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Posted 20 Nov 1999

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Flicker Free Drawing In MFC

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4.96/5 (171 votes)
25 Mar 2002CPOL1 min read
A simple animation example which is used to show CMemDC in several modes

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A simple animation example which is used to show CMemDC in several mode

Introduction

Removing flicker from an MFC application is well-covered territory. You can find the topic addressed in books, and on-line. However, the techniques presented are somewhat complicated and are usually difficult to add to an existing application. One often-presented technique is called double buffering. Double buffering allows the new screen to be drawn in off-screen memory, and then the completed screen is bit-blited back onto the physical screen.

This article presents a class called CMemDC that encapsulates most of the issues associated with writing to off-screen buffers. Adding CMemDC to an existing application or MFC Active X control is nearly trivial.

Modifying an MFC Application to Use CMemDC

  • Add the file memdc.h in your project.
  • Add the line #include "memdc.h" to stdafx.h.
  • Add a windows message handler for WM_ERASEBKGND.
  • Change the code in the message handler as follows:
    C++
    // Change this code
    BOOL CExampleView::OnEraseBkgnd(CDC* pDC) 
    {
          // TODO: Add your message handler code here and/or call default
          return CView::OnEraseBkgnd(pDC);
    }
     
    // To this code
    BOOL CExampleView::OnEraseBkgnd(CDC* pDC) 
    {
          return FALSE;
    }
  • Change your OnDraw code to the following:
    C++
    void CExampleView::OnDraw(CDC* dc)
    {
        CMemDC pDC(dc);
        CExampleDoc* pDoc = GetDocument();
        ASSERT_VALID(pDoc);
        // TODO: add draw code for native data here - use pDC 
         //as the device context to draw to
    }

Compile your code after you've made these changes and you will notice that the flicker you had seen before is gone.

Modifying a MFC Active X Control to Use CMemDC

To add CMemDC support, you follow the instruction for adding the support to an application, however you make one small change in the OnDraw function.

C++
void CParticleTestCtlCtrl::OnDraw(CDC* pdc, const CRect& rcBounds, 
                                  const CRect& rcInvalid)
{
    CMemDC pDC(pdc, &rcBounds);
    // TODO: add draw code for native data here
    // - use pDC as the device context to draw 
}

The only substantial difference is that the rcBounds is passed to the CMemDC constructor.

Source Code

C++
#ifndef _MEMDC_H_
#define _MEMDC_H_
 
//////////////////////////////////////////////////
// CMemDC - memory DC
//
// Author: Keith Rule
// Email:  keithr@europa.com
// Copyright 1996-2002, Keith Rule
//
// You may freely use or modify this code provided this
// Copyright is included in all derived versions.
//
// History - 10/3/97 Fixed scrolling bug.
//               Added print support. - KR
//
//       11/3/99 Fixed most common complaint. Added
//            background color fill. - KR
//
//       11/3/99 Added support for mapping modes other than
//            MM_TEXT as suggested by Lee Sang Hun. - KR
//
//       02/11/02 Added support for CScrollView as supplied
//             by Gary Kirkham. - KR
//
// This class implements a memory Device Context which allows
// flicker free drawing.
 
class CMemDC : public CDC {
private:       
    CBitmap    m_bitmap;        // Offscreen bitmap
    CBitmap*       m_oldBitmap; // bitmap originally found in CMemDC
    CDC*       m_pDC;           // Saves CDC passed in constructor
    CRect      m_rect;          // Rectangle of drawing area.
    BOOL       m_bMemDC;        // TRUE if CDC really is a Memory DC.
public:
    
    CMemDC(CDC* pDC, const CRect* pRect = NULL) : CDC()
    {
        ASSERT(pDC != NULL); 
 
        // Some initialization
        m_pDC = pDC;
        m_oldBitmap = NULL;
        m_bMemDC = !pDC->IsPrinting();
 
        // Get the rectangle to draw
        if (pRect == NULL) {
             pDC->GetClipBox(&m_rect);
        } else {
             m_rect = *pRect;
        }
 
        if (m_bMemDC) {
             // Create a Memory DC
             CreateCompatibleDC(pDC);
             pDC->LPtoDP(&m_rect);
 
             m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), 
                                                  m_rect.Height());
             m_oldBitmap = SelectObject(&m_bitmap);
 
             SetMapMode(pDC->GetMapMode());
 
             SetWindowExt(pDC->GetWindowExt());
             SetViewportExt(pDC->GetViewportExt());
 
             pDC->DPtoLP(&m_rect);
             SetWindowOrg(m_rect.left, m_rect.top);
        } else {
             // Make a copy of the relevant parts of the current 
             // DC for printing
             m_bPrinting = pDC->m_bPrinting;
             m_hDC       = pDC->m_hDC;
             m_hAttribDC = pDC->m_hAttribDC;
        }
 
        // Fill background 
        FillSolidRect(m_rect, pDC->GetBkColor());
    }
    
    ~CMemDC()      
    {          
        if (m_bMemDC) {
             // Copy the offscreen bitmap onto the screen.
             m_pDC->BitBlt(m_rect.left, m_rect.top, 
                           m_rect.Width(),  m_rect.Height(),
                  this, m_rect.left, m_rect.top, SRCCOPY);            
             
             //Swap back the original bitmap.
             SelectObject(m_oldBitmap);        
        } else {
             // All we need to do is replace the DC with an illegal
             // value, this keeps us from accidentally deleting the 
             // handles associated with the CDC that was passed to 
             // the constructor.              
             m_hDC = m_hAttribDC = NULL;
        }       
    }
    
    // Allow usage as a pointer    
    CMemDC* operator->() 
    {
        return this;
    }       
 
    // Allow usage as a pointer    
    operator CMemDC*() 
    {
        return this;
    }
};
 
#endif

License

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


Written By
Software Developer (Senior)
United States United States
I work at Tektronix in Beaverton OR. I've been programming for fun since 1975 (I started while in a Computer Explorer Scout group in Spokane WA). I've been programming in C since 1979 and I've been working professionally since 1983.

I really enjoy www.codeproject.com. It has saved me an incredible amount of time. I only hope my small contributions have given back some of what I've taken.

Comments and Discussions

 
QuestionLicense of MemDC Pin
Member 147604693-Mar-20 1:52
Member 147604693-Mar-20 1:52 
QuestionView not invalidated when child moves partially outside of parent frame Pin
RockyMu13-Oct-18 15:50
RockyMu13-Oct-18 15:50 
GeneralMy vote of 5 Pin
DoDucViet29-Aug-18 3:39
DoDucViet29-Aug-18 3:39 
QuestionThis class remove my background image Pin
TRAORE cheickna28-Jul-17 15:05
TRAORE cheickna28-Jul-17 15:05 
QuestionThank you, it worked! Pin
Member 124957401-May-16 0:57
Member 124957401-May-16 0:57 
Questionstill flickering Pin
Member 109214303-Jul-14 1:47
Member 109214303-Jul-14 1:47 
GeneralMy vote of 5 Pin
Member 1089899126-Jun-14 8:36
Member 1089899126-Jun-14 8:36 
GeneralMy vote of 5 Pin
WebBiscuit15-Sep-13 12:50
WebBiscuit15-Sep-13 12:50 
QuestionIs needed DeleteDC() ? Pin
JiHyung_Lee27-Jan-13 16:36
JiHyung_Lee27-Jan-13 16:36 
QuestionBig Thanks! Pin
sally Fields5-Nov-12 13:42
sally Fields5-Nov-12 13:42 
QuestionProblems with CScrollView Pin
UrbanBlues3-Feb-12 21:48
UrbanBlues3-Feb-12 21:48 
AnswerRe: Problems with CScrollView Pin
AndersBlom29-May-12 23:53
AndersBlom29-May-12 23:53 
GeneralMy vote of 5 Pin
=asm=1-Sep-11 1:24
=asm=1-Sep-11 1:24 
QuestionCo-existing with CMemDC from MFC Pin
Jason Steenstra-Pickens19-Apr-11 22:05
Jason Steenstra-Pickens19-Apr-11 22:05 
AnswerRe: Co-existing with CMemDC from MFC Pin
MatrixDud29-Apr-11 10:40
MatrixDud29-Apr-11 10:40 
GeneralRe: Co-existing with CMemDC from MFC Pin
Ben Aldhouse18-Aug-11 22:27
Ben Aldhouse18-Aug-11 22:27 
GeneralMy vote of 5 Pin
yiguro13-Jan-11 2:34
yiguro13-Jan-11 2:34 
GeneralThanks a lot!!!! Pin
yiguro13-Jan-11 2:32
yiguro13-Jan-11 2:32 
GeneralMy vote of 5 Pin
bogdan3478-Jan-11 23:57
bogdan3478-Jan-11 23:57 
GeneralMemory leak after override OnEraseBkgnd method Pin
Penghui9-Jun-10 7:19
Penghui9-Jun-10 7:19 
GeneralCMemDC and scaling paint Pin
npuleio2-Feb-10 5:35
npuleio2-Feb-10 5:35 
GeneralLicense Concern Pin
aitieng31-Jan-10 16:22
aitieng31-Jan-10 16:22 
GeneralRe: License Concern Pin
Keith Rule31-Jan-10 21:11
professionalKeith Rule31-Jan-10 21:11 
QuestionRe: License Concern Pin
EstelD22-Jun-10 12:14
EstelD22-Jun-10 12:14 
AnswerRe: License Concern Pin
Keith Rule22-Jun-10 12:47
professionalKeith Rule22-Jun-10 12:47 

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