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 files
are available for download from the Microsoft Download
Center:
Cmemdc.exe
For
additional information about how to download Microsoft Support files, click the
following article number to view the article in the Microsoft Knowledge Base:
119591 How to Obtain Microsoft Support Files from Online Services
Microsoft scanned this file for viruses. Microsoft used the most
current virus-detection software that was available on the date that the file
was posted. The file is stored on security-enhanced servers that help to
prevent any unauthorized changes to the file.
NOTE: Use the -d option when running CMEMDC.EXE to
decompress the file and recreate the proper directory structure.