Offscreen DCs

Just a quick question. Can you use GetPixel with an offscreen DC?

Sure, why not?

Well, ive found that it will return -1 if i use a hidden picture box. I'm wondering if it will work if i use an offscreen DC

It will.

If you want to use a picturebox set autoredraw to true..

Same problem appears for any DC if there's no bitmap selected.

Zaei, just create a DC in memory.
I do it in the source code I released/posted in the misc.bas module.

- jamie

Ok, from my experiments, you CANNOT use GetPixel
with an offscreen DC. You can Blt from one, which I
tested, but using GetPixel returns 0 (and when the pixel
referenced is invalid, it returns -1).

I made a memory DC and did a BitBlt of the desktop to it.
Using a For..Next loop with GetPixel() resulted in the colours and not 0 or -1!

Hrmmm.. Post code, please?

Well, here it is my friend:

Code:

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Option Explicit
  'This project needs 1 picturebox
  Private Declare Function BitBlt Lib "gdi32" (ByVal hDestDC As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal dwRop As Long) As Long
  Private Declare Function GetPixel Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long) As Long
  Private Declare Function SetPixelV Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal crColor As Long) As Long
  Private Declare Function CreateCompatibleBitmap Lib "gdi32" (ByVal hdc As Long, ByVal nWidth As Long, ByVal nHeight As Long) As Long
  Private Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long
  Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long
  Private Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long
  Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
  Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long

  Dim W      As Long
  Dim H      As Long
  Dim memDC  As Long
  Dim memBMP As Long

Private Sub Form_Activate()
  Me.Refresh
  BitBlt Me.hdc, 0, 0, W, H, memDC, 0, 0, vbSrcCopy
End Sub

Private Sub Form_DblClick()
  Unload Me
End Sub

Private Sub Form_Load()
  W = Screen.Width
  H = Screen.Height
  Picture1.ScaleMode = vbPixels
  Picture1.BackColor = 0
  Me.ScaleMode = vbPixels
  Me.Move 0, 0, W, H
  W = W / Screen.TwipsPerPixelX
  H = H / Screen.TwipsPerPixelY
  Picture1.Move W - Picture1.Width, H - Picture1.Height
  memDC = CreateCompatibleDC(Me.hdc)
  memBMP = CreateCompatibleBitmap(Me.hdc, W, H)
  SelectObject memDC, memBMP
  BitBlt memDC, 0, 0, W, H, GetDC(0), 0, 0, vbSrcCopy 'Capture desktop...
End Sub

Private Sub Form_MouseMove(Button As Integer, Shift As Integer, x As Single, y As Single)
  Dim C As Long
  Dim i As Integer
  Static BUSY As Boolean
 
  If (BUSY) Then Exit Sub
  BUSY = True
 
  If (x < Picture1.Width) And (y < Picture1.Height) Then
    For i = 0 To 9
      'Get the colour in MEMORY!!! not from the Form!!!
      C = GetPixel(memDC, x + i, y)
      SetPixelV Picture1.hdc, x + i, y, C
      C = GetPixel(memDC, x + i, y + 1)
      SetPixelV Picture1.hdc, x + i, y + 1, C
    Next i
  End If
  BUSY = False
End Sub

Private Sub Form_Unload(Cancel As Integer)
  DeleteDC memDC
  DeleteObject memBMP
  Set Form1 = Nothing
  End
End Sub

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