使用Web客户端获取远程图像生成木纹GIF和不能处理PNG + BMP [英] Using WebClient to get Remote Images Produces Grainy GIFs and Can't Handle PNG+BMP

问题描述

您好！

我创建一个Web表单原型（ImageLaoder.aspx），将返回一个图像，使其可以像这样简单的例子可以使用其他Web窗体/网页：

I'm creating a web form prototype (ImageLaoder.aspx) that will return an image so that it may be used like this simple example other Web Forms/web pages:

<img src="http://www.mydomain.com/ImageLoader.aspx?i=http://images.mydomain.com/img/a.jpg" />

到目前为止，它加载的JPG文件，没有任何问题，但GIF文件相比初代吸血鬼和骨形成蛋白和PNG图像导致以下异常期待颗粒的：

So far, it loads JPGs with no problems, however GIFs look "grainy" compared to the orignals and BMPs and PNGs result in the following exception:

System.Runtime.InteropServices.ExternalException：GDI中发生一般性错误+ 的

System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+

我的code到目前为止是这样的：

My code thus far looks like this:

protected void Page_Load(object sender, EventArgs e)
{
    string l_filePath = Request.QueryString["i"];

    System.Drawing.Image l_image = GetImage(l_filePath);
    if (l_image != null)
    {
        System.Drawing.Imaging.ImageFormat l_imageFormat = DetermineImageFormat(l_filePath);
        WriteImageAsReponse(l_image, l_imageFormat);
    }
}

private System.Drawing.Image GetImage(string filePath)
{
    WebClient l_WebClient = new WebClient();
    byte[] l_imageBytes = l_WebClient.DownloadData(filePath);

    System.Drawing.Image l_image = null;
    using (MemoryStream l_MemStream = new MemoryStream(l_imageBytes, 0, l_imageBytes.Length))
    {
        l_MemStream.Write(l_imageBytes, 0, l_imageBytes.Length);
        l_image = System.Drawing.Image.FromStream(l_MemStream, true);
        l_MemStream.Close();
    }

    return l_image;
}

private System.Drawing.Imaging.ImageFormat DetermineImageFormat(string filePath)
{
    if (filePath.EndsWith(".jpg", StringComparison.OrdinalIgnoreCase))
        return System.Drawing.Imaging.ImageFormat.Jpeg;
    else if (filePath.EndsWith(".gif", StringComparison.OrdinalIgnoreCase))
        return System.Drawing.Imaging.ImageFormat.Gif;
    else if (filePath.EndsWith(".png", StringComparison.OrdinalIgnoreCase))
        return System.Drawing.Imaging.ImageFormat.Png;
    else
        return System.Drawing.Imaging.ImageFormat.Bmp;
}

private void WriteImageAsReponse(System.Drawing.Image image, System.Drawing.Imaging.ImageFormat imageFormat)
{
    if (image == null)
        return;

    System.Drawing.Bitmap l_outputBitMap = new Bitmap(image);

    if (imageFormat == System.Drawing.Imaging.ImageFormat.Jpeg)
        Response.ContentType = "image/jpg";
    else if (imageFormat == System.Drawing.Imaging.ImageFormat.Gif)
        Response.ContentType = "image/gif";
    else if (imageFormat == System.Drawing.Imaging.ImageFormat.Png)
        Response.ContentType = "image/png";
    else
        Response.ContentType = "image/bmp";

    l_outputBitMap.Save(Response.OutputStream, imageFormat);
}

任何想法，为什么GIF格式的颗粒感和PNG格式BMP的和导致异常？

Any ideas why GIFs are grainy and PNGs and BMPs cause exceptions?

推荐答案

你的getImage方法的几点：

A few points about your GetImage method:

• 当你使用Image.FromStream你不应该关闭（或处置）的流


• 如果你在一个流调用Dispose（使用using语句），你不需要调用Close


• 您正在编写到流，但不是倒带它，所以l_image实际上并没有得到任何的数据，据我所看到的（除非Image.FromStream重置本身的位置）。 （这可能是因为GIF / JPG德codeRS倒带流，但BMP / PNG没有，因此错误。）


• 你为什么不只是使用的MemoryStream构造函数，它接受一个字节数组？

总之，我相信你的getImage方法可以替换为：

In short, I believe your GetImage method can be replaced with:

private Image GetImage(string filePath)
{
    WebClient l_WebClient = new WebClient();
    byte[] l_imageBytes = l_WebClient.DownloadData(filePath);
    MemoryStream l_stream = new MemoryStream(l_imageBytes);
    return Image.FromStream(l_stream);
}

现在，更重要的是 - 你为什么加载图像呢？你为什么不只是服务于文件本身作为响应，设定为你已经做的内容类型 - 或可能只是基于扩展？换句话说，所有的code会变成：

Now, more importantly - why are you loading the image at all? Why don't you just serve the file itself as a response, setting the content type as you're already doing - or possibly just based on the extension? In other words, all of your code would become:

protected void Page_Load(object sender, EventArgs e)
{
    string filePath = Request.QueryString["i"];
    string extension = l_filePath.Substring(l_filePath.LastIndexOf('.') + 1);
    Response.ContentType = "image/" + extension;
    byte[] data = new WebClient.DownloadData(filePath);
    Response.OutputStream.Write(data, 0, data.Length);
    Response.End();
}

多一点错误处理（包括这是一个合理的扩展？）将是很好的，但除此之外，我认为这没关系。的实际加载自己的形象，唯一的好处是，你能验证它确实是的图像，而不是病毒或类似的东西。

A bit more error handling (including "is this a reasonable extension?") would be nice, but other than that I think it's okay. The only benefit of actually loading the image yourself is that you get to validate that it really is an image rather than a virus or something like that.

编辑：只是出于兴趣，你有一个很好的理由，为什么你的需要的图像请求通过你的服务器？为什么会网页作者写：

Just out of interest, do you have a good reason why you'd want image requests to go through your server? Why would the web page author write:

<img src="http://www.mydomain.com/ImageLoader.aspx?i=http://images.mydomain.com/img/a.jpg" />

而不是

<img src="http://images.mydomain.com/img/a.jpg" />

有是的一些原因，这可能是有用的，但在许多情况下，它只是一种浪费。

There are some reasons why it might be useful, but in many cases it's just a waste.

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