How to draw a star by using canvas HTML5?

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Am trying to draw a star using canvas, but the code is not running. I want to understand: what are the steps to measure the Y and X coordinate? How to find them? to draw any

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别跟我提以往

2021-01-05 09:06

Those functions, lineTo(), moveTo(), stroke(), etc... belong to the context object, not the canvas object. Are you checking your developer console (f12 in Chrome)? You would see that these functions are undefined.

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耶瑟儿～

2021-01-05 09:07

A star is basically a regular polygon with alternating points on an inner and an outer radius.

Here's an example of a flexible function to draw a star shape.

You can set the position, #spikes and the inner & outer radius of the spikes:

enter image description here

Example code and a Demo: http://jsfiddle.net/m1erickson/8j6kdf4o/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
    body{ background-color: ivory; }
    canvas{border:1px solid red;}
</style>
<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    function drawStar(cx,cy,spikes,outerRadius,innerRadius){
      var rot=Math.PI/2*3;
      var x=cx;
      var y=cy;
      var step=Math.PI/spikes;

      ctx.beginPath();
      ctx.moveTo(cx,cy-outerRadius)
      for(i=0;i<spikes;i++){
        x=cx+Math.cos(rot)*outerRadius;
        y=cy+Math.sin(rot)*outerRadius;
        ctx.lineTo(x,y)
        rot+=step

        x=cx+Math.cos(rot)*innerRadius;
        y=cy+Math.sin(rot)*innerRadius;
        ctx.lineTo(x,y)
        rot+=step
      }
      ctx.lineTo(cx,cy-outerRadius);
      ctx.closePath();
      ctx.lineWidth=5;
      ctx.strokeStyle='blue';
      ctx.stroke();
      ctx.fillStyle='skyblue';
      ctx.fill();
    }

    drawStar(100,100,5,30,15);

}); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

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既然无缘

2021-01-05 09:10

The function can be shorter with coordinate translate:

function strokeStar(x, y, r, n, inset) {
    ctx.save();
    ctx.beginPath();
    ctx.translate(x, y);
    ctx.moveTo(0,0-r);
    for (var i = 0; i < n; i++) {
        ctx.rotate(Math.PI / n);
        ctx.lineTo(0, 0 - (r*inset));
        ctx.rotate(Math.PI / n);
        ctx.lineTo(0, 0 - r);
    }
    ctx.closePath();
    ctx.fill();
    ctx.restore();
}

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不思量自难忘°

2021-01-05 09:14

//function to draw star with N spikes
//centered on a circle of radius R, centered on (cX,cY)
function star(R, cX, cY, N) {
  //star draw
  ctx.beginPath();
  ctx.moveTo(cX + R,cY);
  for(var i = 1; i <= N * 2; i++)
  {
    if(i % 2 == 0){
      var theta = i * (Math.PI * 2) / (N * 2);
      var x = cX + (R * Math.cos(theta));
      var y = cY + (R * Math.sin(theta));
    } else {
      var theta = i * (Math.PI * 2) / (N * 2);
      var x = cX + ((R/2) * Math.cos(theta));
      var y = cY + ((R/2) * Math.sin(theta));
    }

    ctx.lineTo(x ,y);
  }
  ctx.closePath();
  ctx.stroke();
}

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长发绾君心

2021-01-05 09:24

As mentioned by @v-rubinetti, you are trying to call methods incorrectly.

While it can be easy to draw a simple star in canvas by coding, it's complicated for drawing advanced things. You can use a open-source Vector graphics software like Inkscape along with the ink2canvas extension to draw advanced vector graphics and save them to a html5 document.

For example, here is a 25-pointed Star drawn in Inkscape and saved using ink2canvas extension:

<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Inkscape Output</title>
</head>
<body>
    <canvas id='canvas' width='320' height='320'></canvas>
    <script>
    var ctx = document.getElementById("canvas").getContext("2d");
    ctx.save();
    ctx.lineJoin = 'miter';
    ctx.strokeStyle = 'rgb(0, 0, 0)';
    ctx.lineCap = 'butt';
    ctx.lineWidth = 1.000000;
    ctx.fillStyle = 'rgb(255, 255, 0)';
    ctx.beginPath();
    ctx.transform(0.506236, 0.000000, 0.000000, 0.505711, 82.274469, 51.410524);
    ctx.moveTo(342.857130, 449.505040);
    ctx.lineTo(234.764940, 344.516400);
    ctx.lineTo(279.468630, 488.419550);
    ctx.lineTo(200.881970, 359.847880);
    ctx.lineTo(208.393950, 510.347410);
    ctx.lineTo(164.250710, 366.271350);
    ctx.lineTo(134.098980, 513.910810);
    ctx.lineTo(127.172840, 363.383190);
    ctx.lineTo(61.251941, 498.885850);
    ctx.lineTo(91.978093, 351.364870);
    ctx.lineTo(-5.569921, 466.216610);
    ctx.lineTo(60.877887, 330.971560);
    ctx.lineTo(-62.167941, 417.955810);
    ctx.lineTo(35.826363, 303.484630);
    ctx.lineTo(-104.985860, 357.135850);
    ctx.lineTo(18.397601, 270.631190);
    ctx.lineTo(-131.333260, 287.578290);
    ctx.lineTo(9.686712, 234.475540);
    ctx.lineTo(-139.554650, 213.653670);
    ctx.lineTo(10.241036, 197.289490);
    ctx.lineTo(-129.133450, 140.006950);
    ctx.lineTo(20.025741, 161.409550);
    ctx.lineTo(-100.724450, 71.265628);
    ctx.lineTo(38.426018, 129.090210);
    ctx.lineTo(-56.112700, 11.748970);
    ctx.lineTo(64.285711, 102.362200);
    ctx.lineTo(1.898679, -34.803371);
    ctx.lineTo(95.979959, 82.904945);
    ctx.lineTo(69.664621, -65.466342);
    ctx.lineTo(131.517300, 71.941014);
    ctx.lineTo(142.927140, -78.313274);
    ctx.lineTo(168.664780, 70.159311);
    ctx.lineTo(217.082890, -72.536949);
    ctx.lineTo(205.088300, 77.671789);
    ctx.lineTo(287.472380, -48.500313);
    ctx.lineTo(238.499240, 94.006409);
    ctx.lineTo(349.672790, -7.713676);
    ctx.lineTo(266.798250, 118.136810);
    ctx.lineTo(399.775840, 47.260185);
    ctx.lineTo(288.207210, 148.546780);
    ctx.lineTo(434.633360, 112.967060);
    ctx.lineTo(301.380910, 183.325570);
    ctx.lineTo(452.055130, 185.278350);
    ctx.lineTo(305.491610, 220.287880);
    ctx.lineTo(450.946490, 259.650470);
    ctx.lineTo(300.281000, 257.111240);
    ctx.lineTo(431.377070, 331.410340);
    ctx.lineTo(286.076510, 291.481900);
    ctx.lineTo(394.576520, 396.049020);
    ctx.lineTo(263.770630, 321.240230);
    ctx.closePath();
    ctx.fill();
    ctx.stroke();
    ctx.restore();
    </script>
</body>
</html>

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