vb.net print barcode labels Figure 10.3 The Accelerometer Graph shows movement in all three directions. in Objective-C

Creating QR Code 2d barcode in Objective-C Figure 10.3 The Accelerometer Graph shows movement in all three directions.

Figure 10.3 The Accelerometer Graph shows movement in all three directions.
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rhythm of your pace in the
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All of this suggests a simple methodology for detecting basic accelerometer movement: you monitor the accelerometer over the course of movement, saving the largest acceleration in each direction. When the movement has ended, you can report the largest acceleration as the direction of movement. Listing 10.1 puts these lessons together in a program that could easily be used to report the direction of the device s movement (which you could then use to take some action).
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Listing 10.1 Movement reporter that could be applied as a program controller
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- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration { accelX = ((acceleration.x * kFilteringFactor) + (accelX * (1 - kFilteringFactor))); accelY = ((acceleration.y * kFilteringFactor) + (accelY * (1 - kFilteringFactor))); accelZ = ((acceleration.z * kFilteringFactor) + (accelZ * (1 - kFilteringFactor))); float moveX = acceleration.x - accelX; float moveY = acceleration.y - accelY; float moveZ = acceleration.z - accelZ;
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Measures movement
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Positioning: accelerometers, location, and the compass
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if (!starttime) { Marks starttime = acceleration.timestamp; start time } if (acceleration.timestamp > starttime + 1 && (fabs(moveX) >= .3 || fabs(moveY) >= .3 || fabs(moveZ) >= .3)) { if (fabs(moveX) > fabs(moveVector)) { moveVector = moveX; moveDir = (moveVector > 0 @"Right" : @"Left"); } if (fabs(moveY) > fabs(moveVector)) { moveVector = moveY; moveDir = (moveVector > 0 @"Up" : @"Down"); } if (fabs(moveZ) > fabs(moveVector)) { moveVector = moveZ; moveDir = (moveVector > 0 @"Forward" : @"Back"); } lasttime = acceleration.timestamp; } else if (moveVector && acceleration.timestamp > lasttime + .1) { myReport.text = [moveDir stringByAppendingFormat: @": %f.",moveVector]; moveDir = [NSString string]; moveVector = 0; }
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Saves largest movements
You start by creating a low-pass filter B and then taking the inverse of it C in order to get relatively clean movement data. Because the data can be a little dirty at the start, you don t accept any acceleration data sent in the first second D. You could cut this down to a mere fraction of a second. You start looking for movement whenever one of the accelerometers goes above .3 g. When that occurs, you save the direction of highest movement E and keep measuring it until movement drops below .3 g. Afterward, you make sure that at least a tenth of a second has passed, so that you know you re not in a lull during a movement. Finally, you do whatever you want to do with your movement data. This example reports the information in a label, but you d doubtless do something much more intricate in a live program. Cleanup is required to get the next iteration of movement reporting going. This sample program works well, unless the movement is very subtle. In those cases, it occasionally reports the opposite direction because of the force when the device stops its motion. If this type of subtlety is a problem for your application, more work is required. To resolve this, you need to make a better comparison of the start and stop forces for movements; if they re similar in magnitude, you ll usually want to use the first force measured, not necessarily the biggest one. But for the majority of cases, the code in listing 10.1 is sufficient. You now have an application that can accurately report (and take action based on) direction of movement.
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The accelerometers and gestures
Together, gravity and force measurement represent the most obvious things that you can do with the accelerometers, but they re by no means the only things. We suspect that using the accelerometers to measure three-dimensional gestures will be one of their best (and most frequent) uses as the platform matures.
10.3 The accelerometers and gestures
Three-dimensional gestures are one of the coolest results of having accelerometers inside your iPhone or iPad. They let users manipulate your programs without ever having to touch (or even look at) the screen. To recognize a gesture, you must do two things. First, you must accurately track the movements that make up the gesture. Second, you must make sure that in doing so, you don t recognize a random movement that wasn t intended to be a gesture at all. Recognizing a gesture requires only the coding foundation that we ve discussed already. But we ll show one example that puts that foundation into real-world use by creating a method that recognizes a shake gesture. We re defining a shake as a rapid shaking back and forth of the device, like you might shake dice in your hand before you throw them. Apple s Accelerometer Graph is a great tool to use to figure out what s going on. It shows a shake as primarily having these characteristics, presuming a program that s running in Portrait mode: Movement is primarily along the x-axis, with some movement along the y-axis, and even less along the z-axis. There are at least three peaks of movement, with alternating positive and negative forces. All peaks are at least +/-1 g, with at least one peak being +/-2 g for a relatively strong shake. You can use the preceding characteristics to define the average requirements for a shake. If you wanted to tighten them up, you d probably require four or more peaks of movement, but for now, this will do. Alternatively, you might want to decrease the g-force requirements so that users don t have to shake their device quite as much. We ve detailed the code that watches for a shake in listing 10.2.
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