Constraining NURBS curves to joints

Quick and Fast Instructions

• Create your NURBS curve
• Put your Curve into an empty group (Ctrl + G). 
• Make sure you freeze and delete history on both the curve and the group. 
• Modify > Center Pivot for the group
• Select the group, then select the joint you wish to match, then go to Modify > Match Transformations > Match All Transforms. 
• The group should snap to it and the curve should still have its transforms zeroed out in the channel box. The pivots for the joint and curve should also be the same. 
• Finally select your curve, then select the joint, go to Constrain > Parent. 
• Now your curve should control your joint, and you can put it into your curve hierarchy. 

    That's all there is to it. If you want more info + pictures you can continue reading on. 

Setting up the NURBS curve

    When constraining curves to joints, a good practice to keep in mind is to have the pivot of the curve match the pivot of the joint. Start by creating a NURBS curve and scale it to approximately the size you want. Keep it at the origin for now. Then delete history and freeze transformations. Rename the NURBS curve to something descriptive for what it is controlling.

Outliner hierarchy

Making a curve to control my head joint. 

    We want the NURBS curve to be completely clean of any transform, rotate, or scale data. But we need to move it so that it's near the head joint to move it. What we can do is create an Offset Group to put all of the transformation data when moving the curve, while the curve remains clean. While selecting the curve, hit Ctrl + G to create a group.

The outliner's hierarchy

The groups pivot when first created should be at the origin

        It's important that the Offset Group's pivot and the NURBS pivot should be exactly the same (if you haven't translated the NURBS curve, the pivot should still be at the origin). 

Matching all Transformations

    We've completed all of the setup we need for this. Now with the Offset Group selected, find the joint you want to control. You need to add the joint to your selection. This can be done in the outlier with Ctrl+left click or in the viewport Shift + left click. 

Offset Group selected first

Then the joint is added to the selection

Now you can go to Modify > Match Transformations > Match All Transforms.

Under the Modify tab

    Now the Offset Group and the curve snaps to the joint. And you can check the pivots by selecting the curve and then the joint. The translate manipulator should not change direction toggling between the two. 

Head Control Pivot

Joint Pivot

    Here's an example of the pivots not aligning correctly. 

the joints y pivot points up

the control's y pivot is at an angle

    We're using the translate manipulator to check. Notice how the pivot of the joint does not match the pivot of the control. 

    Going back to our head control example. We can also check the the controller's attributes in the channel box. It should still all be zeroed out. Instead all the 'garbage' transform data was put onto the group, and we can check this by selecting the Offset Group and looking in the channel box. 

The Ctrl Curve is clean with no transform data

But when you select the Offset Group, it has all the 'garbage' data used to get the curve into place

    The final step is to select the Curve, then select the joint, go to Constrain > Parent. You can go into the options box and turn off "maintain offset" to check if you've done it correctly. If your curve seems to move a bit when you constrain it you might have something off. If it stays in place you've done everything correctly. 

Changing the shape of the NURBS Curve without changing transformation data

    Now that your curve is in place, you may want to adjust the shape more. You don't want to move the object cause that will move the pivot out of place. What you can do it change the objects shape using the NURBS curve's control vertices.

    Select the NURBS curve. Hold right click and select "Control Vertex" from the radial menu.

The NURBS curve radial menu

    Sometimes this radial menu won't come up initially. It's context specific so make sure you're right clicking over the NURBS curve line. 

NURBS curve control vertecies

    You should see a bunch of purple dots appear around the NURBS curve. These points determine the shape of the curve. You can adjust any of them in anyway, rotate, translate, scale, etc. 

I have an easier time selecting all of them when you turn off 'select joint objects' in the toolbar

select joints turned off

Here I translated and scaled the vertices up

    When you go back into object mode, you can notice that the pivot stays in the original position. 

I've moved up the curves vertices, but the objects pivot stayed the same

That's all there is to it. Have fun making new shapes!