HTML to Image in C#（C#实现Html转换为Image，即网页截图）

原文链接地址：

http://www.codeproject.com/Articles/58605/HTML-to-Image-in-C

Introduction

In this article, I will show you how to capture an HTML document as an image using a WebBrowser object and the IViewObject.Draw method, which according to MSDN draws a representation of an object onto the specified device context. Before we get started, I just want to mention that the obtained results were identical to those obtained using commercial libraries, so I hope this will be useful to someone.

在这篇文章里，我将会演示如何捕获HTML文档并保存为图片，使用WebBrowser对象和IViewObject.Draw方法。

根据MSDN的描述，IViewObject.Draw用户将对象的表现画到制定的设备上下文中。

在我们开始之前，。。。

The IViewObject interface

The very first thing that we must do is to define the IViewObject interface.

我们必须做的首要事情是，我们需要定义IViewObject接口。

ComVisible(true), ComImport()]
[GuidAttribute("0000010d-0000-0000-C000-000000000046")]
[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]
public interface IViewObject
{
    [return: MarshalAs(UnmanagedType.I4)]
    [PreserveSig]
    int Draw(
        [MarshalAs(UnmanagedType.U4)] UInt32 dwDrawAspect,
        int lindex,
        IntPtr pvAspect,
        [In] IntPtr ptd,
        IntPtr hdcTargetDev,
        IntPtr hdcDraw,
        [MarshalAs(UnmanagedType.Struct)] ref Rectangle lprcBounds,
        [MarshalAs(UnmanagedType.Struct)] ref Rectangle lprcWBounds,
        IntPtr pfnContinue,
        [MarshalAs(UnmanagedType.U4)] UInt32 dwContinue);
    [PreserveSig]
    int GetColorSet([In, MarshalAs(UnmanagedType.U4)] int dwDrawAspect, 
       int lindex, IntPtr pvAspect,[In] IntPtr ptd, 
       IntPtr hicTargetDev, [Out] IntPtr ppColorSet);
    [PreserveSig]
    int Freeze([In, MarshalAs(UnmanagedType.U4)] int dwDrawAspect, 
                    int lindex, IntPtr pvAspect, [Out] IntPtr pdwFreeze);
    [PreserveSig]
    int Unfreeze([In, MarshalAs(UnmanagedType.U4)] int dwFreeze);
    void SetAdvise([In, MarshalAs(UnmanagedType.U4)] int aspects, 
      [In, MarshalAs(UnmanagedType.U4)] int advf, 
      [In, MarshalAs(UnmanagedType.Interface)] IAdviseSink pAdvSink);
    void GetAdvise([In, Out, MarshalAs(UnmanagedType.LPArray)] int[] paspects, 
      [In, Out, MarshalAs(UnmanagedType.LPArray)] int[] advf, 
      [In, Out, MarshalAs(UnmanagedType.LPArray)] IAdviseSink[] pAdvSink);
}

Below is a summary description of the parameters that the Draw method takes (this is the only method we will use):

• UInt32 dwDrawAspect - specifies the aspect to be drawn. Valid values are taken from the DVASPECT and DVASPECT2 enumerations. In this example, I'm using DVASPECT.CONTENT so the value passed is 1.
• int lindex - portion of the object that is of interest for the draw operation. Currently, only -1 is supported.
• IntPtr pvAspect - pointer to the additional information.
• IntPtr ptd - describes the device for which the object is to be rendered. We will render for the default target device, so the value passed will be IntPtr.Zero.
• IntPtr hdcTargetDev - information context for the target device indicated by the ptd parameter.
• IntPtr hdcDraw - device context on which to draw.
• ref Rectangle lprcBounds - the size of the captured image.
• ref Rectangle lprcWBounds - the region of the WebBrowser object that we want to be captured.
• IntPtr pfnContinue - pointer to a callback function (not used here).
• UInt32 dwContinue - value to pass as a parameter to the function (not used here).

The HtmlCapture class

Now that we have defined our IViewObject interface, it is time to move on and create a class that will be used to capture a web page as an image.

public class HtmlCapture
{
    private WebBrowser web;
    private Timer tready;
    private Rectangle screen;
    private Size? imgsize=null; 

    //an event that triggers when the html document is captured
    public delegate void HtmlCaptureEvent(object sender, 
                         Uri url, Bitmap image);
    public event HtmlCaptureEvent HtmlImageCapture;
    
    //class constructor
    public HtmlCapture() 
    {
       //initialise the webbrowser and the timer
       web = new WebBrowser();
       tready = new Timer();
       tready.Interval = 2000;
       screen = Screen.PrimaryScreen.Bounds;
       //set the webbrowser width and hight
       web.Width = screen.Width;
       web.Height = screen.Height;
       //suppress script errors and hide scroll bars
       web.ScriptErrorsSuppressed = true;
       web.ScrollBarsEnabled = false;
       //attached events
       web.Navigating += 
         new WebBrowserNavigatingEventHandler(web_Navigating);
       web.DocumentCompleted += new 
         WebBrowserDocumentCompletedEventHandler(web_DocumentCompleted);
       tready.Tick += new EventHandler(tready_Tick);
    }
         
    #region Public methods
    public void Create(string url)
    {
        imgsize = null;
        web.Navigate(url);
    }

    public void Create(string url,Size imgsz) 
    {
        this.imgsize = imgsz;
        web.Navigate(url);
    }
    #endregion 
         
    #region Events
    void web_DocumentCompleted(object sender, 
             WebBrowserDocumentCompletedEventArgs e)
    {
        //start the timer
        tready.Start();
    }

    void web_Navigating(object sender, WebBrowserNavigatingEventArgs e)
    {
        //stop the timer   
        tready.Stop();
    }

    void tready_Tick(object sender, EventArgs e)
    {
        //stop the timer
        tready.Stop();
        
        //capture html as an image
        //...
    }
    #endregion
}

As you can see, I'm using a Timer object to determine if the HTML document is fully loaded and can be captured. The reason I'm doing this is because an HTML document can trigger the DocumentCompleted event multiple times. After the document is fully loaded, the tready_Tick method is called.

void tready_Tick(object sender, EventArgs e)
{
    //stop the timer
    tready.Stop();
    //get the size of the document's body
    Rectangle body = web.Document.Body.ScrollRectangle;

    //check if the document width/height is greater than screen width/height
    Rectangle docRectangle = new Rectangle()
    {
       Location=new Point(0,0),
        Size=new Size(body.Width > screen.Width ? body.Width : screen.Width,
         body.Height > screen.Height ? body.Height : screen.Height)
    };
    //set the width and height of the WebBrowser object
    web.Width = docRectangle.Width;
    web.Height = docRectangle.Height;
    
    //if the imgsize is null, the size of the image will 
    //be the same as the size of webbrowser object
    //otherwise  set the image size to imgsize
    Rectangle imgRectangle;
    if (imgsize == null)
        imgRectangle = docRectangle;
    else
        imgRectangle = new Rectangle()
        {
            Location=new Point(0,0),
            Size =imgsize.Value
        };
    //create a bitmap object 
    Bitmap bitmap = new Bitmap(imgRectangle.Width,imgRectangle.Height);
    //get the viewobject of the WebBrowser
    IViewObject ivo = web.Document.DomDocument as IViewObject;
     
    using (Graphics g = Graphics.FromImage(bitmap))
    {
        //get the handle to the device context and draw
        IntPtr hdc = g.GetHdc();
        ivo.Draw(1, -1, IntPtr.Zero, IntPtr.Zero,
                 IntPtr.Zero, hdc, ref imgRectangle, 
                 ref docRectangle, IntPtr.Zero, 0);
        g.ReleaseHdc(hdc);
    }
     //invoke the HtmlImageCapture event
    HtmlImageCapture(this, web.Url, bitmap);
}

Using the code

HtmlCapture has an overloaded method named Create. If you use the Create(string url) method, the size of the image will be the same as the size of the HTML document. If you want to create a thumbnail image of the HTML document, use Create(string url,Size imgsz).

private void button2_Click(object sender, EventArgs e)
{
    HtmlCapture hc = new HtmlCapture();
    hc.HtmlImageCapture += 
       new HtmlCapture.HtmlCaptureEvent(hc_HtmlImageCapture);
    hc.Create("http://www.codeproject.com");
    //or
    hc.Create("http://www.codeproject.com",new Size(200,300)); 
}

void hc_HtmlImageCapture(object sender, Uri url, Bitmap image)
{
    image.Save("C:/"+ url.Authority+ ".bmp");
}