What Causes the Judder
A credit roll is just an image — a tall image of your text — moving upward at a consistent speed. The key word is consistent. Cinema runs at 24 frames per second. Your credits need to move by a precise, whole number of pixels on every single frame. Two pixels per frame. Three pixels per frame. Any clean whole number will give you a smooth roll.
The problem comes when the maths does not work out neatly. If you divide the total height of the credit graphic by the number of frames and get anything other than a whole number — say you get 2.3 — the software has to compensate. It moves two pixels for most frames, and three on the ones where it needs to catch up. Your eye, in a dark cinema, will catch that catch-up. Every time. Rhythmically. Like a hiccup.
The Premiere Trap
The most common way this happens: someone creates a tall graphic of their credits in Photoshop. They drop it into Adobe Premiere, set a start keyframe at the bottom and an end keyframe at the top, and let Premiere calculate the speed in between. Premiere has no setting to lock the scroll speed to a set number of pixels per frame — it just interpolates between those two keyframes across however many frames the clip lasts. Unless the total height of the graphic divides perfectly by the number of frames at a clean pixel-per-frame rate, it will not be smooth.
On your laptop preview, you might not spot it at all. The stronger the contrast between text and background, the more perceptible the judder becomes — and white text on a black background is the hardest case. Which is exactly what most end credit sequences look like. Put that on a four-metre cinema screen and it is unmissable.
The Fix
The fix must happen in the source before the file comes to us — by the time the motion is rendered, it is baked in. The total pixel height of the credit graphic must be divisible by a clean whole number of pixels per frame. If your graphic is 9,000 pixels tall and you want to move at 3 pixels per frame, your credit roll needs to last exactly 3,000 frames. If the timing does not work at that length, adjust the spacing between the credit lines — not the duration — until the numbers align.
We watch credit rolls on every file we receive. If we spot a judder, we will flag it back to you while there is still time to fix it in the source. An automated service will simply encode what it is given.
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