Imagine the last approval on a film. The picture is locked, the mix is done, and the only thing left to sign off on is the end credits. The post house sends the producer a review link, and the producer watches it that night at home. Halfway through the cast list, the producer frowns. The names aren't gliding. They step and catch, step and catch. The roll goes back to the editor with a note: Credits are jerky. Please fix.
Back at the post house, the editor plays the file. Every frame moves the same distance as the frame before it, exactly, down to the pixel. There is nothing to fix. The note is accurate, and the file is perfect, and both of those things are true at once.
So what's the problem?
The end crawl is the simplest moving picture in a film, and the hardest one to show honestly.
01The one thing the eye follows
Almost nothing in a movie moves evenly. Actors speed up and stop. Cameras drift, push in, and settle. Cuts arrive every few seconds and reset everything. Your eye doesn't follow any of it closely, so small flaws in timing slide past unnoticed.
A crawl is the opposite. It is a column of type moving upward at a constant speed for 6 or 7 minutes, and when the eye sees something moving at a constant speed it does something unusual: it locks on and tracks. Scientists call it smooth pursuit. It's what lets you read the side of a passing train. The eye moves with the type, so the type sits still on the retina, and you can read it.
Which means that every irregularity in the motion lands directly on the retina too. If a line of names arrives a few milliseconds late, the eye, already moving, overshoots it. That is what a stutter is: the eye keeping perfect time and the picture failing to keep up. A crawl is the one picture in a film where the audience is, without knowing it, measuring the timing of every frame.
02Whole pixels
The obvious suspect is still the file. Editors have been wrong before, and the first rule of a smooth crawl is easy to break: move the same whole number of pixels every frame.
Released films crawl at about 1/10 of the screen height a second. On a 1080-line picture at 24 frames a second, that's 4.5 pixels a frame, and a screen can't move anything half a pixel. That leaves 3 choices. Alternate 4 and 5 pixels, and the speed flutters. Draw it at 4.5 pixels anyway, and each letter is smeared across 2 rows of pixels, a little differently every frame, until the names shimmer like heat haze. Or move exactly 4 pixels every frame, or exactly 5, and each frame is the one before, moved up. Final Frame always picks that whole number for you.
This roll was made that way. The file is perfect. So why did the names stutter?
03Two and a half
Back to the producer at home. The answer has nothing to do with the film. It is arithmetic.
Film runs at 24 frames a second, and most computer monitors, laptop screens and office displays refresh 60 times a second. Divide 60 by 24 and you get 2.5. A screen can't show a frame for half a refresh, so it does the only thing it can: it jitters. It shows the first frame for 3 refreshes, the next for 2, the next for 3, then 2, over and over.
Measuring.
So every other frame stays on screen 50% longer than the one beside it: 50 milliseconds, then 33. For most pictures you would never notice. For a crawl, with the audience's eye locked on and tracking, it is the stutter in the note: step and catch, step and catch, 12 times a second.
On a 60 Hz screen, no player can fix it, not ours and not QuickTime. The display is telling the truth about its own limits and lying about your film.
The cinema never had the problem: a projector flashes each frame 2 or 3 times, every flash the same length. A 120 Hz screen, like a recent MacBook Pro or iPad Pro, holds every frame for 5 refreshes, and so does a 120 Hz TV fed a true 24p signal.1 A MacBook Pro can also be set to 48 Hz in its display settings.2 So before a roll comes back with a note about stutter, ask what it was watched on.
A credit roll is the only picture in a film you can judge the screen by.
04The size of the window
A perfect file can still shimmer because of the size of the window it plays in. A 4K roll moving 8 pixels a frame stays sharp only at sizes where those 8 pixels shrink to a whole number, such as 4 at half size. Shrink it to fit a laptop's browser and each step might become 3.7 pixels: the half-pixel problem again, made by the player this time. Drag the corner of the window and the shimmer comes and goes.
Final Frame's review player snaps the picture to a size where every step is a whole number of pixels. We learned this the hard way.
05Nobody notices
The odd thing about all this work is that, done right, it's invisible. Nobody walks out of a cinema saying the credits were smooth. They only notice when the roll is wrong: the shimmer, the stutter, the line that jumps. A technically good crawl is one the audience never thinks about, which is the highest compliment a crawl can get.
And when somebody does notice, the fix may not be in the file at all. It may be in the monitor on the desk at home.
Sources
- RTINGS, "What is 24p playback on TVs?": a 120 Hz panel repeats each 24p frame five times (5:5 pulldown); on 60 Hz sets, 3:2 pulldown holds alternate frames for 50 and 33 milliseconds. https://www.rtings.com/info/what-is-the-24p-playback-on-tvs
- Apple, "Change the refresh rate on your MacBook Pro or Apple display": fixed rates of 47.95, 48, 50, 59.94 and 60 Hz, and "if the content you're viewing is 24 frames per second, select the 48 Hertz refresh rate." https://support.apple.com/en-us/ht210742
