Microsoft KB Archive/119455

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PRB: RoundRect and Ellipse Don't Match Same Shaped Regions

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Q119455

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

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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
When CreateRoundRectrRgn is used to create a region with the shape of a rectangle that has rounded corners and RoundRect is called with the same parameters to draw the same rectangle that has rounded corners, the calculated region does not match the drawn rectangle. The same can be said of the ellipses created by CreateEllipticRgn and Ellipse.

CAUSE
This behavior is because of the design on Windows. The mismatch between fills and frames is because of the way that the boundaries and fills must be specified in order to get polygons to fit together properly. Windows NT duplicates this behavior for compatibility.

RESOLUTION
Perform the fill first, then draw the frame. Some of the frame pixels will overwrite fill pixels and some will not; however, there will be no gap between the frame and the fill, and the fill will not extend past the frame. Use CreateRoundRectRgn or CreateEllipticRgn for the fill and RoundRect or Ellipse, respectively, for the frame. Use the same parameters for both the region API and the filled-shape API.

NOTE: If you use a NULL pen when drawing the filled shape, the pixels will match those drawn by creating a region through the corresponding region API and then calling FillRgn with the same parameters. It draws the frame with the pen from the filled-shape API that causes the discrepancy.

STATUS
This behavior is by design.

Additional query words: 3.10 4.00

Keywords : kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbSDKWin32 kbOSWin95

Issue type : kbprb

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch kbWinSDK310