Microsoft KB Archive/111600

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BUG: Polygons Drawn with Pens of Odd Width (> 5) Misalign

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Q111600

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.1

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SYMPTOMS
When drawing frames with Polygon, if the pen width results in an odd number of pixels (and therefore cannot be centered exactly on its coordinates), the extra pixel of width goes up and/or to the left for widths greater than 5 pixels, whereas it correctly goes down and to the right for widths less than or equal to 5 pixels.

STATUS
Microsoft has confirmed this to be a bug in Windows version 3.1. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION
Using the ShowGDI sample, the following steps reproduce the problem:


 * 1) Start the ShowGDI sample and maximize it.
 * 2) Set the pen width to 5.
 * 3) Choose Polygon from the Draw menu and enter 4 for the number of points. Enter the following ordered pairs for the points 0 through 4: (20,20) (40,20) (40,40) (20,40).

Note that a 5-pixel frame correctly puts the extra pixel of pen width to the right and/or below the 20 and 40 coordinate lines (resulting in two pixels up and left, and three pixels down and right). This is correct (as are 1- and 3-pixel thick lines). However, odd widths of greater than or equal to 7 pixels start to round in the wrong direction:


 * 1) Choose Clear from the View menu.
 * 2) Choose Width from the Pen menu and enter 7 for the width.
 * 3) Repeat step 3 above.

Now the extra pixels goes up and to the left, resulting in 4 up/left and 3 down/right.

Additional query words: buglist3.10 3.10

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Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK310