Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mit-eddie.UUCP Path: utzoo!linus!genrad!mit-eddie!smh From: smh@mit-eddie.UUCP (Steven M. Haflich) Newsgroups: net.micro Subject: Re: Question on the mouse, comments abou - (nf) Message-ID: <389@mit-eddie.UUCP> Date: Mon, 4-Jul-83 11:22:39 EDT Article-I.D.: mit-eddi.389 Posted: Mon Jul 4 11:22:39 1983 Date-Received: Mon, 4-Jul-83 15:48:41 EDT References: <1690@yale-com.UUCP> Organization: MIT, Cambridge, MA Lines: 31 A more complete explication of the `crosshair' method for following a lightpen, cited a while ago by Jerry Leichter: The solution involves displaying a right-angle cross comfortably larger than the aperture of the pen. The cross point displays the cursor position. Following consists of keeping track how far from the cross point in opposite directions the cross is visible to the detector. The tracking algorithm continually adjusts its idea of the cursor location to keep the visible portion of the cross centered. That's all in theory, for there are several practical problems. For some applications, it is necessary to have an acquisition mode in which the display fills the screen with hash in order to pick up the initial position of the pen. (Otherwise, the user must point to the displayed cursor to `pick it up,' analagous to picking up a mouse.) Some sort of smoothing algorithm is necessary on tracking, for detection at the extremes of the aperture tends to be noisy (or rather, statistical). All this, of course, burns a lot of processor time (possibly all of it), although the problem can be moved to a dedicated microprocessor. Remember, a mouse generates no output while stationary. Also, this technique awkward to implement on some lightpen/display hardware. My experience (which is admittedly ancient) is that lightpens work a lot better for pointing to discrete objects, such as menus, than they do for detailed tracing. Like Jerry I cannot provide a reference for first use of the crosshair method. However, it was the common penfollow technique used at the MIT RLE PDP1 in the mid '60s. (If my sources are any good, this this machine with its old point-plotting display, but without the exotic timesharing hardware, is now on display and running at the DEC museum in Marlborough MA.) Steve Haflich genrad!mit-eddie!smh