vb.net print barcode labels Positioning: accelerometers, location, and the compass in Objective-C

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Positioning: accelerometers, location, and the compass
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largely ignored. This example does this for the x- and y-axes because that s all that are used in the example. If you cared about the z-axis, you d need to filter that too. Afterward, you use the average acceleration instead of the raw acceleration when you re changing the position of the ball. The gravity information can be extracted from what looked like an imposing mass of data with a couple of lines of code. As you ll see, looking at only the movement is just as easy.
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10.2.4 Checking for movement
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In the previous example, you isolated the gravitic portion of the accelerometer s data by creating a simple low-pass filter. With that data in hand, it s trivial to create a highpass filter. All you need to do is subtract the low-pass filtered data from the acceleration value; the result is the pure movement data:
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gravX = (acceleration.x * kFilteringFactor) + (gravX * (1 - kFilteringFactor)); gravY = (acceleration.y * kFilteringFactor) + (gravY * (1 - kFilteringFactor)); float moveX = acceleration.x - gravX; float moveY = acceleration.y - gravY;
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This filter doesn t entirely stop gravitic movement, because it takes several iterations for the program to cut out gravity completely. In the meantime, the program is influenced by gravity for a few fractions of a second at startup. If that s a problem, you can tell the program to ignore acceleration input for a second after it loads and after an orientation change. We ll show the first solution in the next example. With that exception, as soon as you start using these new moveX and moveY variables, you re looking at the filtered movement information rather than the filtered gravity information. But when you start looking at movement information, you see that it s trickier to use than gravity information. There are two reasons for this. First, movement information is a lot more ephemeral. It appears for a second, and then it s gone again. If you re displaying some type of continuous movement, as with the red ball example, you need to make your program much more sensitive to detect the movements. You d have to multiply the moveX and moveY values by about 25 to see movement forces applied to the ball in any recognizable manner. Second, movement information is a lot noisier. As you ll see when we look at real movement data, motion occurs in a multitude of directions at the same time, forcing you to parse out the exact information you want. Ultimately, to interpret movement, you have to be more sophisticated, recognizing what are effectively gestures in three-dimensional space.
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10.2.5 Recognizing simple accelerometer movement
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If you want to write programs using acceleration gestures, we suggest that you download the Accelerometer Graph program available from Apple s developer site. This is a nice, simple example of accelerometer use; but more important, it also provides you with a clear display of what the accelerometers report as you make different gestures.
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The accelerometers and movement
Make sure you enable the high-pass filter to get the clearest results. Figure 10.3 shows what the Accelerometer Graph looks like in use (but without movement occurring). As you move the device around, you ll quickly come to see how the accelerometers respond. Here are some details you ll notice about how the accelerometers report information when you look at the Accelerometer Graph: Most gestures cause all three accelerometers to report force; the largest force should usually be in the axis of main movement. Even though there s usually a compensating stop force, the start force is typically larger and shows the direction of main movement. Casual movement usually results in forces of .1 g to .5 g. Slightly forceful movement usually tops out at 1 g. A shake or other more forceful action usually results in a 2 g force. The accelerometers can show things other than simple movement. For example, when you re walking with an iPhone or iPad, you can see the accelerometers.
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