The pen installable device driver, which Windows supplies as the file PENC.DRV, interacts with the virtual pen driver (VPENDC.VXD) and passes pen movement data to Windows. The fact that the pen driver's data may sometimes be needed for on-the-fly handwriting recognition places several constraints on a pen input device:

• The pen driver must be able to report the location of the pen at least 60 times per second. This rate ensures the true path of the pen is reported accurately enough to support the efforts of vector-based recognizers. It also makes the ink, a path of pixels that traces the pen's movement, appear smooth and natural at normal writing speeds. For more information about recognizers, see the "Recognizer" section later in this chapter.

• The pen driver must be able to report pen positions with a resolution of at least 200 points per inch. This degree of resolution ensures ink coordinates are sufficiently fine to let the recognizer make accurate judgments about the path of the pen over the digitizing surface.

• Regardless of the resolution of the device, the pen driver must report the pen position in tablet coordinates of 0.001 inch. This convention ensures that Windows, the recognizer modules, and the application all view the ink at the same scale.

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