Controlling Animation Velocity

Ted Boardman tedb@tbmax.com         http://www.tbmax.com


Giving Your Clients the Run-around

My perception of most "walkthrough" presentations is that someone has placed me in a shopping cart with a plastic bucket on my head from the veterinarian's, and then is running me through the project, flailing around corners.

The speed is generally much too fast and any lateral motion of a single lens camera is very disconcerting. As a rule of thumb, presentations would be more interesting to an audience if a few simple film and video techniques were used. Most forward motion is straight ahead and when turns are needed, the camera slows to a crawl or stops completely while the camera slowly turns.

By editing a series of these short clips together for your presentation you will convey more information and make the viewer more comfortable.

Speed Kills

What is "too fast" and how do you adjust it? Let's look at speed first. There are a few set numbers that happen to make it easy to calculate a good base speed for walking camera motion.

Practically all video in the US uses the NTSC standard playback of 30 frames per second. The average adult step length is around 30 inches and when walking a client through a project one step per second is a comfortable speed. Doing the math results in 1 inch of forward motion for each frame of animation. Therefore you can measure the length of an animation path (use the Utilities/Measure tool) and set the total number of frames in the sequence to the number of inches in the path.

While velocity control of animated objects can be adjusted by playing with the keys created, this is not very intuitive and can quickly lead to frustration even for seasoned animators.

Start…Stop…Backup…Velocity Changes in VIZ and max

One method for controlling the animation velocity of objects is through the use of Ease Curves. Ease Curves give you a clear graphical representation of a base motion that can easily be adjusted as an override to the original motion. If you mess something up it is easy to reset or remove the adjustments without changing the original animation. The process is simple, but there are a few specific steps you have to follow to make it work.

I'll walk you through a simple exercise that will combine the use of a Path Constraint to set the initial animation with an Ease Curve to adjust the velocity of the object. I'm going to keep it very simple to allow you to concentrate on the process. I suggest you try a simple example of your own after learning the process and work up to more complex scenarios gradually.

Scene Setup

In the Perspective viewport, create an Ellipse that is 15'0" in Length and 30'0" in Width. Create a small box that is 1'0" x 2'0" x 1'0". See Figure 1

Figure 1: Create an Ellipse path and a Box object in the Perspective viewport

Note: if you are working in display units other than feet and inches, you can still enter the numbers as shown above and they will be converted to metric or whatever units you are using.

Select the box and, in Motion Panel, Assign Controller rollout, select Position: Bezier, click the Assign Controller button, and double-click Path Constraint in the dialog. See Figure 2.

Figure 2: Assign a Path Constraint to the Box object

In the Motion Panel, Path Parameters rollout, click the Add Path button and pick the Ellipse. Check the Follow option and Constant Velocity. This causes the box to stay perpendicular to the path and to travel at a constant speed. See Figure 3.

		Play the animation back in the Perspective viewport to see the motion of the box. Note: if you are using 3ds max 5, the box will not be traveling at a constant velocity. Because of the new Auto Tangent feature the box will start and end slowly. That can be adjusted later. To calculate the current speed of the box, go to Utilities Panel, click Measure button, and pick the Ellipse in the scene. You will see that it is 72'7 7/8" long. Rounding that to 73 feet, your animation should be around 73 x 12 or 876 frames for a reasonable walking speed. In this exercise you will leave the frame count to 100 to better see velocity changes. Easy Now! To apply Ease Curves to the animation you need to open Graph Editors, Track View (Curve Editor in 3ds max 5). In Track View, expand Box01 to see the Percent option. Path Constraint automatically created two keys, 0 percent along the path at the beginning and 100 percent along the path at the end, to animate the box. See Figure 4.
Figure 3: Check Follow and Constant Velocity in Path Parameters rollout.

Figure 4: Track View with Box01 expanded and keys showing in the right panel.

To apply an Ease Curve you must be viewing the Function Curves of the animation, not the keys. In the Track View toolbar, click the Function Curve button. The displays the Percent as a straight line curve (constant velocity). See Figure 5.

Figure 5: Click the Function Curve button in Track View to view the animation as a straight-line curve.

Now click the Apply Ease Curve button just once. You won't see anything happen, just yet. See Figure 6.

Figure 6: Click the Apply Ease Curve button once.

Click the new plus sign to the left of Percent in the left panel to expand it. Then highlight Ease Curve to display the curve in the right column. It is a copy of the underlying animation so is shown as a straight line, but it has an extra key in the middle of the line.

Finally…You are in Control

Pick the key box in the middle of the curve and you will see in the numeric fields at the bottom of Track View that this point is at frame 50 and is 50.000 percent along the path.

In the right numeric field enter 25 and press Enter. The curve is now flat on the left and steep on the right. See Figure 7. Play the animation. Where the curve is flat the box is stopped. As the curve is steeper the box travels faster.

Figure 7: Enter 25 in the right numeric field of the middle control point.

Click the Add Keys button in the Track View toolbar and pick on the green curve near where it intersects the 60 frame line. See Figure 8. Then, click the Move Keys button so you don't inadvertently create another new key. In the right numeric field, enter 28 and play the animation. The box starts slowly and eases to a pause at frame 50 near 25 percent along the path. It then increases over the rest of the 100 frames.

Figure 8: Click Add Keys button and add a new key on the path at near frame 60 on the graph.

For the key that you added change the numeric fields to be 60 and 25. Then, click the original key and change it to read 40 and 25. Play the animation and you will see that the box slows and backs up slightly, then proceeds at around 25 percent along the path. Whenever the curve slopes down to the right, the object is traveling backward.

Select the key at frame 40, right-click on it and change the Out tangency to Linear. See Figure 9.

Figure 9: Select and right-click on the key at frame 40. Change the Out tangency to Linear with the flyout buttons.

Choose the key at frame 60 and change the In tangency to Linear. The curve should be horizontal between the keys and the box should come to a full stop for 20 frames and progress onward after frame 60. See Figure 10.

Figure 10: By adjusting the outgoing and incoming tangency of keys you can bring an object to a full stop, indicated by a horizontal section of the curve.

Play with the positions of the keys and the tangency settings and you will soon develop a feel for the curves that allows you to accurately adjust your object velocity with ease. Don't forget that the overall animation is still the same so if you slow an object at one point in time it will have to speed up elsewhere to compensate.

To change the overall animation you must use Time Configuration and, because there is existing animation, you must access Rescale Time and lengthen or shorten the total number of frames.

Summary

This method of controlling animation velocity of cameras or objects in a scene is a great way to give you control and visual feedback.

Start with a simple base animation and make your adjustments with the Ease Curves so that if you mess up an animation you can reset or remove the curve without losing the base animation.

My usually setup would be to animate a Dummy object along a path with this method, without using the Follow option in the Path Constraint. Then, link a camera to the dummy and rotate the camera independently by setting keys to have the camera look left and right. This separates the motion from the rotation allowing more control and less confusion with a multitude of keys to keep track of.

Investigate Multiplier Curves, too, in your spare time, of course.

In any case, have fun.

Ted