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HTML to Image in VB

, 21 Jun 2012 CPOL
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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.

Imports System.Runtime.InteropServices

Imports System.Runtime.InteropServices.ComTypes

Imports System.Drawing

<ComVisible(True), ComImport> _
<GuidAttribute("0000010d-0000-0000-C000-000000000046")> _
<InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)> _
Public Interface IViewObject

<PreserveSig()> _
Function Draw(<MarshalAs(UnmanagedType.U4)> dwDrawAspect As UInt32, lindex As Integer, _
         pvAspect As IntPtr, <[In]()> ptd As IntPtr, hdcTargetDev As IntPtr, hdcDraw As IntPtr, _

<MarshalAs(UnmanagedType.Struct)> ByRef lprcBounds As Rectangle, _
              <MarshalAs(UnmanagedType.Struct)> ByRef lprcWBounds As Rectangle, _
              pfnContinue As IntPtr, <MarshalAs(UnmanagedType.U4)> dwContinue As UInt32) _
              As <MarshalAs(UnmanagedType.I4)> Integer

<PreserveSig()> _
Function GetColorSet(<[In](), MarshalAs(UnmanagedType.U4)> dwDrawAspect As Integer, _
   lindex As Integer, pvAspect As IntPtr, <[In]()> ptd As IntPtr, _
   hicTargetDev As IntPtr, <Out()> ppColorSet As IntPtr) As Integer

<PreserveSig()> _
Function Freeze(<[In](), MarshalAs(UnmanagedType.U4)> dwDrawAspect As Integer, _
   lindex As Integer, pvAspect As IntPtr, <Out()> pdwFreeze As IntPtr) As Integer

<PreserveSig()> _
Function Unfreeze(<[In](), MarshalAs(UnmanagedType.U4)> dwFreeze As Integer) As Integer

Sub SetAdvise(<[In](), MarshalAs(UnmanagedType.U4)> aspects As Integer, <[In](), _
  MarshalAs(UnmanagedType.U4)> advf As Integer, <[In](), _
  MarshalAs(UnmanagedType.[Interface])> pAdvSink As IAdviseSink)

Sub GetAdvise(<[In](), Out(), MarshalAs(UnmanagedType.LPArray)> paspects As Integer(), _
    <[In](), Out(), MarshalAs(UnmanagedType.LPArray)> advf As Integer(), _
    <[In](), Out(), MarshalAs(UnmanagedType.LPArray)> pAdvSink As IAdviseSink())

End Interface

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.

Imports System.Windows.Forms
Imports System.Drawing

Public Class HtmlCapture

    Private _Web As WebBrowser
    Private _Timer As Timer
    Private _Screen As Rectangle
    Private _ImgSize As System.Nullable(Of Size) = Nothing

    'an event that triggers when the html document is captured
    Public Delegate Sub HtmlCaptureEvent(sender As Object, url As Uri, image As Bitmap)
    Public Event HtmlImageCapture As HtmlCaptureEvent

    'class constructor
    Public Sub New()
        'initialise the webbrowser and the timer
        _web = New WebBrowser()
        _Timer = New Timer()
        _Timer.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
        AddHandler _web.Navigating, AddressOf web_Navigating
        AddHandler _web.DocumentCompleted, AddressOf web_DocumentCompleted
        AddHandler _Timer.Tick, AddressOf tready_Tick
    End Sub

#Region "Public methods"
    Public Sub Create(url As String)
        _ImgSize = Nothing
    End Sub

    Public Sub Create(url As String, imgsz As Size)
        Me._ImgSize = imgsz
    End Sub
#End Region

#Region "Events"
    Private Sub web_DocumentCompleted(sender As Object, e As WebBrowserDocumentCompletedEventArgs)
        'start the timer
    End Sub

    Private Sub web_Navigating(sender As Object, e As WebBrowserNavigatingEventArgs)
        'stop the timer   
    End Sub

    Private Sub tready_Tick(sender As Object, e As EventArgs)
        'stop the timer
        'get the size of the document's body
        Dim body As Rectangle = _Web.Document.Body.ScrollRectangle

        'check if the document width/height is greater than screen width/height
        Dim docRectangle As New Rectangle() With { _
         .Location = New Point(0, 0), _
         .Size = New Size(If(body.Width > _Screen.Width, body.Width, _Screen.Width), _
                 If(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
        Dim imgRectangle As Rectangle
        If _ImgSize Is Nothing Then
            imgRectangle = docRectangle
            imgRectangle = New Rectangle() With { _
             .Location = New Point(0, 0), _
             .Size = _ImgSize.Value _
        End If
        'create a bitmap object 
        Dim bitmap As New Bitmap(imgRectangle.Width, imgRectangle.Height)
        'get the viewobject of the WebBrowser
        Dim ivo As IViewObject = TryCast(_Web.Document.DomDocument, IViewObject)

        Using g As Graphics = Graphics.FromImage(bitmap)
            'get the handle to the device context and draw
            Dim hdc As IntPtr = g.GetHdc()
            ivo.Draw(1, -1, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero, hdc, _
             imgRectangle, docRectangle, IntPtr.Zero, 0)
        End Using

        'invoke the HtmlImageCapture event
        RaiseEvent HtmlImageCapture(Me, _Web.Url, bitmap)
    End Sub
#End Region

End Class

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.

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 Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    Dim hc As New HtmlCapture()
    AddHandler hc.HtmlImageCapture, AddressOf hc_HtmlImageCapture
    'hc.Create("", New Size(200, 300))
End Sub
Private Sub hc_HtmlImageCapture(sender As Object, url As Uri, image As Bitmap)
    image.Save(OutputDirectory + url.Authority + ".bmp")
End Sub


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Terence Wallace
Founder Arkitech EBC Corporation
United States United States
MS, BBA, software developer, consultant, and trainer. Specializing in building data-centric applications designed for business, university, community & faith based organizations. Started developing Excel VBA macros and never looked back. Freelance developer utilizing VB.Net, SQL Server, Microsoft Access, and ASP.Net.

Comments and Discussions

QuestionHTML to Image for .NET PinmemberFarhomar14-May-14 2:06 
AnswerRe: HTML to Image for .NET PinmemberTL Wallace14-May-14 4:40 
AnswerRe: HTML to Image for .NET PinmemberDewey6-Oct-14 8:40 
GeneralMy vote of 1 PinmemberEdward Nashton8-Jul-13 8:54 
QuestionHTMLCapture Class - HTML element instead of _Web.Document.DomDocument PinmemberLuciana Silvestre24-Aug-12 11:19 
This project is very good and works fine.
But I would like to make a change in HtmlCapture class but I don't know how the interface IViewObject works.
I want to put a HTML element instead of "_Web.Document.DomDocument" but I don't know it is possible.
Your code looks like this:
Dim ivo IViewObject = TryCast (_Web.Document.DomDocument, IViewObject)

I would like to pass only the HTML table element:
Dim element as HtmlElement
Dim ivo IViewObject = TryCast (element, IViewObject)
But when I put the Html Element in the TryCast the variable ivo stay "nothing"
Why It's happening?
Tks Luciana
AnswerRe: HTMLCapture Class - HTML element instead of _Web.Document.DomDocument PinmemberTL Wallace29-Aug-12 9:32 
QuestionCheck this out! PinmemberDadajiIn30-Jul-12 0:35 

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