Threejs - How to offset all points on a 2d geometry by distance - vector

Question

Using Three.js, (although I believe this is more math related) I have a set of 2D points that can create a 2D geometry. such as square, rectangle, pentagon, or custom 2D shape. Based of the original 2D shape, I would like to create a method to offset the points inward or outward uniformly in such a way like the attached image.
I don't know if there is a simple way to offset/grow/shrink all the points (vector3) uniformly on the 2D shape inward or outward. And if so, it'll be cool if I can offset the points by X distance? Kinda of like saying offset the points on the 2D shape outward or inward by X distance.
And no, I'm not referring to scaling from a center point. While scaling may work for symmetrical shapes, it won't work when it comes to non-symmetrical shapes.
see image for example
Thanks in advance.

Answer 1

You can read that forum thread.
I've made some changes with ProfiledContourGeometry and got OffsetContour, so I leave it here, just in case, what if it helps :)
function OffsetContour(offset, contour) {
let result = [];
offset = new THREE.BufferAttribute(new Float32Array([offset, 0, 0]), 3);
console.log("offset", offset);
for (let i = 0; i < contour.length; i++) {
let v1 = new THREE.Vector2().subVectors(contour[i - 1 < 0 ? contour.length - 1 : i - 1], contour[i]);
let v2 = new THREE.Vector2().subVectors(contour[i + 1 == contour.length ? 0 : i + 1], contour[i]);
let angle = v2.angle() - v1.angle();
let halfAngle = angle * 0.5;
let hA = halfAngle;
let tA = v2.angle() + Math.PI * 0.5;
let shift = Math.tan(hA - Math.PI * 0.5);
let shiftMatrix = new THREE.Matrix4().set(
1, 0, 0, 0,
-shift, 1, 0, 0,
0, 0, 1, 0,
0, 0, 0, 1
);
let tempAngle = tA;
let rotationMatrix = new THREE.Matrix4().set(
Math.cos(tempAngle), -Math.sin(tempAngle), 0, 0,
Math.sin(tempAngle), Math.cos(tempAngle), 0, 0,
0, 0, 1, 0,
0, 0, 0, 1
);
let translationMatrix = new THREE.Matrix4().set(
1, 0, 0, contour[i].x,
0, 1, 0, contour[i].y,
0, 0, 1, 0,
0, 0, 0, 1,
);
let cloneOffset = offset.clone();
console.log("cloneOffset", cloneOffset);
shiftMatrix.applyToBufferAttribute(cloneOffset);
rotationMatrix.applyToBufferAttribute(cloneOffset);
translationMatrix.applyToBufferAttribute(cloneOffset);
result.push(new THREE.Vector2(cloneOffset.getX(0), cloneOffset.getY(0)));
}
return result;
}
Feel free to modify it :)

Answer 2

I have some doubts about solutions that do not include number of edges modification.
I faced the same issue in this project where I wanted to ensure a known distance between voronoi cells, and I quickly figured out that scale does not fulfill the use case. But one complication I faced was the disappearance of some edges that I had to handle in a while loop. It was so difficult to debug that I had to create a debug mode that helps see the points and lines, that I also left available. It's possible to activate this debug mode with a checkbox:
Note for the images, I have them as links not embedded as I'm still new contributor (might improve that later).
The edges that shall disappear are shown in red
retraction snapshot1
retraction with edges discard 1
retraction with edges discard 2
Here a link to the function in action, you might have to modify it to have another points format though :
https://github.com/WebSVG/voronoi/blob/8893768e3929ea713a47dba2c4d273b775e0bd82/src/voronoi_diag.js#L278
And here a link to the complete project integrating this function, it has link to a live demo too
https://github.com/WebSVG/voronoi