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.THE IVIEWOBJECT INTERFACE
The very first thing that we must do is to define the
IViewObject
interface.
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[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 theDVASPECT
andDVASPECT2
enumerations. In this example, I'm usingDVASPECT.CONTENT
so the value passed is 1.int lindex
- portion of the object that is of interest for the drawoperation . 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 beIntPtr.
.Zero IntPtr hdcTargetDev
- information context for the target device indicated by theptd
parameter.IntPtr hdcDraw
- device context on which to draw.ref Rectangle lprcBounds
- the size of the captured image.ref Rectangle lprcWBounds
- the region of theWebBrowser
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.
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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 theDocumentCompleted
event multiple times. After the document is fully loaded, the tready_Tick
method is called.
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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)
.
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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");
}
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