SAMPLE: CMEMDC.EXE Memory Bitmap for Scrolling in CScrollV

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1

SUMMARY

Scrolling a complex image in a CScrollView can be very slow and jerky since OnDraw() is called for every WM_VSCROLL and WM_HSCROLL message. While only part of the window is invalidated and redrawn, the whole painting code gets called every time.

This CMemDC sample demonstrates drawing to a memory bitmap instead of the screen. The complex, time-consuming drawing code only needs to be performed when the image changes and in OnInitialUpdate(). During OnPaint(), the only work that needs to be done is a simple BitBlt from the memory bitmap to the screen.

The sample code draws a complex pattern on its client area. A menu option toggle allows the user to see the speed difference between using and not using the offscreen bitmap. Printing and print preview are supported through the normal printing code, and do not use the memory bitmap.

While CMemDC is a 32-bit sample, the same technique can be used in 16-bit MFC applications.

The following file is available for download from the Microsoft Software Library:

 ~ Cmemdc.exe (size: 38424 bytes) 

   ARTICLE-ID: Q119591
   TITLE     : How to Obtain Microsoft Support Files from Online Services

MORE INFORMATION

This sample is a MFC implementation of the Windows SDK sample, MemDC. For more information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE ID: Q130805
   TITLE     : SAMPLE: Drawing to a Memory Bitmap for Faster Performance