A neon glowing line that draws itself

A line that draws itself turns up all over a game: magic circles that trace their runes, neon signs and titles that write on, energy running round a character, a button border with a light chasing round it, a slash that whips across. Video tools do it with a stroke, a trim path and a glow, as the Saber plugin does in After Effects. In ParticleFX Studio it is a Stroke layer: a white-hot core with a soft coloured glow that follows a built-in shape, a path, text or the outline of a sprite, and still loops perfectly.

Magic Circle Draw-OnNeon Text Write-OnNeon Sign FlickerSword Slash ArcEnergy Outline AuraGlowing Button BorderElectric Ring BarrierConstellation Draw-On

The quick way: start from the library

Open Magic Circle Draw-On: six strokes write on in turn, the outer ring in 0.8 s and the pentagram from 0.65 s to 1.5 s, then flash and fade. Or describe yours: build "a neon sign that writes on, then flickers" with the agent (uses 1 build; your first one needs no account).

Learn it hands-on: the Stroke lesson has you write a summoning circle's outer ring on with End keys, give it a pen tip with Pen glow and break a rune ring into ticks with Dash.

Make a line write itself on

1. Press + in the Layers panel and choose Stroke. A glowing circle appears in the middle of the canvas.
2. Pick a Source. Shape traces a circle, polygon or star (Points 5 makes a pentagram); drag the centre handle to move it and the square handle to set the Radius. Path traces a path layer, which you can draw with Draw a path in the same menu. Text traces the outlines of letters you type. Sprite traces the silhouette of an object layer and follows it as it moves.
3. Write it on: click the diamond next to End, set 0 at the start and 1 where the line should be complete. End is where the visible part ends along the shape, from 0 to 1.
4. Write it off: keyframe Start from 0 to 1 after the line is drawn, so the tail chases the head away. For a segment that runs round the shape forever, set Start 0, End 0.2 and a Travel speed in loops per second.
5. Style it under Look: Core and Core width for the white-hot centre, Glow and Glow width for the coloured halo, Glow intensity and Intensity for brightness. Keyframe Intensity to fade the whole line in and out.
6. Add character in All settings. Taper start and Taper end thin the ends to points, Pen glow puts a bright flare on the drawing end, Wobble adds an animated sideways distortion for electric or hand-drawn lines, Flicker drops the brightness at random like a failing neon tube, and Dash with Gap and Dash speed makes marching dashes.
7. Add a Bloom filter under Effect settings > Post filters so the brightest parts bleed light. The library effects use a Strength of 0.6 to 0.8 and a Threshold of 0.2 to 0.25.

Settings from the library

Tips

• Loops close by themselves. Travel speed and a shape's Spin snap to whole laps per loop, so a running light never jumps at the seam. A write-on that loops needs a write-off or an Intensity fade before the loop ends: Magic Circle Draw-On fades from 2.3 s to 2.85 s of its 3 s loop.
• Letters in turn or together. For text and sprites, Outlines on Sequence runs Start and End across every outline in reading order, so letters write one after another; Together draws them all at once.
• Auras. For a sprite source, push the line out with Expand and turn Hole outlines off to keep only the outer silhouette.
• Fade with Intensity. Keyframe Intensity to fade a stroke in and out; the layer's own opacity cannot be keyframed.
• Glow in the engine. The glow is light, so export it on black for additive blending or with transparency, as the blending guide explains.