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Q89641: BUG: Complex Clipping Region Might Cause UAE/GP Fault

Article: Q89641
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kbSDKWin16
Last Modified: 16-JUN-1999

3.00 3.10
WINDOWS
kbprg kbbuglist

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
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SYMPTOMS
========

Drawing lines to a memory DC containing a complex clipping region causes an
unrecoverable application error (UAE) or general protection (GP) fault.

CAUSE
=====

Under certain rare conditions, drawing a single line to a DC containing a
complex clipping region will cause a UAE under Windows 3.0 and a GP fault under
Windows 3.1.

The conditions for this UAE/GP fault to occur are as follows:

- The clipping region for the DC must be a complex region. For example, the
  clipping region must contain multiple rectangles. [The ExcludeClipRect() and
  IntersectClipRect() functions will also tell you whether the region is
  complex.]

- The clipping region must have its top physical coordinate at the Y origin.
  For example, if you are using a memory bitmap in the MM_TEXT mapping mode,
  the clipping region might have an upper-left corner of (0,0).

- The line being drawn must be a "Y-major" line, meaning that the line extends
  in the Y-direction farther than it does in the X- direction.

- The line must start above the top edge of the region (negative Y space) and
  end 1 pixel from the upper-right corner of the clipping region.

For the UAE/GP fault to occur, all the above conditions must be met.

For example, when the following GDI calls are performed on a memory bitmap, a
UAE/GP fault will occur in the VGA display driver:

  IntersectClipRect(hDC, 0, 0, 20, 20); // Top is at Y=0 ExcludeClipRect(hDC,
  5, 5, 15, 30); // Makes region complex MoveTo(hDC, 15,-10); // This line is
  Y-major LineTo(hDC, 19, 0); // Hits upper-right of region

The UAE/GP fault occurs at the display driver level because GDI is sending an
incorrect parameter to the display driver's Output() function. The clipping
rectangle sent to this function contains a -1 for one of the points, and
negative values for clipping rectangles are not valid. A few display drivers,
for example the XGA driver, perform parameter checking on this clipping
rectangle and will not fail. Others, like the V7VGA, VGA, and 8514/a display
drivers do not perform validation on this call and will produce a UAE/GP fault.

RESOLUTION
==========

Because this problem occurs only when the Y origin of your hDC is at the
physical (screen coordinate) Y origin, it is unlikely that this problem will
occur while you are drawing to a standard screen DC. For this UAE/GP fault to
occur on a screen DC, the top of your client area would have to be at the screen
Y origin, which is normally not the case.

On the other hand, memory bitmaps are implemented so that the physical Y origin
is at the top of the bitmap, and the UAE/GP fault is much more likely to occur
when drawing to a memory bitmap. If your application uses complex clipping
regions on a memory DC, do the following to work around this problem:

1. Assuming you're working in the MM_TEXT mapping mode, set your Viewport origin
  to (0,-1) by calling SetViewportOrg(hMemDC, 0,- 1). This will cause your
  logical origin (0,0) to be moved one pixel down in the DC.

2. Make sure that your logical clipping rectangle does not extend above the
  logical Y=0 origin. In other words, the top Y coordinate of your clipping
  rectangle cannot be less than 0 (or less than 1).

To ensure that the second condition is met, you must verify that all calls to
IntersectClipRect() and ExcludeClipRect() (which both take logical coordinates)
do not cause a logical clipping region that extends above the Y origin to be
created.

Note that there is a distinction between logical and physical clipping regions;
the above workaround works only with a logical clipping region. If you create a
region and then select it into your DC as a clipping region, the coordinates for
this region are device coordinates rather than logical coordinates. In this
case, you need to manually offset of the top of the region by 1.

Some of the functions that create physical regions are CreateRectRgn(),
CreateEllipticRgn(), and OffsetRgn(). The functions GetClipBox() and GetRgnBox()
can be used to determine the upper physical-coordinate extent of the regions.

If your application uses only the functions IntersectClipRect() and
ExcludeClipRect(), your clipping regions will be in logical units.

Another workaround is to either not use complex clipping regions in your memory
bitmaps or manually verify that no lines will be drawn above the top of your Y
origin.

NOTE: This UAE/GP fault might be generated for more than just MoveTo()/LineTo()
calls--it can occur for any GDI call that generates lines, including rectangle,
polygon, polypolygon, arc, and vector font output.

STATUS
======

Microsoft has confirmed this to be a bug in Windows versions 3.0, 3.0a, and 3.1.
However, because the conditions for this error to occur are very specific, most
Windows applications will never encounter this problem. We are researching this
problem and will post new information here in the Microsoft Knowledge Base as it
becomes available.

Additional query words: 3.00 3.00a 3.10 OS_POLYLINE buglist3.00 buglist3.00a buglist3.10

======================================================================
Keywords          : kbSDKWin16 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1
Issue type        : kbbug
Solution Type     : kbpending

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