问题
I'm trying to get dimensions from a picture directly from the web using this code:
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;
It works but the performance is terrible because I need to download many images just to get their dimensions.
Is there a more efficient way to do the same thing?
回答1:
In case this is helpful to those coming along later, it does seem that this is indeed possible. A brief review of the JPG, PNG and GIF image formats shows that they all generally have a header at the beginning of the file that contains the image dimensions.
Reddit uses an algorithm to download successive 1024-byte chunks to determine image dimensions without downloading an entire image. The code is in Python but it is in the _fetch_image_size method here: https://github.com/reddit/reddit/blob/35c82a0a0b24441986bdb4ad02f3c8bb0a05de57/r2/r2/lib/media.py#L634
It uses a separate parser in their ImageFile class and successively attempts to parse the image and retrieve the size as more bytes are downloaded. I've loosely translated this to C# in the code below, heavily leveraging the image-parsing code at https://stackoverflow.com/a/112711/3838199.
There may be some cases where retrieving the entire file is necessary but I suspect that applies to a relatively small subset of JPEG images (perhaps progressive images). In my casual testing it seems most image sizes are retrieved via the first 1024-byte retrieval; indeed this chunk size could probably be smaller.
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);
}
}
}
Tests:
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);
}
}
}
回答2:
In a word, no.
In a few more words, you would have to rely on there being a resource containing details of the dimensions of images on a given server. In 99.99% of cases that will simply not exist.
来源：https://stackoverflow.com/questions/30054517/get-image-dimensions-directly-from-url-in-c-sharp