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Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

HTML

            
              <p>Click on the grid to draw a line</p>
<button onClick="clearLines()">Clear lines</button>
<button onClick="init()">Restart</button>
<label>Draw dots: <input class="draw-dots" type="checkbox" /></label>
<label>Draw lines: <input class="draw-lines" type="checkbox" checked /></label>
            
          
!

CSS

            
              body {
  padding: 20px;
  font-family: Helvetica, Arial, sans-serif;
  color: #333;
}

p {
  margin-bottom: 10px;
}

svg {
  overflow: visible;
  margin: 50px auto 20px;
  max-width: 800px;
  display: block;
  position: relative;
}

marker {
  overflow: visible;
}

.coordinate-line {
  stroke: #ddd;
}

marker,
.vector {
  stroke: #abc;
  fill: #abc;
}
            
          
!

JS

            
              const gridStep = 20;
const gridSize = 15;
const maxVelocity = gridStep;
const size = gridSize * gridStep;
const drawDotsInput = document.querySelector('.draw-dots');
const drawDotsLines = document.querySelector('.draw-lines');


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

svg.setAttribute("viewBox", `0 0 ${size} ${size}`);
document.querySelector("body").appendChild(svg);

const vectors = [];

const directionVector = {
  x: maxVelocity * 0.5,
  y: 0
};

function getRandomVector() {
  const xSign = Math.random() > 0.5 ? 1 : -1;
  const ySign = Math.random() > 0.5 ? 1 : -1;

  return {
    x: Math.random() * xSign * maxVelocity + directionVector.x,
    y: Math.random() * ySign * maxVelocity + directionVector.y
  };
}

function roundToDecimal(number, decimalPlaces = 2) {
  return parseFloat(number.toFixed(decimalPlaces));
}

function drawDot(dot, color = "#333") {
  if (drawDotsInput.checked) {
    svg.innerHTML += `
      <circle
        class="generated-dot"
        fill="${color}" 
        cx="${dot.x}" 
        cy="${dot.y}" 
        r="${size / 200}"
      />
    `;
  }
}

function addVectors(v1, v2) {
  return {
    x: v1.x + v2.x,
    y: v1.y + v2.y
  };
}

function multiplyVector(v, factor) {
  return {
    x: v.x * factor,
    y: v.y * factor
  };
}

function getDistance(v1, v2) {
  const x = v1.x - v2.x;
  const y = v1.y - v2.y;

  return Math.sqrt(x * x + y * y) / gridStep;
}

function drawNextDot(dot) {
  const nearVectors = [];

  const searchRadiusFactor = 1.5;

  for (let x = 0; x <= gridSize; x++) {
    const xCoord = x * gridStep;

    for (let y = 0; y <= gridSize; y++) {
      const yCoord = y * gridStep;
      const start = { x: xCoord, y: yCoord };

      if (isDotInCircle(start, dot, gridStep * searchRadiusFactor)) {
        nearVectors.push({
          start,
          vector: vectors[x][y]
        });
      }
    }
  }

  if (nearVectors.length === 0) {
    return;
  }

  let nextDot = { ...dot };

  nearVectors.forEach(vector => {
    const distance = getDistance(dot, vector.start);

    nextDot = addVectors(
      nextDot,
      multiplyVector(vector.vector, searchRadiusFactor - distance)
    );
  });

  drawDot(nextDot);
  line.push([nextDot.x, nextDot.y]);
  dotsDrawn++;

  if (
    nextDot.x > size * 1.1 ||
    nextDot.y > size * 1.1 ||
    nextDot.x < size * -0.1 ||
    nextDot.y < size * -0.1
  ) {
    return;
  }

  if (dotsDrawn < maxDots) {
    drawNextDot(nextDot);
  }
}

function isDotInCircle(dot, circleCenter, radius) {
  const x = dot.x - circleCenter.x;
  const y = dot.y - circleCenter.y;

  return x * x + y * y < radius * radius;
}

function getRandomColor() {
  const goldenRatio = 0.618033988749895 * 360;
  let h = Math.random() * 360;
  
  h += goldenRatio;
  h %= 360;
  return `hsl(${ parseInt(h, 10) }, 70%, 60%)`;
}

const maxDots = 100;
let dotsDrawn = 0;
let line = [];

svg.addEventListener("click", e => {
  const rect = svg.getBoundingClientRect();
  const x = e.pageX - (rect.left + window.pageXOffset);
  const y = e.pageY - (rect.top + window.pageYOffset);
  const factor = rect.width / size;

  dotsDrawn = 0;
  line = [];

  const start = {
    x: x / factor,
    y: y / factor
  };

  line.push([start.x, start.y]);

  drawDot(start);

  drawNextDot(start);

  svg.innerHTML += svgPath(line, bezierCommand);
  /*`
    <path 
      class="generated-line"
      fill="none"
      stroke="${ getRandomColor() }" 
      d="${line.join(" ")}" 
      stroke-width="${(gridStep / 30).toFixed(2)}" 
    />`;
  
  */
});

function clearLines() {
  document.querySelectorAll(".generated-line, .generated-dot").forEach(line => {
    svg.removeChild(line);
  });
}

function init() {
  svg.innerHTML = `
    <marker 
      id="arrow" 
      viewBox="0 0 10 10" 
      refX="5" 
      refY="5"
      markerWidth="${gridStep / 5}" 
      markerHeight="${gridStep / 5}"
      orient="auto-start-reverse"
      stroke-linecap="round"
    >
      <path d="M 0 0 L 10 5 L 0 10 Z" />
    </marker>
  `;

  for (let x = 0; x <= gridSize; x++) {
    vectors[x] = [];

    const coord = x * gridStep;

    svg.innerHTML += `
      <path
        class="coordinate-line"
        d="M ${0} ${coord} L ${size} ${coord}" 
        stroke-width="${gridStep / 100}"
      />
      <path
        class="coordinate-line"
        d="M ${coord} ${0} L ${coord} ${size}" 
        stroke-width="${gridStep / 100}"
      />
    `;

    for (let y = 0; y <= gridSize; y++) {
      vectors[x][y] = getRandomVector();
    }
  }

  vectors.forEach((row, x) => {
    row.forEach((vector, y) => {
      const xCoord = x * gridStep;
      const yCoord = y * gridStep;
      svg.innerHTML += `
        <path 
          class="vector"
          d="M ${xCoord} ${yCoord} L ${(xCoord + vector.x).toFixed(2)} ${ (yCoord + vector.y).toFixed(2) }" 
          marker-end="url(#arrow)"
          stroke-width="${(gridStep / 30).toFixed(2)}"
          stroke-linecap="round"
        />
      `;
    });
  });
}

init();

// The smoothing ratio
const smoothing = 0.15;

// Properties of a line 
// I:  - pointA (array) [x,y]: coordinates
//     - pointB (array) [x,y]: coordinates
// O:  - (object) { length: l, angle: a }: properties of the line
const lineFn = (pointA, pointB) => {
  const lengthX = pointB[0] - pointA[0]
  const lengthY = pointB[1] - pointA[1]
  return {
    length: Math.sqrt(Math.pow(lengthX, 2) + Math.pow(lengthY, 2)),
    angle: Math.atan2(lengthY, lengthX)
  }
}

// Position of a control point 
// I:  - current (array) [x, y]: current point coordinates
//     - previous (array) [x, y]: previous point coordinates
//     - next (array) [x, y]: next point coordinates
//     - reverse (boolean, optional): sets the direction
// O:  - (array) [x,y]: a tuple of coordinates
const controlPoint = (current, previous, next, reverse) => {

  // When 'current' is the first or last point of the array
  // 'previous' or 'next' don't exist.
  // Replace with 'current'
  const p = previous || current
  const n = next || current

  // Properties of the opposed-line
  const o = lineFn(p, n)

  // If is end-control-point, add PI to the angle to go backward
  const angle = o.angle + (reverse ? Math.PI : 0)
  const length = o.length * smoothing

  // The control point position is relative to the current point
  const x = current[0] + Math.cos(angle) * length
  const y = current[1] + Math.sin(angle) * length
  return [x, y]
}

// Create the bezier curve command 
// I:  - point (array) [x,y]: current point coordinates
//     - i (integer): index of 'point' in the array 'a'
//     - a (array): complete array of points coordinates
// O:  - (string) 'C x2,y2 x1,y1 x,y': SVG cubic bezier C command
const bezierCommand = (point, i, a) => {

  // start control point
  const cps = controlPoint(a[i - 1], a[i - 2], point)

  // end control point
  const cpe = controlPoint(point, a[i - 1], a[i + 1], true)
  return `C ${cps[0]},${cps[1]} ${cpe[0]},${cpe[1]} ${point[0]},${point[1]}`
}

// Render the svg <path> element 
// I:  - points (array): points coordinates
//     - command (function)
//       I:  - point (array) [x,y]: current point coordinates
//           - i (integer): index of 'point' in the array 'a'
//           - a (array): complete array of points coordinates
//       O:  - (string) a svg path command
// O:  - (string): a Svg <path> element
const svgPath = (points, command) => {
  if (drawDotsLines.checked) {
    // build the d attributes by looping over the points
    const d = points.reduce((acc, point, i, a) => i === 0
      ? `M ${point[0]},${point[1]}`
      : `${acc} ${command(point, i, a)}`
    , '')
    return `<path class="generated-line" d="${d}" fill="none" stroke="${ getRandomColor() }" />`
  }
}

/*
for (let x = 0; x < gridSize; x++) { 
  const coord = x * gridStep;
      
  svg.innerHTML += `
    <path
      class="coordinate-line"
      d="M ${ 0 } ${ coord } L ${ coord } ${ size }" 
      stroke-width="${ gridStep / 50 }"
    />
    <path
      class="coordinate-line"
      d="M ${ coord } ${ 0 } L ${ size } ${ coord }" 
      stroke-width="${ gridStep / 50 }"
    />
  `;
}
*/

            
          
!
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