一応999角形まで表示できますが、これくらいの大きさだと50角形を超える辺りで全部円にしか見えなくなりますね。
三角形
六角形
十二角形
五十角形
<!DOCTYPE html>
<html>
<head>
<script language="javascript">
var radius = 150;
var center_x = 200;
var center_y = 150;
function draw_polygon(){
var vertex = document.forms["vertex"]["quantity"].value;
var a = 2 * Math.PI / vertex;
var arr_x = [];
var arr_y = [];
for(var i= 0; i < vertex; i++){
var rad =(i + 1) * 2 * Math.PI / vertex;
arr_x[i] = center_x + radius * Math.cos(rad);
arr_y[i] = center_y + radius * Math.sin(rad);
}
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.beginPath();
ctx.moveTo(arr_x[0],arr_y[0]);
for(var i= 1; i < vertex; i++){
ctx.lineTo(arr_x[i],arr_y[i]);
}
ctx.lineTo(arr_x[0],arr_y[0]);
ctx.stroke();
}
</script>
</head>
<body>
<form id = "vertex" onsubmit="draw_polygon();return false;">
Specify the Quantity (between 1 and 999): <input type="number" name="quantity" min="1" max="999" placeholder="4">
<input type="submit">
</form>
<canvas id="myCanvas" width="600" height="300" style="border:1px solid #c3c3c3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
</body>
</html>
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