Avoiding Judder in Motion Graphics
Q: A question?
How do I get smooth playback of my video files? No matter what I do the movement isn't smooth. It looks like I'm dropping frames.
A: An answer
When horizontal or vertical motion refuses to playback smoothly the problem may lie in the framerate and the motion and not playback. Here are the facts:
- CRITICAL SPEEDS: There are certain speeds that sync up with frame rates so that they cause judders, jitters, stuttering or whatever you want to call it. The lower the frame rate the more obvious the problem.
If you've ever watched a western on TV or at the movies you've seen the stagecoach wheels turn backwards at various speeds. This is an example of the problem. It's actually called a stroboscopic effect. Mechanics, in the good old days when cars had carburetors and distributors, used to hook up a timing light to the #1 spark plug so that a light flashed on a timing mark on the crankshaft pulley. This light had the effect of stopping the rotation visually. That's exactly what's happening when you are moving your photograph across the frame at one of the critical speeds. Your eye is fooled and your brain can't process the movement smoothly.
- SUB PIXEL INTERPOLATION: There are also other cases where the sub pixel interpolation between frames causes areas of detail to flicker between frames.
The sub pixel interpolation problem can be easily seen can be seen by viewing your comp a frame at a time while zoomed in to say 800% and observing the edge detail. This problem can be especially nasty when you're trying to do a smooth title or credit roll that uses thin fonts. The horizontal lines that make up fine type can almost completely vanish between fields or frames if the text is moving at the wrong speed. You can test this out by creating a single pixel high horizontal line in AE and animating it from bottom to top of the frame. The only speed where you'll get a nice solid line in every frame or field is one that is an exact multiple of the frame rate. IOW, 1 frame 1 pixel of movement, 1 frame 2 pixels of movement, and so on. Move at 1 frame and 1.33 pixels of movement and the line will appear to completely disappear then return as the line moves up the screen.
- FRAME RATE: If you are working at 24fps progressive you'll find that the juddering caused by motion at critical speeds is much more problematic than projects at 29.97 fps. You may, or should I say will also run into problems with video shot at 24fps if the camera operator is not aware of this problem.
Interlacing helps and gives you a wider range of pleasing motion, but interlacing can introduce more pronounced flickering in the detail in vertical movement. Interlaced output is also only suited to one medium: Television. Along with interlacing goes non square pixels. Video for the internet hates to be interlaced and it hates non square pixels.
- CAMERA TECHNIQUE: As I mentioned above this problem extends to filming as well. Before I learned what I was doing I shot many beautiful sweeping pans of the Olympic mountain range in Washington State on 35mm film that were nearly impossible to watch.
Cinematography manuals contain critical panning speed charts that list the number of degrees per second you can pan with various lens (angle of view) and shutter speed combinations. There isn't a videographer out there that's tried shooting 24P video and not ended up with an unusable pan due to these critical speeds. One basic rule of thumb that almost always works is the "Seven Second Rule." If it takes 7 seconds for the key element in your scene to move from one side of the screen to the other your shot will probably work.
- IMAGE PROCESSING: The solution to successfully animating detailed images is to use the right speed (pixels per second) for your frame rate, add enough motion blur to hide the problem, or reduce the amount of detail in the image.
These are the only solutions that I know of. They all limit design and timing, but what good is your design if it makes your eyes go buggy. Many people have problems with photo montage projects. If you have highly detailed photographs then a good solution is to remove some of that detail with the Dust and Scratches filter in Photoshop.
- CHECKING THE PLAYBACK: The best way I know if to tell if the problem is a playback issue is to use a device or program that tells you when you're dropping frames.
You can use QT (by looking at the playback rate) or in FC or PPro by having the program report dropped frames. If it's a data rate issue you need more horsepower or a better codec. If you have rendered to any of the standard delivery formats at standard datarates and you playback has the judders then it's the speed of the motion that's causing the problem.
- THE SOLUTION: So how do you make sure your project always looks it's best? You follow the rules:
- Fine detail must move at an even number of pixels per frame to avioid the sub pixel problem.
- Aviod critical speeds. The "Seven Second Rule" is a good place to start
- Use motion blur. Use as much as it takes to hide problems.
- Change the frame rate. Does your project benefit from or have to be 24fps?
- ONE LAST THING: If you've spent all night rendering a project and it is full of unacceptable judder bring the rendered footage (hopefully rendered to a lossless codec for archival purposes) into AE, open up the footage interpolation options and conform the footage to a higher frame rate. Change it from 24fps to 30 or 36, or 48. Now drag it into the new composition icon at the bottom of the Project panel. You'll keep every frame you rendered, your sequence will be shorter, but there's a high likelihood that the motion will be smooth. Now open up the Adobe Media Encoder and add your After Effects project to the cue. Pick the re-timed composition and render it using one of the default presets. This will save you a bunch of render time and will do a great job of hiding the problems.
About the contributor
I have been in the television and film business since 1969. I have been an active member and contributor to the After Effects community since I first started useing AE version 3.5. I'm an Emmy award winning cinematographer, producer and writer of news, documentary and commercial projects. I live near Sacramento, California and work anywhere in the world there's a chance to make good pictures.
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Reposted from a legacy community article by @RickGerard
Kevin Monahan - Sr. Community & Engagement Strategist – Pro Video and Audio
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