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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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About the <head>

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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Packages

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Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

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HTML 

              
                <body>
<form>  
  <input type="checkbox" onchange = "deletePointCheck()">Delete point<br/>
  <input type="button" value="clear" onclick="deletePoints()"/>
</form>

<canvas id="drawing" width="600" height="600">Canvas not supported.</canvas>
</body>
!

CSS 

              
                #drawing { 
  border: 1px solid #000;
  background: #fff;  
  display: block;
}
!

JS 

              
                var obj = document.getElementById('drawing');
var ctx = obj.getContext('2d');
var Idint;
var points = [];
var legendPoints = [];
var pointsDel = false;

addEventListener("mousedown", mDown);
addEventListener("mouseup", mUp);
addEventListener("mousemove", mMove);


function deletePoints(){
  points = new Array();
  legendPoints = new Array();
}

function deletePoint(ind){
  points.splice(ind, 1);
  for(let i = ind; i < points.length; i++){
    points[i].legend = "P" + String(i);    
  }
}

function deletePointCheck(){
  pointsDel = !pointsDel;
}

function addPoint(x, y){
  if(x >=0 && x <= obj.width && y >=0 && y<= obj.height) points.push(new Point(x, y, "P" + String(points.length)));
}

var mouseDownOnPoint = false;
var mouseDown = false;
var dragPoint = 0;
function mDown(event){
  mouseDown = true;
  let rect = obj.getBoundingClientRect();
  let x = event.clientX - rect.left;
  let y = event.clientY - rect.top;
  for(let i = 0; i < points.length; i++){
    if(x >= points[i].x - points[i].r && x <= points[i].x + points[i].r && y >= points[i].y - points[i].r && y <= points[i].y + points[i].r){
      points[i].x = x; points[i].y = y; dragPoint = i;
      if(pointsDel) { deletePoint(i); return; }
      mouseDownOnPoint = true;
    }
  }
  if(!mouseDownOnPoint && !pointsDel) addPoint(x, y);
}
function mUp(event){
  mouseDown = false;
  mouseDownOnPoint = false;
}

function mMove(event){
  if(mouseDown){
    if(pointsDel) return;
    if(mouseDownOnPoint){
      let rect = obj.getBoundingClientRect();
      let x = event.clientX - rect.left;
      let y = event.clientY - rect.top;
      let ind = dragPoint;
      if(x < points[ind].r) x = points[ind].r;
      if(x > obj.width - points[ind].r) x = obj.width - points[ind].r;
      if(y < points[ind].r) y = points[ind].r;
      if(y > obj.height - points[ind].r) y = obj.height - points[ind].r;
      points[ind].x = x; points[ind].y = y;
    }
  }
}

function sin(deg){
  return Math.sin(deg * Math.PI / 180);
}
function cos(deg){
  return Math.cos(deg * Math.PI / 180);
}

function circle(x, y, r, color = "white"){
  let oldColor = ctx.fillStyle;
  ctx.beginPath();
  ctx.moveTo(x, y + r);
  for(let t = 0; t <= 360; t += 1){
    ctx.lineTo(r * sin(t) + x, r * cos(t) + y);
  }
  ctx.fillStyle = color; ctx.fill();
  ctx.stroke();
  ctx.fillStyle = oldColor;
}

class Point{
  x = 0; y = 0; legend;
  r = 5; color; fontSize;
  
  constructor(x = 0, y = 0, legend = "", color = "black", r = 5, fontSize = 14){
    this.x = x; this.y = y; this.r = r;
    this.legend = legend; this.fontSize = fontSize
    this.color = color;
  }
  
  copyFrom(point){
    this.x = point.x; this.y = point.y;
    this.r = point.r; this.color = point.color;
    this.legend = point.legend; this.fontSize = point.fontSize;
  }
  
  draw(fillColor = "white"){
    ctx.beginPath();
    ctx.srokeStyle = "white";
    circle(this.x, this.y, this.r, "white")
    ctx.strokeStyle = this.color;
    circle(this.x, this.y, this.r, fillColor)
    ctx.fillStyle = this.color;
    ctx.font = String(this.fontSize) + 'px sherif';
    ctx.fillText(this.legend, this.x - this.r - 2, this.y - this.r - 2);
  }
}

function betaSpline(points, color = "black"){
  let oldStrokeStyle = ctx.strokeStyle;
  ctx.beginPath();
  legendPoints = [];
  for(let i = 1; i < points.length - 2; i++){
    let a3 = (-points[i-1].x + 3*points[i].x - 3*points[i+1].x + points[i+2].x) / 6,
        a2 = (points[i-1].x - 2*points[i].x + points[i+1].x) / 2,
        a1 = (-points[i-1].x + points[i+1].x) / 2,
        a0 = (points[i-1].x + 4*points[i].x + points[i+1].x) / 6;
    let b3 = (-points[i-1].y + 3*points[i].y - 3*points[i+1].y + points[i+2].y) / 6,
        b2 = (points[i-1].y - 2*points[i].y + points[i+1].y) / 2,
        b1 = (-points[i-1].y + points[i+1].y) / 2,
        b0 = (points[i-1].y + 4*points[i].y + points[i+1].y) / 6;    
 
    ctx.fillStyle = "blue";
    ctx.font = '12px sherif';    
    ctx.fillText("Q" + String(i+2), a0 + 0.5*(a1 + 0.5*(a2 + 0.5*a3)) - 2, b0 + 0.5*(b1 + 0.5*(b2 + 0.5*b3)) - 2);
    legendPoints.push(new Point(a0, b0, "t" + String(i+2), "black", 3, 12));
    if(i + 3 == points.length) legendPoints.push(new Point(a0 + a1 + a2 + a3, b0 + b1 + b2 + b3, "t" + String(i+3), "black", 3, 12));
    
    for(let t = 0; t <= 1; t += 0.005){
      let x = ((a3 * t + a2) * t + a1) * t + a0;
      let y = ((b3 * t + b2) * t + b1) * t + b0;
      ctx.lineTo(x, y);
    }
  }
  ctx.strokeStyle = color;
  ctx.stroke();
  ctx.strokeStyle = oldStrokeStyle;
}

function clear(){
  ctx.fillStyle = "white";
  ctx.fillRect(0,0,ctx.canvas.width, ctx.canvas.height);
}

function init(){
  clear();
  betaSpline(points, "red");
  for(let i = 0; i < points.length; i++) points[i].draw();
  for(let i = 0; i < legendPoints.length; i++) legendPoints[i].draw("black");
  Idint = requestAnimationFrame(init);
}

function main(){
  deletePoints();
  addPoint(80, 240);
  addPoint(80, 100);
  addPoint(230, 115);
  addPoint(225, 260);
  addPoint(385, 240);
  addPoint(450, 100);
  addPoint(555, 111);
  addPoint(570, 240);
  init();
}

main();
!
999px

Console