BUG: Complex Clipping Region Might Cause UAE/GP Fault

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

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 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

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).

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.