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Introduction

This article explains how my class CDXSurfaceMgr can be used to facilitate Double Buffered drawing/graphics.

Double Buffered Drawing

Normally when drawing using the GDI you do all your painting straight into the HDC that's provided by either BeginPaint or a CDC (if you are using MFC). Iif you draw a line it immediately appears on the surface of the window you are drawing on. If you draw a lot of graphic items the screen is updated with each item and this can result in flicker or stuttering as the surface is filled with the drawing. Double Buffering can be used to create smooth updates, you build a "frame" of graphics in a non visible place first then copy the whole thing onto the main windows surface.

There a lots of ways to implement double buffering, one way is to create a compatible HDC in memory somewhere and draw into that then Blit (copy) that memory into the visible windows HDC. My approach uses a subset of DirectX called DirectDraw which can be used to draw 2d Graphics. CDXSurfaceMgr is written using the DirectDraw interfaces supplied in VC6 (ddraw.h/lib) and should work on 95/98/Me/2k/XP and NT 4.0 (sp3 - I think).

Using CDXSurfaceMgr

Create an instance of CDXSurfaceMgr

#include "DXSurfaceMgr.h"
...
...
CDXSurfaceManager_NBP dxMgr_;

Initialise the instance like so

int CMfcdxView::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
 if (CView::OnCreate(lpCreateStruct) == -1)
         return -1;
 // Initialise CDXSurfaceMgr
 if(!dxMgr_.Initialise(m_hWnd)) return -1;

 return 0;
}

This should be done just once, the parameter to the Initialise method is the Handle to the window you want to draw on. Then when handling the WM_PAINT message (the place you would normally do the drawing) call the BeginPaint Method

void CMfcdxView::OnDraw(CDC* pDC)
{
 CMfcdxDoc* pDoc = GetDocument();
 ASSERT_VALID(pDoc);
 // TODO: add draw code for native data here

 HDC& hdc = dxMgr_.BeginPaint(GetSysColor(COLOR_3DFACE));
 if(NULL !=hdc)
 {
 ....

The parameter to BeginPaint is a COLORREF that BeginPaint will use to fill the background with. BeginPaint returns an HDC that that you can then use to draw things with, either via the standard GDI API;

 ::SetBkMode(hdc,TRANSPARENT);
 ::TextOut(hdc,10,10,"Please don't call me reg its not my name", 40);

Or if you are using MFC;

 CDC* cdc = CDC::FromHandle(hdc);
 cdc->SetBkMode(TRANSPARENT);
 cdc->(10,10, CString("All aboard Brendas iron sledge"));

Note all the usual GDI rules apply - if you select a brush remember to reselect the old one etc. When you have finished drawing, call the EndPaint method like so;

dxMgr_.EndPaint();

This will "flip" your drawing onto the main windows surface. and the drawing will instantaneously appear.

What CDXSurfaceMgr does

CDXSurfaceMgr creates a primary surface - one that is visible - and a secondary off screen surface that is the same size as the primary one but invisible. When you call BeginPaint an HDC attached to the secondary surface is returned; you do all your drawing into this; then EndPaint Blits(copies) the whole of the secondary surface onto the primary one and it appears on the window. If you have a capable graphics adapter on your machine then the Blit is very very fast, if you don't then the operation is obviously not as fast, but still very functional.

Demos

There are three demos included one that uses WFC and draws rectangles and ellipses moving up and down the screen, one that uses MFC, and draws a few lines and a bit of text, and one that does exactly the same again but is just a Win32 program.