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，因为我需要下载许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？

最佳答案

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，它们通常在文件开头都有一个包含图像尺寸的标题。

Reddit 使用一种算法下载连续的 1024 字节 block 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

它在其 ImageFile 类中使用一个单独的解析器，并在下载更多字节时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#，大量利用了位于 https://stackoverflow.com/a/112711/3838199 的图像解析代码。 .

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 block 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://stackoverflow.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://stackoverflow.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);
        }
    }
}

关于c# - C#中直接从URL获取图片尺寸，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30054517/