在 C# 中直接从 URL 获取图像尺寸

Question

我正在尝试使用以下代码直接从网络上获取图片的尺寸：

string image = @"http://www.hephaestusproject.com/.../csharp3.png";
byte[] imageData = new WebClient().DownloadData(image);
MemoryStream imgStream = new MemoryStream(imageData);
Image img = Image.FromStream(imgStream);

int wSize = img.Width;
int hSize = img.Height;

它可以工作，但性能很糟糕，因为我需要下载很多图像才能获得它们的尺寸。

有没有更有效的方法来做同样的事情？

Answer 1

一句话，没有

简而言之，您将不得不依赖于在给定服务器上存在包含图像尺寸详细信息的资源。在 99.99% 的情况下根本不存在。

Answer 2

如果对后来的人有帮助的话，看来确实是可以的。对 JPG、PNG 和 GIF 图像格式的简要回顾表明，它们通常在包含图像尺寸的文件开头都有一个 header。

Reddit 使用算法下载连续的 1024 字节块来确定图像尺寸，而无需下载整个图像。代码在 Python 中，但在此处的 _fetch_image_size 方法中：https://github.com/reddit/reddit/blob/35c82a0a0b24441986bdb4ad02f3c8bb0a05de57/r2/r2/lib/media.py#L634

它在他们的 ImageFile class 中使用一个单独的解析器，并随着更多字节的下载连续尝试解析图像并检索大小。我在下面的代码中将其粗略地转换为 C#，大量利用 https://whosebug.com/a/112711/3838199.

处的 image-parsing 代码

在某些情况下，可能需要检索整个文件，但我怀疑这适用于相对较小的 JPEG 图像子集（可能是渐进式图像）。在我的随意测试中，似乎大多数图像尺寸都是通过第一个 1024 字节检索来检索的；事实上，这个块大小可能会更小。

using System;
using System.Collections.Generic;
using System.Drawing; // note: add reference to System.Drawing assembly
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace Utilities
{
    // largely credited to https://whosebug.com/a/112711/3838199 for the image-specific code
    public static class ImageUtilities
    {
        private const string ErrorMessage = "Could not read image data";
        private const int ChunkSize = 1024;

        private static readonly HttpClient Client = new HttpClient();
        private static readonly 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>
        /// Retrieve the dimensions of an online image, downloading as little as possible
        /// </summary>
        public static async Task<Size> GetWebDimensions(Uri uri)
        {
            var moreBytes = true;
            var currentStart = 0;
            byte[] allBytes = { };

            while (moreBytes)
            {
                try
                {
                    var newBytes = await GetSomeBytes(uri, currentStart, currentStart + ChunkSize - 1).ConfigureAwait(false);
                    if (newBytes is null || newBytes.Length < ChunkSize)
                        moreBytes = false;
                    
                    if(new bytes != null)
                        allBytes = Combine(allBytes, newBytes);

                    return GetDimensions(new BinaryReader(new MemoryStream(allBytes)));
                }
                catch
                {
                    currentStart += ChunkSize;
                }
            }

            return new Size(0, 0);
        }

        private static async Task<byte[]?> GetSomeBytes(Uri uri, int startRange, int endRange)
        {
            var request = new HttpRequestMessage { RequestUri = uri };
            request.Headers.Range = new RangeHeaderValue(startRange, endRange);
            try
            {
                var response = await Client.SendAsync(request).ConfigureAwait(false);
                return await response.Content.ReadAsByteArrayAsync().ConfigureAwait(false);
            }
            catch 
            {

            }
            return new byte[] { };
        }

        /// <summary>
        /// Gets the dimensions of an image.
        /// </summary>
        /// <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, nameof(binaryReader));
        }

        // from https://whosebug.com/a/415839/3838199
        private static byte[] Combine(byte[] first, byte[] second)
        {
            byte[] ret = new byte[first.Length + second.Length];
            Buffer.BlockCopy(first, 0, ret, 0, first.Length);
            Buffer.BlockCopy(second, 0, ret, first.Length, second.Length);
            return ret;
        }

        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 || marker == 0xc1 || marker == 0xc2)
                {
                    binaryReader.ReadByte();

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

                binaryReader.ReadBytes(chunkLength - 2);
            }

            throw new ArgumentException(ErrorMessage);
        }
    }
}

测试：

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Utilities;
using System.Drawing;

namespace Utilities.Tests
{
    [TestClass]
    public class ImageUtilitiesTests
    {
        [TestMethod]
        public void GetPngDimensionsTest()
        {
            string url = "https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png";
            Uri uri = new Uri(url);
            var actual = ImageUtilities.GetWebDimensions(uri);
            Assert.AreEqual(new Size(272, 92), actual);
        }

        [TestMethod]
        public void GetJpgDimensionsTest()
        {
            string url = "https://upload.wikimedia.org/wikipedia/commons/e/e0/JPEG_example_JPG_RIP_050.jpg";
            Uri uri = new Uri(url);
            var actual = ImageUtilities.GetWebDimensions(uri);
            Assert.AreEqual(new Size(313, 234), actual);
        }

        [TestMethod]
        public void GetGifDimensionsTest()
        {
            string url = "https://upload.wikimedia.org/wikipedia/commons/a/a0/Sunflower_as_gif_websafe.gif";
            Uri uri = new Uri(url);
            var actual = ImageUtilities.GetWebDimensions(uri);
            Assert.AreEqual(new Size(250, 297), actual);
        }
    }
}

在 C# 中直接从 URL 获取图像尺寸

Get image dimensions directly from URL in C#

c#

image

download

dimensions