4x4 matrix rotation unexpected: 200° 45°=115°-CodePudding

I would like to make a rotation using 4x4 matrix in Swift, but it has unexpected behavior: 200 degrees 45 degrees = 115 degrees, and not 245

    let degree200 = Angle(degrees: 200).radians
    let degree45 = Angle(degrees: 45).radians
    // 200 degrees   45 degrees
    let rotationMatrix = float4x4(simd_quatf(angle: Float(degree200 degree45), axis: SIMD3<Float>(0, 1, 0)))
    // it prints 115 degree, and not 245
    print(Angle(radians: Double(simd_quatf(rotationMatrix).angle)).degrees)

CodePudding user response：

I assume that's a typo, and you in fact meant -115 degrees? (remainder(245, 360)) When using quaternions & Matrices to express orientations, you can only expect to see values of -180 to 180 degrees when converting those values back to Euler angles.

In general it is impossible to convert back to Euler angles from either a quaternion or matrix and get the original input values back. You either store the original Euler angles and present those to the user, or you will have to have a known starting Euler value and apply an Euler filter to obtain approximately correct results.

The only correct way to get your expected result is to NOT print the value after conversion to quats:

print((degree200   degree45). degrees)

CodePudding user response：

Well I know 115 and 245 are 360. Just a guess but maybe you're rotating the wrong way?? Maybe try negative values and see what happens.