In this tutorial, Animation Mentor mentor and Pixar Animator Anthony Wong explains how to animate a convincing bouncing ball by separating translation from rotation and then correctly linking them. The goal is to make your ball feel physically grounded instead of arbitrary, using motion trails, spacing, and clear decay patterns. Watch the full video above.
Meet Anthony Wong
Anthony Wong has been a professional animator since 1991. Anthony started as a character layout artist on The Simpsons and animated on some of Disney’s classics like Hercules, Mulan, and Tarzan. As a 3D character animator at Pixar he has worked on Inside out 2, Cars, Ratatouille, Toy Story 3 and many others. Anthony mentors students in Animation Basics, the first 3D Character Animation Course at Animation Mentor.
Big Idea: Treat Axes Independently
Anthony’s core concept is that the ball’s up-and-down motion (TY) and its forward motion (TZ) are independent. You can adjust one without automatically changing the other.
- TY (vertical) is where you describe gravity and how the bounces lose height over time.
- TZ (forward) is where you describe how far and how long the ball travels along the ground.
He shows that you can scale or slide the TZ curve in the graph editor—making the ball travel less, more, or over a longer time—without touching TY. Likewise, you can change the number or height of bounces in TY without having to rebuild TZ.
Using Motion Trails to Read the Pattern
Turning on a motion trail for the ball’s root control gives you a visual “signature” of the bounce. Each arc is a bounce, and the horizontal gaps between contacts represent timing.
Key points he emphasizes:
- The gaps between impacts should form a kind of countdown (14, 12, 10, 8, 6, 4, 2 frames, for example) as the ball loses vertical energy.
- The peak heights should decrease consistently; if you draw a line through the tops of the bounces, it should be straight or slightly curved downward, not random. A more concave line reads as a heavier ball.
He also recommends turning on spacing markers on the trail and adjusting their color/size so you can clearly see how far the ball moves between frames.
Gravity vs Friction: TY and TZ
Anthony distinguishes the forces at work on each axis:
- TY (up/down) has to obey gravity and energy loss at each contact, so each bounce is shorter in both height and time than the last.
- TZ (forward) mainly represents friction, so its curve is typically “fast out, ease in” — similar to a car coasting and slowly stopping.
If you have done basic car or ball roll exercises, the same kind of forward curve applies here: strong initial movement that gradually eases as the ball runs out of steam.
Correct Spacing into and out of the Ground
Using the motion trail’s spacing markers, he focuses on the direction of spacing changes at each impact:
- Spacing into the ground should be larger than spacing coming out. This shows that the ball comes in with more energy, loses some on impact, and leaves with less.
- If you flip that (small spacing coming in, big spacing going out), the motion reads as a jump rather than a bounce—even if the timing and contact frames are the same.
This is a common beginner mistake: timing looks okay, but spacing is reversed, so the physical idea is unclear.
Matching Rotation to Forward Travel
Once the translation feels right, the next step is to match the ball’s rotation to how far it has rolled so it does not look like it is sliding. Anthony points out that the rotation curve and TZ curve are essentially the same shape, just at different scales.
A simple workflow he suggests:
- Turn off or delete any existing rotation keys.
- Key the rolling axis (usually RX) at the start of the forward movement.
- Go to the frame where TZ stops moving.
- With auto-key on, rotate the ball until the roll visually matches the distance travelled.
From there you can refine the rotation curve, but the main idea is that the amount of rotation must be consistent with the forward distance, especially in the rolling phase after the last bounce.
Adjusting Weight with Decay and Duration
By stretching or compressing the TZ curve, you can make the ball roll farther or stop sooner without changing the vertical motion. Combined with how quickly the peak heights decay in TY, this becomes your main “weight dial”:
- Slower height decay and longer forward travel → lighter ball.
- Faster height decay and shorter travel → heavier ball.
The important thing is that both the bounce heights and the time between bounces decrease in a clean, logical way so the energy loss feels intentional.
Common Pitfalls to Avoid
Anthony highlights several frequent issues he sees in student work:
- Bounce heights that do not follow any clear decaying pattern.
- TY curves that look noisy or random, even when the contact timing is technically correct.
- Symmetrical spacing into and out of the ground, which flattens the sense of impact.
Regularly checking the motion trail, drawing imaginary lines through bounce peaks, and adjusting spacing near impacts will help you clean these problems up.
How to Apply This in Your Own Practice
You can use these ideas in any bouncing-ball exercise:
- Animate TY and TZ as separate problems, then refine how they support each other.
- Use motion trails to judge timing, spacing, and energy decay instead of relying only on the curves.
- Make spacing into the ground bigger than spacing out to show energy loss.
- Once translation is solid, match rotation to forward distance so the ball convincingly rolls to a stop.
Practicing with this level of discipline on a simple ball will make your later character and prop animation feel far more grounded and intentional.
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