The rotation property of the object3d class in threejs stores and instance of the THREE.Euler class and stores the current rotation, or orientation of an object. This rotation property is a key value pair of the base class known as Object3d so then it can be used to set the rotation of Mesh Objects, Groups, Cameras, and just about anything else that is based off of the Object3D class including event a whole Scene Object.
When it comes to just setting the local rotation of an object by way of this property, one way is by using something like the set method of the Euler class to set the rotation of the object to a given set of Euler angles in terms of radian values. In other worlds values between 0, and Math.PI * \2. If you have a hard time thinking in radians there are tools that can be used to preform a quick conversion in the MathUtils object. There is a bit more to it than just that though, for example in some cases I might want to set the orientation of a geometry of a mesh object rather than the mesh object itself. There are other related tools and topics that I should also take a moment to cover while I am at it.
When I first wrote this post I was using r135 of threejs, which was still a fairly new version of threejs at the time I started this post. I take a moment to always mention what version I was using when wrote a post on threejs because I have been using it long enough to know that code breaking changes are made to the library often.
The source code examples that I am writing about in this post are up on Gitub.
To start out with rotation and the object3d class I made this quick static scene example that involves just a few mesh objects created with the box geometry constructor and the normal material. After setting up the scene object, camera, and renderer I then have a simple create cube helper that will create and return one of these cubes. I am then just calling the array for each method off of an array if values that I want to use for rotations of the y value for each cube that I will be creating and adding to the scene in the body of a function that I give to array for each.
Now for a simple animation example using the request animation frame method in the body of a loop function. Also while I am at it I also made the cubes all children of a group rather than the scene object. So then in this animation example I am using the rotation property of the object3d class to rotate each child of the group over time, as well as the group as a whole.
There is working with the rotation property of an object directly, but then there is also using the lookAt method of the object3d class as a way to set rotation. This look at method works by just passing a position in terms of a set of three numbers that are values for x, y and z values, or an instance of Vector3 such as the position property of an object that is the position that I want an object to look at.
For this example then I am creating a group of cubes by using a while loop to create and position each cube that I want. I am then looping over the children of the group and calling the look at method for each cube having each child look at the child in front of it, or behind in the case of the last child in the group. In the body of the function that I give to the for each method that I call of the array of children of the group I am just using the index argument as a way to know what I am dealing with in order to get the desired index value of the other cube in the collection that I want to have the current child cube look at.
The end result is more or less what I had in mind for this example, but I think that I will need to write about at least one if not more examples about this sort of thing. The situation here works okay because each of the cubes look more or less the same on each side, for some other mesh objects this might not always be the case with the nature of the geometry. The thing about this is that the rotation property and the look at method are ways of setting the rotation of the object, but not the geometry of a mesh object. In some cases I might want to adjust the rotation not just of a mesh object, but also the geometry at least once in order to change what the ‘front’ of the geometry is.
The rotation property effects just the local rotation of the object in which I set the rotation property. In addition to this there is also setting the rotation properties of any nested children, or in the case of Mesh objects there is rotating or changing he state of the geometry that is used with the Mesh.
The rotation property is then what I often used in order to set the rotation of an object such as a mesh object, group or camera. There is also the position property of the object3d class that holds an instance of the Vector3 class that is what is used to store and change the position of the object as well. There are a whole lot of other properties as well as method to be aware of in the object3d class that come into play allot when making one or more threejs projects such as the scale property and the lookAT method just to name a few.