diff --git a/spatial/r3/euler_example_test.go b/spatial/r3/euler_example_test.go new file mode 100644 index 00000000..fc69aaa0 --- /dev/null +++ b/spatial/r3/euler_example_test.go @@ -0,0 +1,77 @@ +// Copyright ©2021 The Gonum Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package r3_test + +import ( + "fmt" + "math" + + "gonum.org/v1/gonum/num/quat" + "gonum.org/v1/gonum/spatial/r3" +) + +// euler returns an r3.Rotation that corresponds to the Euler +// angles alpha, beta and gamma which are rotations around the x, +// y and z axes respectively. The order of rotations is x, y, z; +// there are many conventions for this ordering. +func euler(alpha, beta, gamma float64) r3.Rotation { + // Note that this function can be algebraically simplified + // to reduce floating point operations, but is left in this + // form for clarity. + var rot1, rot2, rot3 quat.Number + rot1.Imag, rot1.Real = math.Sincos(alpha / 2) // x-axis rotation + rot2.Jmag, rot2.Real = math.Sincos(beta / 2) // y-axis rotation + rot3.Kmag, rot3.Real = math.Sincos(gamma / 2) // z-axis rotation + + return r3.Rotation(quat.Mul(rot3, quat.Mul(rot2, rot1))) // order of rotations +} + +func ExampleRotation_eulerAngles() { + // It is possible to interconvert between the quaternion representation + // of a rotation and Euler angles, but this leads to problems. + // + // The first of these is that there are a variety of conventions for + // application of the rotations. + // + // The more serious consequence of using Euler angles is that it is + // possible to put the rotation system into a singularity which results + // in loss of degrees of freedom and so causes gimbal lock. This happens + // when the second axis to be rotated around is rotated to 𝝿/2. + // + // See https://en.wikipedia.org/wiki/Euler_angles for more details. + + pt := r3.Vec{1, 0, 0} + + // For the Euler conversion function in this example, the second rotation + // is around the y-axis. + const singularY = math.Pi / 2 + + arb := math.Pi / 4 + + fmt.Printf("rotate around x-axis: %.2f\n", euler(arb, 0, 0).Rotate(pt)) + fmt.Printf("rotate around y-axis: %.2f\n", euler(0, arb, 0).Rotate(pt)) + fmt.Printf("rotate around z-axis: %.2f\n", euler(0, 0, arb).Rotate(pt)) + fmt.Printf("rotate around x+y-axes: %.2f\n", euler(arb, arb, 0).Rotate(pt)) + fmt.Printf("rotate around x+z-axes: %.2f\n", euler(arb, 0, arb).Rotate(pt)) + fmt.Printf("rotate around y+z-axes: %.2f\n", euler(0, arb, arb).Rotate(pt)) + + fmt.Printf("rotate around y-axis to singularity: %.2f\n", euler(0, singularY, 0).Rotate(pt)) + fmt.Printf("rotate around x+y-axes with singularity → gimbal lock: %.2f\n", euler(arb, singularY, 0).Rotate(pt)) + fmt.Printf("rotate around z+y-axes with singularity → gimbal lock: %.2f\n", euler(0, singularY, arb).Rotate(pt)) + fmt.Printf("rotate around all-axes with singularity → gimbal lock: %.2f\n", euler(arb, singularY, arb).Rotate(pt)) + + // Output: + // + // rotate around x-axis: {1.00 0.00 0.00} + // rotate around y-axis: {0.71 0.00 -0.71} + // rotate around z-axis: {0.71 0.71 0.00} + // rotate around x+y-axes: {0.71 0.00 -0.71} + // rotate around x+z-axes: {0.71 0.71 0.00} + // rotate around y+z-axes: {0.50 0.50 -0.71} + // rotate around y-axis to singularity: {0.00 0.00 -1.00} + // rotate around x+y-axes with singularity → gimbal lock: {0.00 0.00 -1.00} + // rotate around z+y-axes with singularity → gimbal lock: {0.00 0.00 -1.00} + // rotate around all-axes with singularity → gimbal lock: {0.00 0.00 -1.00} +}