Manually building spines usually involves using the Path tool.

1. In the viewport, press ⇥ Tab and type Path.

2. Draw the curve in the viewport.

3. Make sure the curve is selected, press ⇥ Tab in the viewport, and type Bones from curve.

4. In the operation controls toolbar, set the Kinematics to Follow Curve.

If you move the handles, the bones will follow; however, they will not stretch with the spine. To make a stretchy spine you need to manipulate the length of all the bones with respect to the changing length of the curve. In order to do this you must use the arclen() hscript expression pointing to the curve.

Making bones stretch with the spine ¶

On the Bone tab of one of the bones, use the arclen() expression in the Bone Length field. Each bone has to have a length equal to the curve diced up to the number of bones.

For example, in the following expression the division by fifteen represents one fifteenth of the length of the curve.

arclen("/obj/path1/output_curve", 0, 0, 1) / 15