Math Behind Quaternion to Gravity Conversion

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hi everyone,

i using mpu6050 calculate y gravity vector quaternions. equation used in i2cdev library

gravity.y = 2 * (q.w * q.x + q.y * q.z);

where q.w scalar component , rest vector quaternions.

i wanted know how math here gives gravity vector , if there other way compute y gravity vector using least possible number of quaternion components.

regards.

sorry, meaningless out of context, can show entire operation, or point source code?


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