Wirebook › Bonus lesson
Why does a game judder when it can't keep up?
A screen is a flip book. A thumb turns the page on a steady beat, 60 times a second on most TVs and phones, 120 or 144 on fast ones. The game is an artist drawing the pages as fast as it can. So what happens when the artist can't keep up with the thumb?
Why does a page get shown twice?
The thumb never waits. Every 16.7 thousandths of a second (at 60 Hz) it flips, whether a new page is ready or not. If the artist is still drawing, the thumb has nothing new to show, so the old page stays up for another beat. At 50 pages a second on a 60 Hz screen, that happens about 10 times every second. Most pages are up for one beat, a few for two, and the stick man moves in uneven steps: step, step, step-and-a-half. That stumble is called judder or stutter, and your eyes are very good at spotting it, especially when the picture pans sideways.
Notice the strange part: 50 pages a second can look worse than 30. At 30, every page gets exactly two beats, so the steps are bigger but perfectly even. That's why some games offer a "30 fps cap" or "half refresh rate" setting. Try the 🧢 cap in the box above: the wobble drops to zero.
What's a screen tear?
A screen doesn't swap the whole picture in one go. It paints it from top to bottom, line by line, during each beat. If the game says "here's a new page!" while the screen is halfway down, and nobody makes it wait, the screen carries on painting with the new page. The top of the picture is from one moment and the bottom from a slightly later one, with a sharp line where they meet. That's a screen tear. Pick "90 on 60, V-Sync off" and play it: the post and the stick man are cut and shifted along the dashed lines.
V-Sync fixes tearing by making the game hand over pages only between beats. The catch is the judder above, and a little extra delay, because finished pages sometimes wait for the next beat.
The magic thumb: VRR
What if the thumb waited for the artist? That's VRR, variable refresh rate (Nvidia calls it G-Sync, AMD calls it FreeSync, and consoles and TVs use HDMI 2.1 VRR). The screen flips the moment each page is finished, so a page that took a bit longer simply stays up a bit longer. There's no tearing, almost no waiting, and 50 pages a second looks nearly as smooth as 60.
VRR isn't truly magic, though. It has a range, for example 40 to 144 flips a second. The thumb can't flip faster than the top of the range, and it can't wait forever: below the bottom of the range, the picture would start to fade, so the screen shows the old page again. And if the artist has a big hiccup, a page that takes three times as long, no thumb can hide it. Then the only fix is a lighter page: lower the graphics detail so every page is quick to draw.
Why do films judder on some TVs?
Most films are made at 24 pictures a second. 60 doesn't divide by 24, so a 60 Hz TV has to show the pages for 3 beats, then 2, then 3, then 2. Grown-ups call it "3:2 pulldown". In slow pans across a room you can see the uneven steps. 120 is 24 × 5, so at 120 Hz every film page gets exactly 5 beats and the judder disappears. Many TVs also have a "match frame rate" setting that switches the screen to 24 Hz for films. Try the 🎬 button above and then switch the thumb to 120.
So what happens when the artist can't keep up?
The thumb keeps flipping and shows the old page again, so the movement goes step, step, stumble. Nothing is lost and the game hasn't slowed down; the pages just arrive out of step with the beat. You can fix it three ways: draw pages faster (lower the detail), make the steps even (cap the frame rate at a number that fits the beat), or get a thumb that waits for you (VRR). And if the thumb doesn't wait and the artist doesn't either, you get a torn page instead.
Related lessons: how a game controller talks to a console · how online games stay in sync · what happens when you squish a video too much · why a video goes blurry, then sharp · why phones get hot and slow down.
Grown-up help on Tooltern: screen tearing: how to fix it, game stutters even with high FPS, VRR, G-Sync and FreeSync explained, 60 vs 120 vs 144 Hz: can you tell the difference?, Nvidia Reflex and AMD Anti-Lag, TV game mode and input lag and the input lag calculator. Glossary: refresh rate, frame rate, VRR, V-Sync, screen tearing.