Css - Calculate position of point along edge of regular polygon?

Lance
June 1, 2024
115 views
1 vote
2 Answers

Say I have a pentagon (and we number the sides, like moving around a clock):

Starting from the center of the polygon, how do you compute the position of a point in these spots (along the line of the edge of the polygon):

At the vertex between sides 2 and 3 (this is the max distance from the center).
At the midpoint of 4 (this is the min distance from the center).
At a point 2/3 across side 3, moving clockwise (randomly chosen distance from the center).

Knowing how to compute the x/y coordinates relative to the center will mean I can plot points along the straight line-segments of an arbitrary polygon (ranging from say 3 to 20 sides). I am having a really time conceptualizing how to do this, let alone getting it to work in code. Doesn’t matter what language it is in, but preferably JavaScript/TypeScript, or else Python or C (or self-explanatory pseudocode).

Here is a combination of what I conjured up. The polygon layout works properly, but the point positioning isn’t working. How do you layout these 3 points?

const ANGLE = -Math.PI / 2 // Start the first vertex at the top center

function computePolygonPoints({
  width,
  height,
  sides,
  strokeWidth = 0,
  rotation = 0,
}) {
  const centerX = width / 2 + strokeWidth / 2
  const centerY = height / 2 + strokeWidth / 2
  const radiusX = width / 2 - strokeWidth / 2
  const radiusY = height / 2 - strokeWidth / 2
  const offsetX = strokeWidth / 2
  const offsetY = strokeWidth / 2

  const rotationRad = (rotation * Math.PI) / 180

  const points = Array.from({ length: sides }, (_, i) => {
    const angle = (i * 2 * Math.PI) / sides + ANGLE
    const x = centerX + radiusX * Math.cos(angle)
    const y = centerY + radiusY * Math.sin(angle)

    // Apply rotation around the center
    const rotatedX =
      centerX +
      (x - centerX) * Math.cos(rotationRad) -
      (y - centerY) * Math.sin(rotationRad)
    const rotatedY =
      centerY +
      (x - centerX) * Math.sin(rotationRad) +
      (y - centerY) * Math.cos(rotationRad)

    return { x: rotatedX, y: rotatedY }
  })

  const minX = Math.min(...points.map(p => p.x))
  const minY = Math.min(...points.map(p => p.y))

  const adjustedPoints = points.map(p => ({
    x: offsetX + p.x - minX,
    y: offsetY + p.y - minY,
  }))

  return adjustedPoints
}

function vertexCoordinates(n, R, vertexIndex) {
  const angle = 2 * Math.PI * vertexIndex / n - Math.PI / 2; // Adjusting to start from the top
  return {
    x: R * Math.cos(angle),
    y: R * Math.sin(angle),
  }
}

function midpointCoordinates(x1, y1, x2) {
  return {
    x: (x1 + x2.x) / 2,
    y: (y1 + x2.y) / 2,
  }
}

function fractionalPoint(x1, y1, x2, fraction) {
  return {
    x: x1 + fraction * (x2.x - x1),
    y: y1 + fraction * (x2.y - y1),
  }
}

const pentagonPoints = computePolygonPoints({ width: 300, height: 300, sides: 5 })

const svg = document.createElementNS("http://www.w3.org/2000/svg", "svg");
svg.setAttribute("width", 300)
svg.setAttribute("height", 300);

const pentagon = document.createElementNS("http://www.w3.org/2000/svg", "polygon");
pentagon.setAttribute('fill', 'cyan')
pentagon.setAttribute('points', pentagonPoints
  .map((p) => `${p.x},${p.y}`)
  .join(" "))

svg.appendChild(pentagon)
document.body.appendChild(svg);

const n = 5 // Number of sides for a pentagon
const width = 300; // Width of the pentagon
const R = width / (2 * Math.cos(Math.PI / n)); // Radius of the circumscribed circle

const centerX = 150; // Center of the canvas
const centerY = 150;

// Vertex between sides 2 and 3
const vertex23 = vertexCoordinates(n, R, 2)
const vertex23Adjusted = {
    x: centerX + vertex23.x, // subtract radius too?
    y: centerY + vertex23.y
};
console.log('Vertex between sides 2 and 3:', vertex23Adjusted)

const circle23 = document.createElementNS("http://www.w3.org/2000/svg", "circle");
circle23.setAttribute('fill', 'magenta')
circle23.setAttribute('r', 16)
circle23.setAttribute('cx', vertex23Adjusted.x)
circle23.setAttribute('cy', vertex23Adjusted.y)
svg.appendChild(circle23)

// Midpoint of side 4
const vertex4_1 = vertexCoordinates(n, R, 3)
const vertex4_2 = vertexCoordinates(n, R, 4)
const mid4 = midpointCoordinates(vertex4_1.x, vertex4_1.y, vertex4_2)
console.log('Midpoint of side 4:', mid4)

const mid4Circle = document.createElementNS("http://www.w3.org/2000/svg", "circle");
mid4Circle.setAttribute('fill', 'magenta')
mid4Circle.setAttribute('r', 16)
mid4Circle.setAttribute('cx', mid4.x)
mid4Circle.setAttribute('cy', mid4.y)
svg.appendChild(mid4Circle)

// Point 2/3 across side 3, moving clockwise
const vertex3_1 = vertexCoordinates(n, R, 2)
const vertex3_2 = vertexCoordinates(n, R, 3)
const frac3 = fractionalPoint(
  vertex3_1.x,
  vertex3_1.y,
  vertex3_2,
  2 / 3,
)
console.log('Point 2/3 across side 3:', frac3)

const frac3Circle = document.createElementNS("http://www.w3.org/2000/svg", "circle");
frac3Circle.setAttribute('fill', 'magenta')
frac3Circle.setAttribute('r', 16)
frac3Circle.setAttribute('cx', frac3.x)
frac3Circle.setAttribute('cy', frac3.y)
svg.appendChild(frac3Circle)

I’d like to be able to solve this for any polygon from 3 to 20 sides, not just for the pentagon.

Answers

Chosen as BEST ANSWER

Here is the answer, I finally figured it out.

export function calculatePolygonDotPosition({
  polygonRadius,
  polygonSideCount,
  polygonEdgeNumber,
  polygonEdgePositionRatio, // from 0 to 1
  gap = 0,
  dotRadius,
  rotation = 0,
  offset = 0,
}: {
  polygonRadius: number
  polygonSideCount: number
  polygonEdgeNumber: number
  polygonEdgePositionRatio: number
  gap?: number
  dotRadius: number
  rotation?: number
  offset?: number
}) {
  const n = polygonSideCount
  const R = polygonRadius
  const e = polygonEdgeNumber
  const t = polygonEdgePositionRatio
  const o = gap

  const rotationAngle = (rotation * Math.PI) / 180

  const V1 = rotatePoint(getPolygonVertex(e - 1, n, R), rotationAngle)
  const V2 = rotatePoint(getPolygonVertex(e % n, n, R), rotationAngle) // Wrap around using modulo

  // Interpolate position along the edge
  const P = {
    x: (1 - t) * V1.x + t * V2.x,
    y: (1 - t) * V1.y + t * V2.y,
  }

  // Calculate the edge vector and the normal vector
  const dx = V2.x - V1.x
  const dy = V2.y - V1.y
  const edgeLength = Math.sqrt(dx * dx + dy * dy)

  // Unit normal vector (rotate by 90 degrees counter-clockwise)
  const normal = {
    x: -dy / edgeLength,
    y: dx / edgeLength,
  }

  // Offset the point by the gap distance
  const P_offset = {
    x: P.x + (-o - dotRadius + offset) * normal.x,
    y: P.y + (-o - dotRadius + offset) * normal.y,
  }

  return {
    x: P_offset.x + R,
    y: -P_offset.y + R,
  }
}

// Calculate vertex positions
export function getPolygonVertex(i: number, n: number, R: number) {
  const angle = (2 * Math.PI * i) / n + Math.PI / 2
  return { x: R * Math.cos(angle), y: R * Math.sin(angle) }
}

function rotatePoint(
  { x, y }: { x: number; y: number },
  angle: number,
) {
  const cos = Math.cos(angle)
  const sin = Math.sin(angle)
  return {
    x: x * cos - y * sin,
    y: x * sin + y * cos,
  }
}

Usage:

calculatePolygonDotPosition({
  polygonRadius: 300,
  polygonSideCount: 5,
  polygonEdgeNumber: 2,
  polygonEdgePositionRatio: 0.5,
  gap: 4,
  dotRadius: 3,
  offset: 4, // strokeWidth
})

Here are 4 points laid out just outside the outer edge.

(Edit)

The problem with your trigonometrical computation is that the circumscribed circle is too big. You can see it if you add

<circle cx="«centerX»" cy="«centerY»" r="«R»"/>

to your diagram:

But you need not do the computation yourself, you can let SVG do it for you with the path.getPointAtLength(length) function. In your cases,

the vertex between sides 2 and 3 is at length=2
the midpoint of 4 is at length=3.5
a point 2/3 across side 3, moving clockwise, is at length=2.67.

var path = document.querySelector("polygon");
var circle = document.querySelector("circle");
var control = document.querySelector("input[type=range]");

function init(n) {
  control.setAttribute("max", n);
  var d = [];
  for (var i = 0; i < n; i++) {
    var alpha = 2 * i * Math.PI / n;
    d.push(`${Math.sin(alpha)},${-Math.cos(alpha)}`);
  }
  path.setAttribute("points", d.join(" "));
  control.value = 0;
  move();
}

function move() {
  var p = path.getPointAtLength(path.getTotalLength() * control.value / control.getAttribute("max"));
  circle.setAttribute("cx", p.x);
  circle.setAttribute("cy", p.y);
  control.nextElementSibling.textContent = control.value;
}

init(5);

polygon {
  fill: yellow;
  stroke: black;
  stroke-width: 0.01;
}

circle {
  fill: red;
}

<input type="number" value="5" onchange="init(this.value)" />
<svg viewBox="-1 -1 2 2">
    <polygon/>
    <circle r="0.1"/>
</svg>
<input type="range" step="0.01" onchange="move()" /><span></span>

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Css – Calculate position of point along edge of regular polygon?

Answers