How to Make a Line Boil in After Effects

In After Effects, a line boil is made with Turbulent Displace set to a small amount and a large size, with the Evolution property driven by an expression that changes only every second or third frame instead of animating smoothly.

After Effects will do this, and the technique is worth understanding whether or not you use it, because it shows exactly what the effect is made of.

The effect to reach for

Turbulent Displace. It generates a fractal noise field and pushes pixels around according to it, which is the whole mechanism of a line boil.

Starting points, for a 1080p comp:

Note that Amount is not measured in pixels. A value of 10 at Size 60 moves edges a few pixels; the same Amount at Size 5 tears them apart. The two properties are not independent, which is the most common source of confusion with this effect.

Why it looks wrong out of the box

Animate Evolution linearly and you get a smooth, continuous churn. It looks like the image is underwater.

Real animation does not churn. It changes, holds, changes again. Each drawing is a discrete event.

The fix, which is one expression

Put this on Evolution:

```

hold = 2;

seed = 40;

Math.floor(time * (1 / thisComp.frameDuration) / hold) * seed

```

That takes the current frame, divides by how many frames you want to hold, floors it, and multiplies by an arbitrary step. The result is a value that jumps to a new number every second frame and sits perfectly still in between.

Change hold to 3 or 4 for a slower, heavier feel. The seed value only needs to be large enough that consecutive steps land on visibly different parts of the noise field; 40 works, so does 100.

Without the floor, you have a wobble. With it, you have a redraw. That single function is the difference.

Two refinements

Confine it to the edges. Turbulent Displace hits the whole layer, so flat interiors move along with the outlines. Duplicate the layer, run Find Edges on the copy, blur it slightly, and use it as a luma matte on an adjustment layer holding the displace. Now only the lines move.

Vary it across the frame. A single displace applies one noise field to everything, so distant parts of the image move in a correlated way. Two instances at different Sizes, or a slight Offset animation, breaks that up.

What it costs

Turbulent Displace with a matte is not cheap, and the edge-detection pass doubles the layer count. On a long sequence, expect to prerender.

This is also the honest reason a dedicated tool exists. The After Effects route gives you a boil inside a comp where you can composite it against everything else, which is the right call when the shot needs that. If you only want a clip boiled and exported, a pass with edge weighting, hold, and variation as four numbers is faster.

Boiler does exactly that pass, free and open source, and you can compare settings on a single frame before committing to a render. The After Effects method is better when the boil is one layer in a larger composite. Neither replaces the other.

If you use another tool

The same three ingredients apply anywhere: a noise field, a displacement driven by it, and a step function on the noise so it changes on twos instead of continuously. In Blender it is a Displace node fed by a noise texture with a stepped Value node. In Nuke it is IDistort with a noise input. The names change; the floor function does not.

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