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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

            
              # Debug utilities
stringifyQuaternion = (quaternion) ->
  radToDeg = 180 / Math.PI
  euler = new THREE.Euler()
  euler.setFromQuaternion(quaternion, 'XYZ')
  return "x: #{(euler.x * radToDeg).toFixed(1)}°, y: #{(euler.y * radToDeg).toFixed(1)}°, z: #{(euler.z * radToDeg).toFixed(1)}°"

# The method in question
calculateRelativeRotation = (from, to) ->
  return from.clone().inverse().multiply to

# Sane case - relative rotation works as expected
# Just some rotations around the z-axis
qFrom = (new THREE.Quaternion()).setFromAxisAngle new THREE.Vector3(0, 0, 1), 2 * (Math.PI / 6)
qTo = (new THREE.Quaternion()).setFromAxisAngle new THREE.Vector3(0, 0, 1), 5 * (Math.PI / 6)
qExp = (new THREE.Quaternion()).setFromAxisAngle new THREE.Vector3(0, 0, 1), 3 * (Math.PI / 6)
qDelta = calculateRelativeRotation(qFrom, qTo)

# First rotate around qFrom, then around qDelta to achieve qTo
qResult = qDelta.clone().multiply(qFrom)
console.log "Sane case results (delta):  expected #{stringifyQuaternion(qExp)}, got #{stringifyQuaternion(qDelta)}"
console.log "Sane case results (result): expected #{stringifyQuaternion(qTo)}, got #{stringifyQuaternion(qResult)}"

# Problematic case: (target) VS (from + delta) don't match
dToR = Math.PI / 180

eFrom = new THREE.Euler(0, 170 * dToR, 10 * dToR, 'XYZ')
eTo = new THREE.Euler(0, 170 * dToR, -10 * dToR, 'XYZ')
eExp = new THREE.Euler(0, 0, -20 * dToR, 'XYZ')

qFrom = new THREE.Quaternion()
qFrom.setFromEuler eFrom

qTo = new THREE.Quaternion()
qTo.setFromEuler eTo

qExp = new THREE.Quaternion()
qExp.setFromEuler eExp

qDelta = calculateRelativeRotation(qFrom, qTo)
qCombined = qDelta.clone().multiply(qFrom)

console.log "Problematic case results (delta): expected #{stringifyQuaternion(qExp)}, got #{stringifyQuaternion(qDelta)}"
console.error "Problematic case results (result): expected #{stringifyQuaternion(qTo)}, got #{stringifyQuaternion(qCombined)}"
console.error "expected result quaternion (qTo): #{JSON.stringify(qTo)}"
console.error "actual result quaternion: #{JSON.stringify(qCombined)}"
            
          
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