Following a Spline

Introduction

If you are using version 0.61.08 or lower, please see the old version of this tutorial, since this version is about the 0.61.09 version.

This tutorial will demonstrate how to make an object follow the path of an arbitrary curve, rotating to face the direction of travel.

Summary

We're going to:

• Draw a curve and an arrow
• Link the arrow's Origin and Rotation to the Spline so the arrow follows the curve

Tutorial

This is a brief tutorial giving an example of how to use it:

Create the Animation

File > New

Create the Layers

Select the Spline Tool

enable just the Outline checkbox

draw a spline that you want the arrow to move along

click the "Make Spline" icon in the bottom left of the Tool Options Panel to create the spline.

still in the Spline Tool, enable "Create Outline" and "Create Region" checkboxes in tool options

draw an arrow or whatever, pointing to the right.

Switch to the Transform Tool

select the outline, hit control-a to select all its handles except the green position handle

drag the handle so that the arrow is centred around the green position handle

Add a Rotate Layer above the outline and region

Group the rotate, outline, and region layers

so now you've got 2 top-level layers: a curved path, and a group containing an arrow and a rotate layer

Make the Arrow Move and Rotate

select the group layer by clicking it in the Layers panel

select its green position handle by clicking on it in the canvas window

additionally select the Rotate layer by holding Control and clicking it in the Layers panel

additionally select the blue "rotation amount" handle by holding Control and clicking on it in the canvas window

so now we should have 2 layers selected, and one handle from each of those 2 layers selected

now additionally select the curved spline layer (it should be the last layer in the Layer panel's list) by holding Control and clicking on it

right-click on the dotted line that indicates the position of the curved spline - not on any handle, but on the dotted line between handle

from the context menu that pops up, select "Link to Spline"

the arrow group should move so that its green position handle is on the spline, and it should rotate so that the arrow points along the spline at that point

select just the group layer, and drag its green handle around. you'll see that the handle is constrained now to line on the spline, and that dragging it also affects the rotation of the arrow as expected

we can now animate the arrow. turn on "animate edit mode" by clicking the icon in the bottom right of the canvas window.

at time 0f, drag the group layer's green position handle to one end of the spline

at time 5s, drag the same position handle to the other end of the spline

Try File > Preview or View > Play to watch the animation.

Results

This is the animation I ended up with: Arrow-follows-bline.sifz‎

Controlling the linear velocity

By default, the arrow travels the whole spline with a constant velocity, independently of the spline structure.

If you select the group layer and look at the Parameters Panel, then you'll see that its Origin parameter is converted to "Spline Vertex" type. This is done automatically when you do "Link to Spline" action. You can disable the "Homogenous" subparameter and then the speed of the arrow will depend on the spline structure - it will take the same time to move along each segment of the spline. So if there's a long straight part then a bendy complex part, the arrow will move much faster along the straight parts (since there will be less vertices in that part). In physics terms, the linear velocity (that is, the speed over the spline) is not constant.

For illustration of the "Homogenous" effect see this demo.