Create Expression Driven Arc

Today we’re going to build an expression-driven arc stroke.

Start with creating any kind of shape. For example ellipse.

Now go to your shape layer and expand the contents. You can delete the Ellipse 1 path. Right next to Contents, you should see an Add option with a right arrow next to it.

Now click on this arrow and add a path.

Initially of course your path will be empty. Expand your path options to see the time-watch icon.

Now alt-click the stop-watch icon. This will add an expression to your path property.

Now delete the current expression and add the one below.

JavaScript

// Expression for a shape layer's Path property to create an arc as a single-stroke

// Slider controls
var startAngle = effect("Start Angle")("Slider"); // Starting angle (in degrees)
var arcAngle = effect("Angle Control")("Slider"); // Arc angle (in degrees)
var radius = effect("Radius")("Slider"); // Radius of the arc
var resolution = effect("Resolution")("Slider"); // Number of points along the arc

// Ensure resolution is at least 2
if (resolution < 2) { resolution = 2; }

var pts = []; // Array to store points on the path
var inTangents = []; // Array for in tangents (set to zero for a straightforward stroke)
var outTangents = []; // Array for out tangents (set to zero as well)

// Convert angles to radians for trigonometric functions
var startRads = degreesToRadians(startAngle);
var endRads = degreesToRadians(startAngle + arcAngle);

// Generate points along the arc
for (var i = 0; i < resolution; i++){
var alpha = i / (resolution - 1);
var theta = startRads + alpha * (endRads - startRads);
var x = radius * Math.cos(theta);
var y = radius * Math.sin(theta);
pts.push([x, y]);
// No tangents needed since we are drawing a straight stroke along calculated points
inTangents.push([0, 0]);
outTangents.push([0, 0]);
}

// Create the path from points, with no tangents, and set as open (false)
createPath(pts, inTangents, outTangents, false);