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    <p>As you are not allowed to create an instance of CMAttitude on your own, the only way is to create a new quaternion and take this for any further calculations. I recommend using your own customised quaternions class instead of the simple struct <a href="http://developer.apple.com/library/ios/documentation/CoreMotion/Reference/CMAttitude_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40009943-CH1-SW1" rel="nofollow">CMQuaternion</a>. A good starting point is the <a href="http://code.google.com/p/cocoamath/" rel="nofollow">cocoamath</a> project where you can take Quaternion.m, Quaternion.h and QuaternionOperations.m from <a href="http://code.google.com/p/cocoamath/source/browse/trunk" rel="nofollow">trunk</a> and I will use it for this sample code.</p> <p>First you have to save an instance from the <a href="http://content.gpwiki.org/index.php/OpenGL%3aTutorials%3aUsing_Quaternions_to_represent_rotation#The_complex_conjugate_of_a_quaternion" rel="nofollow">complex conjugate</a> of your reference quaternion (taken from <code>CMAttitude.quaternion</code>) to use for all subsequent multiplications i.e.</p> <p>MyClass.h</p> <pre><code>Quaternion* inverseReferenceQuaternion; </code></pre> <p>MyClass.c</p> <pre><code>// initialising code run only once after each call to [motionManager startDeviceMotionUpdates]; CMAttitude firstAttitude = [motionManager deviceMotion].attitude; CMQuaternion q = firstAttitude.quaternion; inverseReferenceQuaternion = [[Quaternion alloc] initWithRe:q.w i:-q.x j:-q.y k:-q.z]; </code></pre> <p>In your timer loop (or handler block) you need a <a href="http://content.gpwiki.org/index.php/OpenGL%3aTutorials%3aUsing_Quaternions_to_represent_rotation#Multiplying_quaternions" rel="nofollow">quaternion multiplication</a>:</p> <pre><code>// on every update CMAttitude attitude = [motionManager deviceMotion].attitude; CMQuaternion current = attitude.quaternion; Quaternion currentMultiplied = initAsProductOf:inverseReferenceQuaternion And:current </code></pre> <p>Now <code>currentMultiplied</code> should contain what you are looking for.</p> <p>Quaternions are pretty if you are new to this subject, but it's worth to read some tutorials like for example <a href="http://content.gpwiki.org/index.php/OpenGL%3aTutorials%3aUsing_Quaternions_to_represent_rotation" rel="nofollow">OpenGL:Tutorials:Using Quaternions to represent rotation</a> or <a href="http://www.gamedev.net/page/resources/_/technical/math-and-physics/quaternion-powers-r1095" rel="nofollow">Quaternion Powers</a>.</p>
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