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Flicker Free Drawing In MFC

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25 Mar 2002CPOL1 min read 1.2M   18.1K   314   234
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

 
GeneralError in SetWindowExt and SetViewportExt used in OnPrepareDC Pin
zhuoran4-Jan-04 15:04
zhuoran4-Jan-04 15:04 
QuestionAre there improvements to this somewhere? Pin
prcarp15-Dec-03 2:44
prcarp15-Dec-03 2:44 
GeneralGreat! Pin
tbrammer27-Nov-03 0:14
professionaltbrammer27-Nov-03 0:14 
GeneralJust get a blank window! Pin
Leif Bloomquist25-Nov-03 3:44
Leif Bloomquist25-Nov-03 3:44 
GeneralRe: Just get a blank window! Pin
rw10426-Nov-03 1:48
rw10426-Nov-03 1:48 
GeneralRe: Just get a blank window! Pin
Leif Bloomquist26-Nov-03 3:18
Leif Bloomquist26-Nov-03 3:18 
GeneralRe: Just get a blank window! Pin
Anonymous5-Apr-04 15:12
Anonymous5-Apr-04 15:12 
QuestionCRectTracker Disappears with CMemDC? Pin
cdsmith21-Nov-03 8:27
cdsmith21-Nov-03 8:27 
I have a view I have been using for a while. It is a WYSIWYG editing view. I use CRectTracker to allow the user to move items around on the view. When I changed over to CMemDC the resize handles for the CRectTracker and the dotted line border disappeared. I've tried moving the handles inside the tracking rect and outside the tracking rect, no luck. Anybody else see this? Anybody know what I am doing wrong? I would love to use CMemDC with CRectTrackers.

Thanks for any help!
AnswerRe: CRectTracker Disappears with CMemDC? Pin
trevium:)14-Jun-04 8:04
trevium:)14-Jun-04 8:04 
QuestionHow would you integrate/modify CMemDC to work with GDI+? Pin
Salvador Dali30-Oct-03 2:36
Salvador Dali30-Oct-03 2:36 
GeneralWorks like a plug-in almost Pin
Grape-Man20-Oct-03 19:01
Grape-Man20-Oct-03 19:01 
QuestionCan someone help Pin
Member 6236969-Oct-03 13:00
Member 6236969-Oct-03 13:00 
GeneralNooB Question Pin
Ruination9-Sep-03 17:13
Ruination9-Sep-03 17:13 
GeneralRe: WM_ERASEBKGND Pin
Ruination10-Sep-03 12:08
Ruination10-Sep-03 12:08 
GeneralRe: NooB Question Pin
Anonymous5-Apr-04 15:16
Anonymous5-Apr-04 15:16 
GeneralOvelapped CWnds Pin
Mark Ruzon19-Aug-03 7:05
Mark Ruzon19-Aug-03 7:05 
GeneralDrawing transparent image Pin
6-Aug-03 19:16
suss6-Aug-03 19:16 
QuestionWhat happened to Microsoft's CMemDC? Pin
Anonymous2-Aug-03 14:35
Anonymous2-Aug-03 14:35 
AnswerRe: What happened to Microsoft's CMemDC? Pin
Steve Mayfield20-Sep-03 20:27
Steve Mayfield20-Sep-03 20:27 
GeneralWindows CE / Pocket PC Pin
bothari25-Jun-03 12:08
bothari25-Jun-03 12:08 
GeneralRe: Windows CE / Pocket PC Pin
João Paulo Figueira18-Sep-03 1:26
professionalJoão Paulo Figueira18-Sep-03 1:26 
Generalerror , please help~ Pin
cclai10-Jun-03 19:41
cclai10-Jun-03 19:41 
GeneralCrash within CMemDC Pin
Blue Solitare4-Jun-03 16:11
Blue Solitare4-Jun-03 16:11 
GeneralCoordinates Pin
Frank Hald2-Jun-03 2:23
Frank Hald2-Jun-03 2:23 
GeneralA little more explanation Pin
Mohammed El Shoukry1-Jun-03 5:32
Mohammed El Shoukry1-Jun-03 5:32 

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