Capturing a still image from a Webcam in .net

Standard

image 

What if you want to capture a single image from a webcam?  It turns out that this is quite straight forward to do using a couple of Win32 api calls to the avicap32.dll library.

I encapsulated these api calls in a simple class.

To write a file called “snapshot.jpg” out to the desktop from a single webcam, simply declare the class and call a the TakePicture method.

Dim loSnap As New WebcamSnap
loSnap.TakePicture(0)

The devices collection shows a list of all webcam devices on your machine so you can populate a combo with them.

Note: This example has no decent error handling, which I suspect will be needed in the “real world” of multiple devices.

Here’s the WebcamSnap class:

Option Explicit On
Imports System.IO
Imports System.Runtime.InteropServices
''' <summary>
''' This class takes a snapshot from any or all attached webcams
''' </summary>
''' <remarks></remarks>
Public Class WebcamSnap
    Const WM_CAP As Short = &H400S

    Const WM_CAP_DRIVER_CONNECT As Integer = WM_CAP + 10
    Const WM_CAP_DRIVER_DISCONNECT As Integer = WM_CAP + 11
    Const WM_CAP_EDIT_COPY As Integer = WM_CAP + 30
    Const WS_CHILD As Integer = &H40000000
    Const WS_VISIBLE As Integer = &H10000000
    Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
        (ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, _
        <MarshalAs(UnmanagedType.AsAny)> ByVal lParam As Object) As Integer

    Declare Function capCreateCaptureWindowA Lib "avicap32.dll" _
        (ByVal lpszWindowName As String, ByVal dwStyle As Integer, _
        ByVal x As Integer, ByVal y As Integer, ByVal nWidth As Integer, _
        ByVal nHeight As Short, ByVal hWndParent As Integer, _
        ByVal nID As Integer) As Integer

    Declare Function capGetDriverDescriptionA Lib "avicap32.dll" (ByVal wDriver As Short, _
        ByVal lpszName As String, ByVal cbName As Integer, ByVal lpszVer As String, _
        ByVal cbVer As Integer) As Boolean

    Public Devices As New List(Of String)
    Public Height As Integer = 480
    Public Width As Integer = 640
    Public OutputPath As String = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory)
    Public FilenamePrefix As String = "snapshot"
    Public Sub New()
        mLoadDeviceList()
    End Sub
    Private Sub mLoadDeviceList()
        Dim lsName As String = Space(100)
        Dim lsVers As String = Space(100)
        Dim lbReturn As Boolean
        Dim x As Integer = 0

        Do
            '   Get Driver name and version
            lbReturn = capGetDriverDescriptionA(x, lsName, 100, lsVers, 100)

            ' If there was a device add device name to the list
            If lbReturn Then Devices.Add(lsName.Trim)
            x += 1
        Loop Until lbReturn = False
    End Sub
    Public Sub TakePicture()
        For i = 0 To Me.Devices.Count - 1
            Dim lsFilename As String = Path.Combine(OutputPath, Me.FilenamePrefix & i & ".jpg")
            TakePicture(i, lsFilename)
        Next
    End Sub
    Public Sub TakePicture(ByVal iDevice As Integer)
        Me.TakePicture(iDevice, Path.Combine(OutputPath, Me.FilenamePrefix & ".jpg"))
    End Sub
    Public Sub TakePicture(ByVal iDevice As Integer, ByVal filename As String)

        Dim lhHwnd As Integer ' Handle to preview window

        ' Create a form to play with
        Using loWindow As New System.Windows.Forms.Form

            ' Create capture window
            lhHwnd = capCreateCaptureWindowA(iDevice, WS_VISIBLE Or WS_CHILD, 0, 0, Me.Width, _
               Me.Height, loWindow.Handle.ToInt32, 0)

            ' Hook up the device
            SendMessage(lhHwnd, WM_CAP_DRIVER_CONNECT, iDevice, 0)
            ' Allow the webcam apeture to let enough light in
            For i = 1 To 10
                Application.DoEvents()
            Next

            ' Copy image to clipboard
            SendMessage(lhHwnd, WM_CAP_EDIT_COPY, 0, 0)

            ' Get image from clipboard and convert it to a bitmap
            Dim loData As IDataObject = Clipboard.GetDataObject()
            If loData.GetDataPresent(GetType(System.Drawing.Bitmap)) Then
                Using loBitmap As Image = CType(loData.GetData(GetType(System.Drawing.Bitmap)), Image)
                    loBitmap.Save(filename, Imaging.ImageFormat.Jpeg)
                End Using
            End If

            SendMessage(lhHwnd, WM_CAP_DRIVER_DISCONNECT, iDevice, 0)

        End Using

    End Sub

End Class

Links

I found the original code sample here:

http://www.experts-exchange.com/Programming/Languages/.NET/Visual_Basic.NET/Q_22482234.html

(Although I’ve seen it all over the web in various guises)

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