Tracing along Shape Path with Rounded Corners

[Rob Mcdougall]

Working on this a little more, I figure out how to draw a path using expressions only, but I can’t figure out a way of rounding the corners. I can add a tangent but that’s not right… Does anyone know how to add rounded corners as an expression?

[Filip Vandueren]

Folllowing a question on reddit, I reverse engineered the rounding function, so I might as well add it here for future reference:

function roundCorners(aPath, rounding) {
	// Mimic After Effects' native round corner shape modifier but return an actual new Path
        // Filip Vandueren 2023
	
	const pts = aPath.points();
	const it = aPath.inTangents(); const ot = aPath.outTangents(); const cl = aPath.isClosed();

	let newPts = []; let newIt = []; let newOt = [];

	for (let i=0; i<pts.length; i++) {
			
		if ((!cl && (i==0 || i==pts.length-1)) || length(it[i])!=0 || length(ot[i])!=0) {
			// re-use the first and last vertex unaltered if the path is not closed
			// OR if this vertex originally had tangents, it's also not discarded and chamfered, but kept:
			newPts.push(pts[i]);
			newIt.push(it[i]);
			newOt.push(ot[i]);	
		} else {
			// add chamfer vertices, discarding the original vertex
			// giving these new vertices tangents creates the rounded corners
			
			// look at previous vertex
			let j = i-1; 
			if (j<0) j+=pts.length; // loop around in closed paths
			let segment = (pts[j]+ot[j]-pts[i]); // consider a linesegment from this vertex to the prev outTangent
			// where should we place the chamferpoint:
			// on that segment, at a point that's 'roundingvalue' away from the current vertex
			// but on a straight linesegment it should never be further than 1/2 the distance
			// (the rounding is limited by the edges length)
			let n = normalize(segment)*Math.min(rounding, length(segment)/(length(ot[j])==0 ? 2 : 1));
			let chamferPoint = pts[i] + n;
			
			// add the point and it's tangent, using the magic number 0.55 
			// it's on the same linesegment
			newPts.push(chamferPoint);
			newIt.push([0,0]);
			newOt.push(-n*0.55);
			

			// look at next vertex
			j = (i+1)%pts.length;  // loop around in closed paths			
			segment = (pts[j]+it[j]-pts[i]);
			// same logic as above
			n = normalize(segment)*Math.min(rounding, length(segment)/(length(it[j])==0 ? 2 : 1));
			chamferPoint = pts[i] + n;

			newPts.push(chamferPoint);
			newIt.push(-n*0.55);
			newOt.push([0,0]);
		}
	}

	return createPath(newPts, newIt, newOt, cl);
}

roundCorners(thisProperty, 50);