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Another extremely useful preset in our MassFX Constraint options is the twist constraint. This can be particularly handy if we need to set up objects that spin or twist around a particular axis. As an example, we might think of the blades on a garden windmill ornament, or as we will create here, a spinning target. We are no doubt by now becoming fairly familiar with the MassFX constraints workflow, so we can just jump straight into setting this up. As you can see, we have two small knob objects that represent the central spindle around which our target panel will spin.
Now, we could constrain the panel to just one of these pieces of geometry and that would, in most situations, work just fine. In this case though, we are going to assume that we need to use both, which will give us the opportunity to walk through the process for setting that up. The first thing we want to do is attach our two pieces of spindle geometry together. So, let's select one of them, right-click on it, and choose the Attach command from the Quad menu. Then of course, we can click on the second spindle object, which will attach or join them together as a single piece of geometry.
Don't forget to then right-click so as to exit Attach mode. We have already mentioned the importance of pivot point placement. Generally speaking, it is a good idea for us to check that the current location of our pivot points will work for the type of constraints we are wanting to add. Naturally, we want to do this before we add any constraints. We can do that now by adding the target panel to our selection by holding down the Ctrl key and clicking on it. Next, we can right-click in the viewport and use the Isolate Selection command.
To reliably check the placement of pivots, we will want to work in an orthographic view, so let's switch over to a front view by using the F key on the keyboard. Then we can just use the Zoom Extents Selected tool to get a close-up view. From here, we can select each of the objects in turn and visually check the alignment of our pivots. From this view, things look as if they should be okay. Even though the pivots are not in the same location, things appear to line up along the currently all-important vertical axes.
However, if we just hit the T key on our keyboard to switch to our top view and again use Zoom Extents Selected and perform the same check, you can see we do have a definite misalignment. The fact that this exists as we look down what will be our spin axes means that our objects will have a slight tilt to them as they spin instead of being perfectly upright. If we want everything to work correctly here, we do have a little bit of work to do. First of all, let's select the panel geometry, come over to the Hierarchy tab, and click the Effect Pivot Only option.
As we ideally want the panel's pivot to be in the same location from this view as our spindles, let's go up to the main toolbar and select the Align tool. Now, we can click the spindle object and align the pivots along the X and Y axes. This gives us a precise rotational center around which our target panel can spin. With that done, we can of course exit the Pivot Only and Isolation modes. We can also use the C key to get back to our target close-up camera. Then we can use Ctrl+Click to select our spindle and panel and then add a twist constraint from the MassFX toolbar.
Again, of course, we need to apply the rigid body modifiers and size our constraint helper appropriately for our scene. We are going to need our spindle geometry to remain fixed in place, so again, let's go and set the Rigid Body Modifier Type to Kinematic. If we run the simulation at this point, we can see that things are working pretty well. Finally, if we just select our constraint helper and come over to the Command panel, we can see that the default settings for the twist constraint have swing Y and Z locked, whilst the twist action is set to run free.
This of course is pretty much what we want. We do, however, need to restrict the spinning motion of our panel a little. Otherwise, it will just continue to turn perpetually. To do that, let's set the Spring to Resting Twist Damping value to something as low as 0.025. One final test shows that our damping is having quite an obvious effect. Hopefully, by now we have discerned that setting up a wide range of dynamic motions can be handled very easily inside the MassFX constraint system.
Not that we've finished yet. We can now use everything that we've looked at so far in this chapter to set up what I will call a crazy target, one that will give us a much wider range of motions than the fixed presets that we've looked at so far.
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