FCP 7: Using Bezier Controls for Smoother Motion Paths

[ This article was first published in the July, 2004, issue of
Larry’s Monthly Final Cut Studio Newsletter. Click here to subscribe.
Updated August 22, 2004 with a better technique suggested by Tom Wolsky,
illustrated at the end of this article. ]

I’ve been promising this Technique for months, but it kept getting pushed off the keyboard. However, last month’s surveys convinced me I needed to write it — which reminds me that I really do pay attention to your answers on these surveys!

Sometimes, when you are creating a motion effect, or other effect with keyframes, you may want to smooth out the corners, so that your object moves more smoothly across the frame. This technique shows you how to do it.

1. Start with an image you want to move around the screen. We’ll use our puppy to illustrate. To keep this simple, I’ll use a still frame, but the process is the same whether you use a still frame or video clip.

    

   

 

2. Load your clip into the Viewer, scale it back to, say 40%

    

   

 

3. Switch the Canvas to Image/Wireframe by clicking in the third pop-up menu at the top of the Canvas

 

4. Drag the image to the top left corner of the Canvas and set a keyframe for it’s starting position.

    

   

 

5. Move the Playhead 30 frames, or so, ahead and move the image to the middle of the right side. Moving the image’s location automatically sets a new Center keyframe in the Motion tab.

    

   

 

6. Move the Playhead another second, or so, ahead and move the image to the bottom left corner. This, too, will set a new keyframe.

    

   

 

7. Play the animation you just created and you’ll see a nice sharp corner where the image moves around the middle keyframe.

 

8. But we don’t WANT a nice sharp corner. We want a nice, SMOOTH corner.

    

   

 

9. In this case, Control-click on the middle keyframe and set it to”linear.” (Linear means that the speed of the clip through the keyframe will be consistent.)

    

   

 

10. The motion path turns into a nice, smooth curve with lines flying off from it. Those lines are called, “Bezier handles.” Grab the dots at the end and watch how moving them adjusts the shape of the curve. Notice that these handles move together, move one handle and the other moves in a corresponding way.

 

11. If you look closely, there are two purple dots midway along each line. These control the speed into and out of the curve. Using “linear” means the speed of movement of your clip remains constant. Selecting, “Ease In Ease Out” slows your clip down as it approaches a keyframe and speeds up as it leaves. If you want to change the velocity of your object as it turns the corner, move the velocity dots in the middle of the line.

     

    

 

12. You can adjust the SHARPNESS of one side of the curve by holding the Shift key down and dragging a line.

 

13. You can adjust the ANGLE of one side of the curve by holding the Control key down and dragging a line.

Update

Naturally, as soon as this article was published, a better way to do this surfaced, courtesy of Tom Wolsky.

The problem with this technique, as I’ve illustrated it, is that unless you set your keyframe in the EXACT center of the move, there’s a real chance your motion will have one speed before the center keyframe and a different speed after it, due to having an uneven number of frames on each side of the keyframe.

Tom suggested a much simpler approach: Put Bezier control points on each end-point, then bend the end-points so that your motion path follows a nice smooth curve through the frame.

Here is one example, emulating the curve I illustrated in Step 9.

(By the way, I explained “linear” earlier, but not “Ease-in.” Selecting this menu option means that as you clip approaches the keyframe it will change speed, generally, this means it will slow down, then speed up as it leaves the keyframe. Both these settings are adjustable.)

Here’s another example, where setting Bezier control points provides a nice smooth split “S” through the frame.

The key idea behind Tom’s comment is to avoid putting keyframes in the middle of a movement, unless it is absolutely necessary.

Thanks, Tom, for the suggestion.

There is a lot you can do with Bezier curves in Final Cut. This technique simply shows you where to look. Virtually every keyframe in both the Motion tab and Effect Filters has Bezier control, though in many instances, changing the angle of the curve of a filter may not make much sense.

Still, knowledge is power. Enjoy.

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