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【JS】js学习笔记之canvas基本绘图

2017-12-14 14:38 477 查看

准备工作：

①首先得在html加一个canvas标签，如：<canvas id="preview_canvas" width="600px" height="370px">

②在js中得到画布canvas：

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

1.绘制三角形：

ctx.beginPath();
ctx.moveTo(135,135);
ctx.lineTo(150,150);
ctx.lineTo(135,165);
ctx.fill();

2.添加颜色:

ctx.strokeStyle = "orange"; //设置图形的填充颜色。
ctx.fillStyle = "orange";//设置图形轮廓的颜色。

3.动画小例子：来源：https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>

<body>
<canvas id="canvas" width="500px" height="500px"></canvas>
<canvas id="canvas1" width="500px" height="500px"></canvas>
</body>
<script>
var sun = new Image();
var moon = new Image();
var earth = new Image();
function init(){
sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
window.requestAnimationFrame(draw);
}

function draw() {
var ctx = document.getElementById('canvas').getContext('2d');

ctx.globalCompositeOperation = 'destination-over';
ctx.clearRect(0,0,300,300); // clear canvas

ctx.fillStyle = 'rgba(0,0,0,0.4)';
ctx.strokeStyle = 'rgba(0,153,255,0.4)';
ctx.save();
ctx.translate(150,150);

// Earth
var time = new Date();
ctx.rotate( ((2*Math.PI)/60)*time.getSeconds() + ((2*Math.PI)/60000)*time.getMilliseconds() );
ctx.translate(105,0);
ctx.fillRect(0,-12,50,24); // Shadow
ctx.drawImage(earth,-12,-12);

// Moon
ctx.save();
ctx.rotate( ((2*Math.PI)/6)*time.getSeconds() + ((2*Math.PI)/6000)*time.getMilliseconds() );
ctx.translate(0,28.5);
ctx.drawImage(moon,-3.5,-3.5);
ctx.restore();

ctx.restore();

ctx.beginPath();
ctx.arc(150,150,105,0,Math.PI*2,false); // Earth orbit
ctx.stroke();

ctx.drawImage(sun,0,0,300,300);

window.requestAnimationFrame(draw);
}

init();

function clock(){
var now = new Date();
var ctx = document.getElementById('canvas1').getContext('2d');
ctx.save();
ctx.clearRect(0,0,150,150);
ctx.translate(75,75);
ctx.scale(0.4,0.4);
ctx.rotate(-Math.PI/2);
ctx.strokeStyle = "black";
ctx.fillStyle = "white";
ctx.lineWidth = 8;
ctx.lineCap = "round";

// Hour marks
ctx.save();
for (var i=0;i<12;i++){
ctx.beginPath();
ctx.rotate(Math.PI/6);
ctx.moveTo(100,0);
ctx.lineTo(120,0);
ctx.stroke();
}
ctx.restore();

// Minute marks
ctx.save();
ctx.lineWidth = 5;
for (i=0;i<60;i++){
if (i%5!=0) {
ctx.beginPath();
ctx.moveTo(117,0);
ctx.lineTo(120,0);
ctx.stroke();
}
ctx.rotate(Math.PI/30);
}
ctx.restore();

var sec = now.getSeconds();
var min = now.getMinutes();
var hr = now.getHours();
hr = hr>=12 ? hr-12 : hr;

ctx.fillStyle = "black";

// write Hours
ctx.save();
ctx.rotate( hr*(Math.PI/6) + (Math.PI/360)*min + (Math.PI/21600)*sec )
ctx.lineWidth = 14;
ctx.beginPath();
ctx.moveTo(-20,0);
ctx.lineTo(80,0);
ctx.stroke();
ctx.restore();

// write Minutes
ctx.save();
ctx.rotate( (Math.PI/30)*min + (Math.PI/1800)*sec )
ctx.lineWidth = 10;
ctx.beginPath();
ctx.moveTo(-28,0);
ctx.lineTo(112,0);
ctx.stroke();
ctx.restore();

// Write seconds
ctx.save();
ctx.rotate(sec * Math.PI/30);
ctx.strokeStyle = "#D40000";
ctx.fillStyle = "#D40000";
ctx.lineWidth = 6;
ctx.beginPath();
ctx.moveTo(-30,0);
ctx.lineTo(83,0);
ctx.stroke();
ctx.beginPath();
ctx.arc(0,0,10,0,Math.PI*2,true);
ctx.fill();
ctx.beginPath();
ctx.arc(95,0,10,0,Math.PI*2,true);
ctx.stroke();
ctx.fillStyle = "rgba(0,0,0,0)";
ctx.arc(0,0,3,0,Math.PI*2,true);
ctx.fill();
ctx.restore();

ctx.beginPath();
ctx.lineWidth = 14;
ctx.strokeStyle = '#325FA2';
ctx.arc(0,0,142,0,Math.PI*2,true);
ctx.stroke();

ctx.restore();

window.requestAnimationFrame(clock);
}

window.requestAnimationFrame(clock);
</script>
</html>效果：

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