Touch is the primary way a user will interact with iOS devices. One of the most natural and obvious functionalities these devices are expected to provide is allowing the user to draw on the screen with their finger. There are many freehand drawing and note-taking apps currently in the App Store, and many companies even ask customers to sign an iDevice when making purchases. How do these applications actually work? Let's stop and think for a minute about what's going on "under the hood".
When a user scrolls a table view, pinches to enlarge a picture, or draws a curve in a painting app, the device display is rapidly updating (say, 60 times a second) and the application run-loop is constantly sampling the location of the user's finger(s). During this process the "analog" input of a finger dragging across the screen must be converted to a digital set of points on the display, and this conversion process can pose significant challenges. In the context of our painting app, we have a "data-fitting" problem on our hands. As the user scribbles away merrily on the device, the programmer essentially must interpolate missing analog information ("connect-the-dots") that has been lost among the sampled touch points that iOS reported to us. Further, this interpolation must occur in such a way that the result is a stroke that appears continuous, natural, and smooth to the end user, as if he had been sketching with a pen on a notepad made of paper.
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