The initial versions of the Tablet PC SDK were notable in presenting an elegant and capable object model for working with high-resolution ink data. Data is collected with either an InkOverlay or InkCollector object, which provide the programmer Ink, Strokes, and Stroke data. These and related classes – InkDivider, RecognizerContext, and so forth – make it very easy to program for the Tablet PC Edition of Windows XP. However, these classes do not provide the "pedal to the metal" performance that might be desired by graphics programs or games. These types of applications need to reduce lag between the high-resolution, high-speed data being generated by the digitizer and what is displayed on screen. The "RealTimeStylus" APIs introduced in the Microsoft Windows XP Tablet PC Edition Development Kit 1.7 provide higher-performance access to a lower-level flow of data from the digitizer.
As good a job as the early versions of the SDK did, an application performing complex synchronous transforms on the stream of data coming from the digitizer could introduce "lags and jags" into the ink. Having said that, in practice the pen has to be moving mighty fast for this to be noticeable – the strokes of an artistic sketch being one of the only common places where the lag is detectable. No one's yet written a game for the Tablet PC that requires fast-twitch response, which would be another place that one might expect to see an issue. So where's the need for a new set of APIs?
There seem to be two issues. First, Microsoft wants graphics programs and games that go beyond mouse data and exploit the advantages of high-resolution digitizers. Second, data collection within the Tablet PC platform itself benefits from the RealTimeStylus architecture by lowering computational requirements and thereby perhaps subtly increasing battery longevity, which is one of the great obsessions of the platform team.
The RealtimeStylus architecture conceives of three performance domains: the digitizer domain, the realtime synchronous domain, and a slower-performing asynchronous domain. Figure 1 shows the flow of data through the important parts of a RealTimeStylus object.
09-22-2004, 08:01 AM
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