Q118472: PRB: SelectClipRgn() Cannot Grow Clip Region in WM_PAINT

Article: Q118472
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Operating System(s): 
Keyword(s): kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 _IK kbSDKWin16
Last Modified: 12-MAY-2001

3.10 4.00 | 3.50 3.51 4.00
WINDOWS   | WINDOWS NT
kbgraphic kbprb docerr

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
- Microsoft Win32 Application Programming Interface (API) 
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SYMPTOMS
========

Setting a smaller clipping region in WM_PAINT by using SelectClipRgn() works
fine; however, setting a larger clipping region seems to have no effect.
GetClipBox() can be used to verify this after calling SelectClipRgn().

CAUSE
=====

When you call SelectClipRgn() within a BeginPaint()/EndPaint() block in an
application's WM_PAINT case, the maximum size to which you can set your clipping
region is the size of the update region of your paint structure. This is because
the resulting clip region is the intersection of the update region and the
region specified in the call to SelectClipRgn(). In other words, you can use
SelectClipRgn() to shrink your update region, but not to grow it. This behavior
is by design.

RESOLUTION
==========

Invalidate the clipping region area you want before calling BeginPaint(). For
example:

     case WM_PAINT:

     InvalidateRect(hWnd, ....); // Invalidate the size you'll want

                                 // for the clip region.
     BeginPaint()
     SelectClipRgn();
     ... paint away ...
     EndPaint();
     break;

Something similar could be done in the Microsoft Foundation Classes (MFC), such
as:

     void CMyView::OnPaint()

     {

     InvalidateRect(...); // Invalidate the size you'll want.
     CPaintDC dc(this);   // CPaintDC wraps BeginPaint()/EndPaint().
     // Do drawing here...

     }

STATUS
======

This behavior is by design.

MORE INFORMATION
================

This is addressed in the documentation for the Windows NT SDK version 3.1
[Section 20.1.5, "Window Regions" in Chapter 20, "Painting and Drawing" in the
"Microsoft Win32 Programmer's Reference, Volume 1" or in the Win32 API Reference
online help (search on "Window Regions")] which states:

In addition to the update region, every window has a visible region that defines
the window portion visible to the user. The system changes the visible region
for the window whenever the window changes size or whenever another window is
moved such that it obscures or exposes a portion of the window. Applications
cannot change the visible region directly, but Windows automatically uses the
visible region to create the clipping region for any display DC retrieved for
the window.

The clipping region determines where the system permits drawing. When the
application retrieves a display DC using the BeginPaint, GetDC, or GetDCEx
function, the system sets the clipping region for the DC to the intersection of
the visible region and the update region. Applications can change the clipping
region by using functions such as SelectClipPath and SelectClipRgn, to further
limit drawing to a particular portion of the update area.

Additional query words: 3.10 4.00 SelectClipRegion big small large

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Keywords          : kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 _IK kbSDKWin16 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Issue type        : kbprb

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