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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Filtering and the accelerometer
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It may seem that the acceleration data is mushed together, but it s easy to isolate exactly the data you need using basic electronics techniques. A low-pass filter passes low-frequency signals and attenuates high-frequency signals. That s what you use to reduce the effects of sudden changes in your data, such as those caused by an abrupt motion. A high-pass filter passes high-frequency signals and attenuates low-frequency signals. That s what you use to reduce the effects of ongoing forces, such as gravity. You ll see examples of these two filtering methods in the upcoming sections.
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READING ACCELERATION INFORMATION
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The following code shows how you can use the accelerometers to modify redBall, a UIImage picture of a red ball, created in Interface Builder and initially set in the middle of the screen:
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- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration { CGPoint curCenter = [redBall center]; float newX = 3 * acceleration.x + curCenter.x; float newY = -3 * acceleration.y + curCenter.y; if (newX < 25) newX = 25; if (newY < 25) newY = 25; if (newX > 295) newX = 295; if (newY > 455) newY = 455; redBall.center = CGPointMake(newX,newY); }
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Any accelerometer program begins with the accelerometer:didAccelerate: method, which you access by setting the current program as a delegate of the Accelerometer shared action. You then mark the current position of the redBall. To access the accelerometer, all you do is look at the x and y coordinates of the UIAcceleration object and prepare to modify the redBall s position based on those. The acceleration is multiplied by 3 here to keep the ball s movement from being snaillike. There s also a z property for the third axis and a timestamp property indicating when the UIAcceleration object was created, none of which you need in this example. Movement has a limited effect on the example anyway, because an abrupt movement doesn t change the ball s slow roll much. After acquiring your gravitic information, you make sure the 50 x 50 red ball stays within the bounds of the screen. If you wanted to be fancy, you could introduce vectors and bounce the ball when it hits the edge, but that s beyond the scope of this example. After that check, you move the ball. Figure 10.2 shows what this program looks like on the iPad. With a minimal amount of work, you ve created a program that s acted on by gravity. This program could easily be modified to act as a leveler tool for pictures (by
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The accelerometers and movement
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Figure 10.2 Gravity test as shown on the iPad. The ball falls as if pulled by gravity and responds accordingly to changes in the orientation of the device.
having it move along only one of the three axes) or could be turned into a game where a player tries to move a ball from one side of the screen to the other, avoiding pits on the way. Now, what would it take to make this example totally functional by filtering out all movement The answer, it turns out, is not much more work at all.
FILTERING OUT MOVEMENT
To create a low-pass filter that lets through gravitic force but not movement, you need to average out the acceleration information you re receiving, so that at any time the vast majority of your input is coming from the steady force of gravity. This is shown in the following code, which modifies the previous example:
gravX + gravY + float float = (acceleration.x * kFilteringFactor) (gravX * (1 - kFilteringFactor)); = (acceleration.y * kFilteringFactor) (gravY * (1 - kFilteringFactor)); newX = 3 * gravX + curCenter.x; newY = -3 * gravY + curCenter.y;
This example depends on three predefined variables: kFilteringFactor is a constant set to .1, which means that only 10 percent of the active movement is used at any time; gravX and gravY each maintain a cumulative average for that axis of movement as the program runs. You filter things by averaging 10 percent of the active movement with 90 percent of the average. This smoothes out any bumps, which means sudden acceleration is
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