Lesson 7 - 2-Axis Rigging
In lesson 6 we discussed how to rig a mesh that has an angled axis of rotation. This lesson goes over some of the differences surrounding rotations about 2 axes. The process is nearly identical just with a repeat of the some steps to process the second angle.
You can download this example model I am using to follow along with this lesson. If you have been following along from previous lessons you will want to re-download this model as some changes have been made to help highlight the differences between 1-axis and 2-axis rigging.
Summary of Differences
The main difference between 1-axis and 2-axis rigging is order and careful note taking and/or on-the-go preparation. Whereas the 1-axis rigging only required you to remember one number, you will now need to remember the rotation order and the values for them. I have some handy tricks for this, however in the end process and order will still matter.
In most meshes that model real-world aircraft, almost every animation rigging process will involve 2-axis rigging. It is nearly impossible to accurately animate without learning this skill.
Identifying When 2-Axis Rigging Is Needed
identifying when you need to perform 2-axis rigging is straight forward. Using the front, top, and side views, examine the mesh object in question and observe how the desired axis of rotation is angled as highlighted in the images below for the elevators on our example model. You may need to zoom in closely to see some angles. If your first pass at only rigging following the 1-axis rigging procedure does not produce a clean animation, you may need to remove that rigging and perform a 2-axis rigging.
If the axis is not aligned with 2 global axes, then you need to use 2-axis rigging.
General Procedure for 2-Axis Rigging
Complete the object center setup and moving procedure found in Lesson 6.
Align the mesh object with 1 global axis, record the rotation angle and zero out the object rotation (CTRL + a in Object Mode).
Align the mesh object with the second global axis, record the the rotation angle and zero out the object rotation (CTRL + a in Object Mode).
Establish the parent-child relationship with a new empty object located at the mesh object's axis of origin (best place is at the mesh object center).
Rotate the parent empty by the two angles recorded from the first and second global axis alignment.
Re-position empty parent to ensure the mesh object aligns with its original position.
The main trick I use when working on 2-axis rigging comes into play once we have isolated the mesh object into the isolated layer. Here I insert a temporary empty object that will not end up as the parent object. This temporary empty object is only used to mirror the rotations being made to the mesh object. Once I make a rotation to align the mesh object with one of the global axes, the temporary empty is rotated about the same axis the same amount. This serves two purposes.
Helps me remember which axis I have rotated about (very helpful for small alignments with a global axis)
Lets me copy-paste the rotations back to the parent empty once the parent-child relationship is established.
This makes the process of mirroring a rigging from one side of the aircraft to another a lot easier as you do not have to re-invent the wheel and re-align the second mesh object by hand. The key thing is that this only works if the aircraft is perfectly mirrored.
Unless designed for single-axis rigging, most models will need two-axis rigging to resolve the complex geometries that make up an aircraft. Wings are especially notorious for adding complexity like this. Expanding from one to two-axis rigging isn't the hardest transition, but it emphasizes how important it is to follow a good process every time. While in some circumstances the process I've shown you will not be the fastest, but 9 times out of 10, it will be.