Acceleration / Orientation Visual Reference

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Acceleration / Orientation Visual Reference

Postby Terrormaster » Sun Mar 07, 2010 5:46 pm

I don't know about anyone else here but I'm more a hands on kinda person when it comes to doing things. I can read documentation until my eyes fall out and a lot of times I just won't get it until I see it or do it. Keep in mind before I started Android development I was already familiar with the basics of 3D game development - translation and transformation. And really that's exactly what acceleration and orientation mean, I just wasn't getting it because of the different terminology.

Orientation is just a rotation transformation while acceleration is the amount of force that caused a transformation. To understand lets look at transformations. On it's simplest level a transformation is changing a 3D object by either rotation (ie: in this case abstracted by the term "Orientation") or scaling. A translation is the movement of an object in 3D space from one coordinate to another.

In its simplistic form Orientation is what it says it is - how the device is oriented around it's X/Y/Z coordinate and is expressed in three angles: azimuth, pitch, and roll. To see this illustrated lay your phone flat on the table.

Azimuth represents the angle between true north and the top of the phone. So if the top is pointing towards the north the angle is 0. If the top of the phone is pointing south the angle is 180. This angle is always measured 0-359 (ie: 360 degrees).

Pitch is rotation around the X axis and is measured -180 to 180 in degrees. To illustrate lay your phone flat on the table with the top facing away (ie: normal use). This is 0 degrees. Now grab the phone at the bottom by the sides and flip it away from you. The pitch will increase positively until it is face down at 180 degrees. However if you had grabbed the top and flipped it towards you the pitch will degrees negatively.

Roll is a little trickier since it's not measured in any sense at a full 360 degrees. In fact it only measures 180 degrees of motion ranging from -90 to 90. Using our example of the phone laying flat on the table we are now going to flip it side to side instead of front to back. Thats's simple roll. If the phone is face up rolling to the left returns 0 to 90 while rolling to the right returns 0 to -90.

So what happens when the roll exceeds 90 degrees in either direction? At that point the device is considered face down and the measurement are the same but measured from that perspective. So for example if you're monitoring the angle and you're rolling to the right (ie: negative angle). You'll notice once you exceed -90 the angle will start increasing back towards 0. This is because zero degrees now represents the phone face down.

You can check if the phone is face down by checking the pitch value. If the pitch is a positive value the roll is measuring from a face down position. If pitch is negative roll is measuring from a face up position.

Image

Acceleration on the other hand simply measures the force of a translation in one of X Y or Z directions. It has absolutely nothing to do with orientation. Nor does it measure the force in change of orientation.

To illustrate lay your phone flat on the table again. Sliding it across the table left or right results in force along the X axis. Pushing it to the right yields a positive force while sliding to the left yields negative force.

The same is true for the Y axis. Pushing the phone away yields a positive Y axis force while sliding it towards you gives a negative Y axis force.

If you lift the phone off the table you'll get positive force along the Z axis. While putting the phone back down on the table returns a negative force on the Z axis.

The faster or slower you do these actions determines the amount of force applied. Fast movement results in larger numbers while slow movements yield smaller numbers.

Image

I put the above two illustrations together because sometimes it helps in the visualization. Most of the information though can be found in the Android documentation with just a little digging. So instead of digging just point your browsers here: http://developer.android.com/reference/ ... Event.html

Hope all this was helpful and maybe clear up some of the confusion I've seen with Orientation and Acceleration.

-TM
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Re: Acceleration / Orientation Visual Reference

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