140 The Design of Everyday Thingscould become standard and easy to apply. It will happen, but itmay take considerable time.Alas, like many things that change, new technologies willbring virtues and deficits. The controls are apt to be throughtouch-sensitive screens, allowing excellent natural mapping to thespatial layouts involved, but lacking the physical affordances ofphysical switches. They cant be operated with the side of the arm�or the elbow while trying to enter a room, hands loaded with packagesor cups of coffee. Touch screens are fine if the hands are free.Perhaps cameras that recognize gestures will do the job.ACTIVITY-CENTERED CONTROLSSpatial mapping of switches is not always appropriate. In manycases it is better to have switches that control activities: activitycenteredcontrol. Many auditoriums in schools and companieshave computer-based controls, with switches labeled with suchphrases as video, computer, full lights, and lecture. When�� �� ����carefully designed, with a good, detailed analysis of the activitiesto be supported, the mapping of controls to activities worksextremely well: video requires a dark auditorium plus control ofsound level and controls to start, pause, and stop the presentation.Projected images require a dark screen area with enough light inthe auditorium so people can take notes. Lectures require somestage lights so the speaker can be seen. Activity-based controls areexcellent in theory, but the practice is difficult to get right. When itis done badly, it creates difficulties.A related but wrong approach is to be device-centered ratherthan activity-centered. When they are device-centered, differentcontrol screens cover lights, sound, computer, and video projection.This requires the lecturer to go to one screen to adjust thelight, a different screen to adjust sound levels, and yet a differentscreen to advance or control the images. It is a horrible cognitiveinterruption to the flow of the talk to go back and forth among thescreens, perhaps to pause the video in order to make a commentor answer a question. Activity-centered controls anticipate this needand put light, sound level, and projection controls all in one location.four: Knowing What to Do: Constraints, Discoverability, and Feedback 141I once used an activity-centered control, setting it to present myphotographs to the audience. All worked well until I was asked a
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question. I paused to answer it, but wanted to raise the room lightsso I could see the audience. No, the activity of giving a talk withvisually presented images meant that room lights were fixed at adim setting. When I tried to increase the light intensity, this tookme out of giving a talkactivity, so I did get the light to where I��wanted it, but the projection screen also went up into the ceilingand the projector was turned off. The difficulty with activity-basedcontrollers is handling the exceptional cases, the ones not thoughtabout during design.
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