INFO: Drawing a Rubber Rectangle (114471)


The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) 3.1
  • Microsoft Win32 Application Programming Interface (API)
This article was previously published under Q114471
3.10 4.00 | 3.50 3.51 WINDOWS | WINDOWS NT kbgraphic

SUMMARY

Most drawing software uses what is termed a "rubber rectangle". This term is used to describe the situation where

  1. the left mouse button is held down, defining one corner of the rectangle
  2. the mouse is dragged and released at the point defining the opposite corner of the rectangle
  3. the rectangle is drawn while the mouse is being dragged, so that it looks like the rectangle is being stretched and contracted, like a rubber band

MORE INFORMATION

The key to making this work is in the following call, which should be made in the WM_LBUTTONDOWN case: 
<CALL>
   SetROP2( hDC, R2_NOT )
On each WM_MOUSEMOVE message, the rectangle is redrawn in its previous position. Because of the ROP code, the rectangle appears to be erased. The new position for the rectangle is calculated and then the rectangle is drawn.

Note that Windows will only let you draw in the invalid area of the window if you use a DC returned from BeginPaint(). If you want to use the DC returned from BeginPaint(), you must first call InvalidateRect() to specify the region to be updated.

With the DC returned from GetWindowDC(), Windows will restrict your drawing to the client and non-client areas. With the hDC returned from CreateDC(), you can write on the entire display, so you must be careful.
Modification Type:MajorLast Reviewed:10/23/2003
Keywords:kbinfo KB114471
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