Sequencing Rotations


Sequencing Rotations

When you use the rotation method on a mesh then the rotation is applied in local space first around the y axis, then the x axis and finally about the z axis. How then do you rotate a mesh around a custom sequence of axes? This involves rotation quaternions either implicitly or explicitly.

Add Rotation

The simplest way is to use the addRotation method, addRotation(x, y, z) with two zero parameters, for example both of these sets of code

mesh.rotation.addRotation(Math.PI / 2, 0, 0);
mesh.rotation.addRotation(0, 0, Math.PI / 3);
mesh.rotation.addRotation(0, Math.PI / 8, 0);
mesh.rotation.addRotation(Math.PI / 2, 0, 0).addRotation(0, 0, Math.PI / 3).addRotation(0, Math.PI / 8);

will form the current rotation of the mesh further rotate it π/2 about the x axis, then π/3 about the z axis then π/8 about the y axis.

Using non zero x, y and z values with addRotation will add rotations in the order y, x, z.

The internal calculations for addRotations convert the Euler angles to rotation quaternions and back again.

Sequence using addRotation: Sequence Rotations Using addRotation


Imagine a disc with an axis through its center. The disc is able to rotate about the axis. The image below shows the disc at several different rotation points around the axis.

For all rotations of the disc the axis can be tilted as seen in the diagram below.

Specifying a direction vector for an axis along with an angle is an alternative way of producing a rotation. This is how the rotate method is used either in world space or local space.

mesh.rotate(new BABYLON.Vector3(1, 0 -1), Math.PI / 3, BABYLON.Space.WORLD);
mesh.rotate(new BABYLON.Vector3(1, 0 -1), Math.PI / 3, BABYLON.Space.LOCAL);

Three useful vectors are predefined


The rotate method is also additive, for example both this set of codes

mesh.rotate(new BABYLON.Vector3(2, -3, 7), Math.PI / 3, BABYLON.Space.LOCAL);
mesh.rotate(BABYLON.Axis.Y, -Math.PI / 2, BABYLON.Space.WORLD);
mesh.rotate(new BABYLON.Vector3(5.6, 7.8, - 3.4), 1.5 * Math.PI, BABYLON.Space.WORLD);
mesh.rotate(BABYLON.Axis.Z, -Math.PI, BABYLON.Space.LOCAL);

will start with the current orientation of the mesh, the add to this a rotation of π/3 about the given local space axis, then add a rotation of -π/2 about the world y axis, the add a rotation of 1.5π about the given world axis and finally add a rotation of -π about the local z axis.


Earth rotates on tilted axis: Earth Rotating On A Tilted Axis Using mixed rotate World and Local: Using Mixed Rotate World and Local Two cubes one rotates in World Space other in Local Space: 2 Cubes Rotating in World and Local Space Purple rotates in World Space, brown Local Space: Purple and Brown rotation in World and Local Space

The use of rotate sets the orientation of the mesh using a rotation quaternion and subsequently there can then be issues trying to set the orientation of the mesh using rotation.

About time to look further at rotation quaternions.