电子绝缘电阻表hi什么意思

書(shū)名： HTML5 Canvas Cookbook
作者名： Eric Rowell
本章字?jǐn)?shù)： 262字
更新時(shí)間： 2021-08-27 12:08:04

Drawing a spiral

Caution, this recipe may induce hypnosis. In this recipe, we'll draw a spiral by connecting a series of short lines to form a spiral path.

How to do it...

Follow these steps to draw a centered spiral:

Define a 2D canvas context and initialize the spiral parameters:

window.onload = function(){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
    
    var radius = 0;
    var angle = 0;

Set the spiral style:

context.lineWidth = 10;
    context.strokeStyle = "#0096FF"; // blue-ish color
    context.beginPath();
    context.moveTo(canvas.width / 2, canvas.height / 2);

Rotate about the center of the canvas three times (50 iterations per full revolution) while increasing the radius by 0.75 for each iteration and draw a line segment to the current point from the previous point with lineTo(). Finally, make the spiral visible with stroke():

for (var n = 0; n < 150; n++) {
        radius += 0.75;
        // make a complete circle every 50 iterations
        angle += (Math.PI * 2) / 50;
        var x = canvas.width / 2 + radius * Math.cos(angle);
        var y = canvas.height / 2 + radius * Math.sin(angle);
        context.lineTo(x, y);
    }
    
    context.stroke();
};

Embed the canvas tag inside the body of the HTML document:

<canvas id="myCanvas" width="600" height="250" style="border:1px solid black;">
</canvas>

How it works...

To draw a spiral with HTML5 canvas, we can place our drawing cursor in the center of the canvas, iteratively increase the radius and angle about the center, and then draw a super short line from the previous point to the current point. Another way to think about it is to imagine yourself as a kid standing on a sidewalk with a piece of colored chalk. Bend down and put the chalk on the sidewalk, and then start turning in a circle (not too fast, though, unless you want to get dizzy and fall over). As you spin around, move the piece of chalk outward away from you. After a few revolutions, you'll have drawn a neat little spiral.

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HTML5 Canvas Cookbook

Drawing a spiral

How to do it...

How it works...