"Stuttering" when panning/zooming a still image...?

Question

Hello.
I'm working on a small project which is basically a series of zooms/pans on still images. The pans/zooms are quite slow and take place over about 6-8 seconds. There's nothing too fancy going on here, but I notice that when I do a RAM preview or render out a comp, I get small "stutters" in some of them. Does anyone have any ideas why this would be happening and any suggestions on making these animations a little more "smooth". I mean, these things aren't complex at all, and overall they're quite smooth, but there's still times when I get these stutters and I can't figure out why they'd be happening? Would appreciate some advice!
Thanks,
Kristin.

Answer 1

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 slower 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.
There are also other cases where the sub pixel interpolation between frames causes areas of detail to flicker between frames. The latter problem 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. 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.
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. Interlacing, as was mentioned before, helps and gives you a wider range of motion, but interlacing can introduce more pronounced flickering in the detail if you are right at one of these critical speeds.
This problem extends to shooting as well. 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.
The solution to successfully animating detailed images is to use the right speed (pixels per second) for your frame rate, add extra 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.
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.