How To: Fake a png alpha-blending background...

[mtlca401]

I have a way to fake a png background for a form. I am sure other people have thought of this though it works perfect for stationary windows or splash screens.

It looks like this:



I can't upload my actual source code because the attach file icon is X'ed out.


paste this into a module:

I don't take credit for the screenshot code. I downloaded it from codeproject.com I think but I'm not sure.

'Imports System.Runtime.InteropServices
'Imports System.Text

Public Class modScreenCapture

    ' The ScreenCapture class allows you to take screenshots (printscreens)
    ' of the desktop or of individual windows.
    '
    ' Usage:
    '
    ' PictureBox1.Image = ScreenCapture.GrabScreen()
    ' PictureBox1.Image = ScreenCapture.GrabActiveWindow()
    ' PictureBox1.Image = ScreenCapture.GrabWindow(SomeHwnd)
    '
    ' PictureBox1.Image = ScreenCapture.GrabScreen(X, Y, Width, Height)
    ' PictureBox1.Image = ScreenCapture.GrabScreen(Rect)
    ' PictureBox1.Image = ScreenCapture.GrabScreen(Location, Size)


#Region "Constants"

    Private Const HORZRES As Integer = 8
    Private Const VERTRES As Integer = 10
    Private Const SRCCOPY = &HCC0020
    Private Const SRCINVERT = &H660046

    Private Const USE_SCREEN_WIDTH = -1
    Private Const USE_SCREEN_HEIGHT = -1

#End Region

#Region "API's"

    Private Structure RECT
        Public Left As Int32
        Public Top As Int32
        Public Right As Int32
        Public Bottom As Int32
    End Structure

    Private Declare Function CreateDC Lib "gdi32" Alias "CreateDCA" (ByVal lpDriverName As String, ByVal lpDeviceName As String, ByVal lpOutput As String, ByVal lpInitData As String) As Integer
    Private Declare Function CreateCompatibleDC Lib "GDI32" (ByVal hDC As Integer) As Integer
    Private Declare Function DeleteDC Lib "GDI32" (ByVal hDC As Integer) As Integer
    Private Declare Function GetWindowDC Lib "user32" Alias "GetWindowDC" (ByVal hwnd As Integer) As Integer
    Private Declare Function ReleaseDC Lib "user32" Alias "ReleaseDC" (ByVal hwnd As Integer, ByVal hdc As Integer) As Long
    Private Declare Function GetDeviceCaps Lib "gdi32" Alias "GetDeviceCaps" (ByVal hdc As Integer, ByVal nIndex As Integer) As Integer
    Private Declare Function CreateCompatibleBitmap Lib "GDI32" (ByVal hDC As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer) As Integer
    Private Declare Function SelectObject Lib "GDI32" (ByVal hDC As Integer, ByVal hObject As Integer) As Integer
    Private Declare Function DeleteObject Lib "GDI32" (ByVal hObj As Integer) As Integer
    Private Declare Function BitBlt Lib "GDI32" (ByVal hDestDC As Integer, ByVal X As Integer, ByVal Y As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer, ByVal hSrcDC As Integer, ByVal SrcX As Integer, ByVal SrcY As Integer, ByVal Rop As Integer) As Integer
    Private Declare Function GetForegroundWindow Lib "user32" Alias "GetForegroundWindow" () As Integer
    Private Declare Function IsWindow Lib "user32" Alias "IsWindow" (ByVal hwnd As Integer) As Long
    Private Declare Function GetWindowRect Lib "user32.dll" (ByVal hWnd As Integer, ByRef lpRect As RECT) As Int32
    Private Declare Function MoveWindow Lib "user32" (ByVal hwnd As Integer, ByVal x As Integer, ByVal y As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer, ByVal bRepaint As Integer) As Long

#End Region

#Region "Public Methods."

    Public Shared Function GrabScreen() As Bitmap

        Return GrabScreen(0, 0, USE_SCREEN_WIDTH, USE_SCREEN_HEIGHT)

    End Function

    Public Shared Function GrabScreen(ByVal Rect As Rectangle) As Bitmap

        Return GrabScreen(Rect.X, Rect.Y, Rect.Width, Rect.Height)

    End Function

    Public Shared Function GrabScreen(ByVal Location As System.Drawing.Point, ByVal Size As System.Drawing.Size) As Bitmap

        Return GrabScreen(Location.X, Location.Y, Size.Width, Size.Height)

    End Function

    Public Shared Function GrabScreen(ByVal X As Integer, ByVal Y As Integer, ByVal Width As Integer, ByVal Height As Integer) As Bitmap

        Dim hDesktopDC As Integer
        Dim hOffscreenDC As Integer
        Dim hBitmap As Integer
        Dim hOldBmp As Integer
        Dim MyBitmap As Bitmap

        ' Get the desktop device context.
        hDesktopDC = CreateDC("DISPLAY", "", "", "")
        If hDesktopDC Then
            ' Adjust width and height.
            If Width = USE_SCREEN_WIDTH Then
                Width = GetDeviceCaps(hDesktopDC, HORZRES)
            End If
            If Height = USE_SCREEN_HEIGHT Then
                Height = GetDeviceCaps(hDesktopDC, VERTRES)
            End If
            ' Create an offscreen device context.
            hOffscreenDC = CreateCompatibleDC(hDesktopDC)
            If hOffscreenDC Then
                ' Create a bitmap for our offscreen device context.
                hBitmap = CreateCompatibleBitmap(hDesktopDC, Width, Height)
                If hBitmap Then
                    ' Copy the image and create an instance of the Bitmap class.
                    hOldBmp = SelectObject(hOffscreenDC, hBitmap)
                    BitBlt(hOffscreenDC, 0, 0, Width, Height, hDesktopDC, X, Y, SRCCOPY)
                    MyBitmap = Bitmap.FromHbitmap(New IntPtr(hBitmap))
                    ' Clean up.
                    DeleteObject(SelectObject(hOffscreenDC, hOldBmp))
                End If
                DeleteDC(hOffscreenDC)
            End If
            DeleteDC(hDesktopDC)
        End If
        ' Return our Bitmap instance.
        Return MyBitmap

    End Function

    Public Shared Function GrabActiveWindow() As Bitmap

        Return GrabWindow(GetForegroundWindow())

    End Function

    Public Shared Function GrabWindow(ByVal hWnd As Integer) As Bitmap

        Dim hWindowDC As Integer
        Dim hOffscreenDC As Long
        Dim rec As RECT
        Dim nWidth As Long
        Dim nHeight As Long
        Dim hBitmap As Long
        Dim hOldBmp As Long
        Dim MyBitmap As Bitmap


        ' Verify if a valid window handle was provided.
        If hWnd <> 0 And IsWindow(hWnd) Then
            ' Get the window's device context.
            hWindowDC = GetWindowDC(hWnd)
            If hWindowDC Then
                ' Get width and height.
                If GetWindowRect(hWnd, rec) Then
                    nWidth = rec.Right - rec.Left
                    nHeight = rec.Bottom - rec.Top

                    ' Create an offscreen device context.
                    hOffscreenDC = CreateCompatibleDC(hWindowDC)
                    If hOffscreenDC Then
                        ' Create a bitmap for our offscreen device context.
                        hBitmap = CreateCompatibleBitmap(hWindowDC, nWidth, nHeight)
                        If hBitmap Then
                            ' Copy the image and create an instance of the Bitmap class.
                            hOldBmp = SelectObject(hOffscreenDC, hBitmap)
                            BitBlt(hOffscreenDC, 0, 0, nWidth, nHeight, hWindowDC, 0, 0, SRCCOPY)
                            MyBitmap = Bitmap.FromHbitmap(New IntPtr(hBitmap))
                            ' Clean up.
                            DeleteObject(SelectObject(hOffscreenDC, hOldBmp))
                        End If
                        DeleteDC(hOffscreenDC)
                    End If
                End If
                ReleaseDC(hWnd, hWindowDC)
            End If
        End If
        ' Return our Bitmap instance.
        Return MyBitmap

    End Function

#End Region

End Class

and put this in a timer event:

set the timer event to 100

set the transparency key below to whatever it is on your form.

Me.TransparencyKey = Color.Magenta
Me.BackgroundImage = modScreenCapture.GrabScreen(Me.Left, Me.Top, Me.Width, Me.Height)

make sure the transparency is set on your form and the background color

add a picturebox to your form and load a png, set the background color to transparent. Make sure that the png file doesn't have a background color already in it.


All this is doing is taking a screenshot of the desktop and setting it to the background image of the form. Use this for a splashscreen or something because if you try to move the form it doesn't update fast enough.

I know this is a round about way of doing this, but it's easy to do and understand. This is just a work around for me until I understand GDI+ and setting pixels. It works for me.