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Matthias Pospiech > Programming > C++ > MFC > Enhanced CMemDC

Enhanced CMemDC

Flicker Free drawing onto a dc can be realised by using the class CMemDC by Keith Rule (Flicker free drawing using memory DC)

I changed the Class in such a way that it supports the automatic resizing of images that have higher or lower size than the dc that it is painted onto. The change is thus from using BitBlt to StretchBlt.

#ifndef _MEMDC_H_
#define _MEMDC_H_
// CMemDC - memory DC
// Author: Keith Rule
// Email:
// 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.
// Changed by Matthias Pospiech (2007) with additional support 
// of stretching a bitmap onto a dc.
class CMemDC : public CDC {
	CBitmap		m_bitmap;		// Offscreen bitmap
	CBitmap*	m_oldBitmap;	// bitmap originally found in CMemDC
	CDC*		m_pDC;			// Saves CDC passed in constructor
	CRect		m_rect_dest;	// Rectangle of drawing area.
	CRect		m_rect_source;  // Rectangle of source area.
	BOOL		m_bMemDC;		// TRUE if CDC really is a Memory DC.
	CMemDC(CDC* pDC, const CRect* pSourceRect = NULL, const CRect* pDestRect = NULL) : CDC()
		// Some initialization
		m_pDC = pDC;
		m_oldBitmap = NULL;
		m_bMemDC = !pDC->IsPrinting();
		// Get the rectangle to draw
		if (pDestRect == NULL) {
		} else {
			m_rect_dest = *pDestRect;
		// Get the rectangle to draw
		if (pSourceRect == NULL) {
		} else {
			m_rect_source = *pSourceRect;
		if (m_bMemDC) {
			// Create a Memory DC
			m_bitmap.CreateCompatibleBitmap(pDC, m_rect_dest.Width(), m_rect_dest.Height());
			m_oldBitmap = SelectObject(&m_bitmap);
		} else {
			// Make a copy of the relevent 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_dest, pDC->GetBkColor());
		if (m_bMemDC) {
				m_rect_dest.left, // x-coord of destination upper-left corner,  // y-coord of destination upper-left corner
				m_rect_dest.Width(), // width of destination rectangle
				m_rect_dest.Height(), // height of destination rectangle
				this, // handle to source DC
				0, // x-coord of source upper-left corner
				0, // y-coord of source upper-left corner
				m_rect_source.Width(), // width of source rectangle
				m_rect_source.Height(),  // height of source rectangle
			//Swap back the original bitmap.
		} else {
			// All we need to do is replace the DC with an illegal value,
			// this keeps us from accidently 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;

It is used the same way as the original, just with an additional parameter for the size of the source.

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  1. Matthias Pospiech :: MFC: Flicker Free example with dialog - [...] Here is an example done by myself. I used the CMemDC Class posted in Enhanced CMemDC. [...]

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