Double buffering with Memory DC

[Rick H]

I am writing a graph program that plots data in real time. At the moment I am using two picture boxes, I draw the data block (50ms worth of data) onto the background picturebox and then use BitBlt to copy this block onto the front picbox. This works great. However I am trying to replace to background picturebox with a memory dc.

This is the code I am using:

dim dc As Long            ' Memory DC for double buffering
dim mempic As PictureBox  ' Memory picture box used for double buffering

dc = CreateCompatibleDC(UserControl.hdc)
If dc = vbNull Then StatusBar1.SimpleText = "Failed to create memory DC"

Set mempic = PicMain ' copy picmain attributes
SelectObject dc, mempic

picmain is my front picturebox. I cant get this to work though. If I run run the usercontrol it comes up blank. Any ideas?

Cheers

[Fox]

Try this function to load an empty DC:

'Types
    Type tRect
        'Coordinates
        x As Long
        y As Long
        
        X2 As Long
        Y2 As Long
    End Type

'Declares
    Private Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc 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 SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long
    
    Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
    Public Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long
    
    Public Declare Function FillRect Lib "user32" (ByVal hdc As Long, lpRect As tRect, ByVal hBrush As Long) As Long

'Create a DC
Function CreateDC(iW As Long, iH As Long) As Long
    Dim Temp As Long
    Dim Rect As tRect
    
    'Create
    CreateDC = CreateCompatibleDC(frmMain.hdc)
    Temp = CreateCompatibleBitmap(frmMain.hdc, iW, iH)
    
    'Get the DC
    SelectObject CreateDC, Temp
    DeleteObject Temp

    'Clear
    Rect.X2 = iW
    Rect.Y2 = iH
    
    FillRect CreateDC, Rect, 0
End Function

And keep up the good work! (use DCs instead of pics )

[Mad Compie]

I always use this kind of code:

Option Explicit  
  Dim memDC       As Long
  Dim memBMP      As Long

Sub Form_Load()
  memDC = CreateCompatibleDC(0)
  memBMP = CreateCompatibleBitmap(Me.hdc, ScrWidth, ScrHeight)
  SelectObject memDC, memBMP
  '... Do some actions
  'i.e. this Blt copies the desktop into the memory DC
  BitBlt memDC, 0, 0, ScrWidth, ScrHeight, GetDC(0), 0, 0, vbSrcCopy
  '...
End Sub

Private Sub Form_Unload(Cancel As Integer)
  DeleteDC memDC
  DeleteObject memBMP
End Sub

[dangerousdave]

Hmm,

Check Autoredraw properties are correct.

Also I noticed you didnt actually blit anything to the new picturebox.

I cant really put anything else that the other 2 havnt already put. Try using their methods. From my reckoning that should work.... I aint used BitBlt and the like in a while though so I could be rusty.