ADR 0016: The attitude is levelled offline

Status: accepted (2026-10-20)

Context

The Mahony filter (ADR 0005) nudges the integrated gyro toward the accelerometer at every sample. In freefall the accelerometer reads drag and every change of the camera's motion, from under 1 to over 40 m/s² on ONE R footage, and with a proportional gain of 1 rad/s the filter chases it: measured against plain gyro integration, it adds 0.27 to 0.44 degrees of rotation per frame on average (up to 1.8), while the camera itself turns about 0.5 degrees a frame in steady freefall. Video stabilized by it comes out shakier than with the gyro alone. A rendition has the whole recording, so the vertical does not have to be decided as it goes.

Decision

• motion/levelOrientation integrates the gyro and levels it slowly. Each accelerometer reading is turned into the integrated frame and weighted by how near its magnitude is to one g (a Gaussian of a fifth of g), the readings are summed in tenth-of-a-second bins and averaged with a Gaussian of four seconds both ways in time, and the attitude is turned by the smallest tilt that takes that averaged up to the world's. Heading is the gyro's.
• The rendition uses it in place of fuseOrientation, which stays for streaming uses that cannot look ahead.

Consequences

• On ONE R footage the rotation added per frame is 0.02 to 0.06 degrees, ten times less, and the whole recording levels in under two tenths of a second.
• The vertical follows the gyro's drift, not the camera's linear motion. Where the accelerometer is far from one g for long (leaving the plane) the vertical comes from the readings around it, so the first seconds of freefall level only as well as the plane ride and the freefall after say.
• The averaged vertical is the accelerometer's: a constant tilt between the IMU and the lenses, or an accelerometer bias, shows as a constant tilt of the horizon, which this does not remove.