();
            }
            return BitConverter.ToInt16(bytes, 0);
        }

        private static int ReadLittleEndianInt32(this BinaryReader binaryReader)
        {
            byte[] bytes = new byte[sizeof(int)];
            for (int i = 0; i < sizeof(int); i += 1)
            {
                bytes[sizeof(int) - 1 - i] = binaryReader.ReadByte();
            }
            return BitConverter.ToInt32(bytes, 0);
        }

        private static Size DecodeBitmap(BinaryReader binaryReader)
        {
            binaryReader.ReadBytes(16);
            int width = binaryReader.ReadInt32();
            int height = binaryReader.ReadInt32();
            return new Size(width, height);
        }

        private static Size DecodeGif(BinaryReader binaryReader)
        {
            int width = binaryReader.ReadInt16();
            int height = binaryReader.ReadInt16();
            return new Size(width, height);
        }

        private static Size DecodePng(BinaryReader binaryReader)
        {
            binaryReader.ReadBytes(8);
            int width = binaryReader.ReadLittleEndianInt32();
            int height = binaryReader.ReadLittleEndianInt32();
            return new Size(width, height);
        }

        private static Size DecodeJfif(BinaryReader binaryReader)
        {
            while (binaryReader.ReadByte() == 0xff)
            {
                byte marker = binaryReader.ReadByte();
                short chunkLength = binaryReader.ReadLittleEndianInt16();

                if (marker == 0xc0)
                {
                    binaryReader.ReadByte();

                    int height = binaryReader.ReadLittleEndianInt16();
                    int width = binaryReader.ReadLittleEndianInt16();
                    return new Size(width, height);
                }

                binaryReader.ReadBytes(chunkLength - 2);
            }

            throw new ArgumentException(errorMessage);
        }
    }
}

希望代码是相当明显的。要添加新的文件格式，您可以将其添加到 imageFormatDecoders 中，键是出现在给定格式的每个文件开头的“魔法位”数组，值是提取的函数来自流的大小。大多数格式都足够简单，唯一真正糟糕的是 jpeg。