Docs: describe turn-table gimble lock mitigation
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@ -758,7 +758,32 @@ static void viewrotate_apply(ViewOpsData *vod, const int event_xy[2])
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quat_to_mat3(m, vod->curr.viewquat);
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invert_m3_m3(m_inv, m);
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/* avoid gimble lock */
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/* Avoid Gimble Lock
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*
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* Even though turn-table mode is in use, this can occur when the user exits the camera view
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* or when aligning the view to a rotated object.
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*
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* We have gimble lock when the user's view is rotated +/- 90 degrees along the view axis.
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* In this case the vertical rotation is the same as the sideways turntable motion.
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* Making it impossible to get out of the gimble locked state without resetting the view.
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*
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* The logic below lets the user exit out of this state without any abrupt 'fix'
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* which would be disorienting.
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*
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* This works by blending two horizons:
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* - Rotated-horizon: `cross_v3_v3v3(xaxis, zvec_global, m_inv[2])`
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* When only this is used, this turntable rotation works - but it's side-ways
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* (as if the entire turn-table has been placed on it's side)
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* While there is no gimble lock, it's also awkward to use.
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* - Un-rotated-horizon: `m_inv[0]`
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* When only this is used, the turntable rotation can have gimbal lock.
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*
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* The solution used here is to blend between these two values,
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* so the severity of the gimbal lock is used to blend the rotated horizon.
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* Blending isn't essential, it just makes the transition smoother.
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*
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* This allows sideways turn-table rotation on a Z axis that isn't world-space Z,
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* While up-down turntable rotation eventually corrects gimble lock. */
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#if 1
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if (len_squared_v3v3(zvec_global, m_inv[2]) > 0.001f) {
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float fac;
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