html5畫布canvas

zxc123e發表於2016-01-06
原文網址 : https://blog.csdn.net/zxc123e/article/details/50470944
HTMLCanvas

canvas 元素用於在網頁上繪製圖形。
建立 Canvas 元素

<canvas id="myCanvas" width="200" height="100"></canvas>

通過 JavaScript 來繪製

<script type="text/javascript">
    var c=document.getElementById("myCanvas");
    var cxt=c.getContext("2d");
    cxt.fillStyle="#FF0000";
    cxt.fillRect(0,0,150,75);
</script>

getContext(“2d”) 物件是內建的 HTML5 物件，擁有多種繪製路徑、矩形、圓形、字元以及新增影象的方法。

canvas變換
translate（偏移）：以起始點為基準點，移動當前座標位置
rotate（旋轉）：引數為弧度
scale（縮放）

例項（漫天飛雪）

<!doctype html>
<html lang="en">
 <head>
  <meta charset="UTF-8">
  <meta name="Generator" content="EditPlus®">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">
  <title>Document</title>
  <style type="text/css">
* {margin: 0; padding: 0;}</p> <p>body {
/*You can use any kind of background here.*/
background: #6b92b9;
}
canvas {
    display: block;
}
body{
    background:#6b92b9;
}
</style>
</head>
<body>

<button id="btn">Hello</button>

<canvas id="canvas" width="100%" height="100%" style="position:fixed; top:0px;left:0px;z-index:80;pointer-events:none;"></canvas>

<script>
window.onload = function(){

    //canvas init
    var canvas = document.getElementById("canvas");
    var btn = document.getElementById("btn");
    btn.onclick = function()
    {
        //alert(this.innerHTML);
        this.innerHTML="pressed";
    }
    var ctx = canvas.getContext("2d");

    //canvas dimensions
    var W = window.innerWidth;
    var H = window.innerHeight;
    canvas.width = W;
    canvas.height = H;

    //snowflake particles
    var mp = 500; //max particles
    var particles = [];
    for(var i = 0; i < mp; i++)
    {
        particles.push({
        x: Math.random()*W, //x-coordinate
        y: Math.random()*H, //y-coordinate
        r: Math.random()*3+1, //radius
        d: Math.random()*mp //density
        })
    }

    //Lets draw the flakes
    function draw()
    {
        ctx.clearRect(0, 0, W, H);

        ctx.fillStyle = "rgba(255, 255, 255, 0.8)";
        /* ctx.fillStyle = "#FF0000";*/
        ctx.beginPath();
        for(var i = 0; i < mp; i++)
        {
            var p = particles[i];
            ctx.moveTo(p.x, p.y);
            ctx.arc(p.x, p.y, p.r, 0, Math.PI*2, true);
        }
        ctx.fill();
        update();
    }

    //Function to move the snowflakes
    //angle will be an ongoing incremental flag. Sin and Cos functions will be applied to it to create vertical and horizontal movements of the flakes
    var angle = 0;
    function update()
    {
        angle += 0.01;
        for(var i = 0; i < mp; i++)
        {
            var p = particles[i];
            //Updating X and Y coordinates
            //We will add 1 to the cos function to prevent negative values which will lead flakes to move upwards
            //Every particle has its own density which can be used to make the downward movement different for each flake
            //Lets make it more random by adding in the radius
            p.y += Math.cos(angle+p.d) + 1 + p.r/2;
            p.x += Math.sin(angle) * 2;

            //Sending flakes back from the top when it exits
            //Lets make it a bit more organic and let flakes enter from the left and right also.
            if(p.x > W || p.x < 0 || p.y > H)
            {
                if(i%3 > 0) //66.67% of the flakes
                {
                    particles[i] = {x: Math.random()*W, y: -10, r: p.r, d: p.d};
                }
                else
                {
                    //If the flake is exitting from the right
                    if(Math.sin(angle) > 0)
                    {
                        //Enter fromth
                        particles[i] = {x: -5, y: Math.random()*H, r: p.r, d: p.d};
                    }
                    else
                    {
                        //Enter from the right
                        particles[i] = {x: W+5, y: Math.random()*H, r: p.r, d: p.d};
                    }
                }
            }
        }
    }

    //animation loop
    setInterval(draw, 50);
}
</script>
 </body>
</html>

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