canvas绘制二次贝塞尔曲线----演示二次贝塞尔四个参数的作用

canvas中绘制二次贝塞尔曲线的方法为ctx.quadraticCurveTo(x1,y1,x2,y2);

四个参数分别为两个控制点的坐标.开始点即当前canvas中目前的点,如果想从指定的点开始,需要使用ctx.moveTo(x,y)方法

演示效果如下图

canvas绘制二次贝塞尔曲线----演示二次贝塞尔四个参数的作用

上代码:

html

<!doctype html>

<html>

<head>

<meta charset="utf-8">

<title>无标题文档</title>

<style>

*{ margin:0; padding:0;}

#info{ width:800px; height:30px; line-height:30px; margin:50px auto 0 auto;}

#canvas{ display:block; border:1px solid #ccc; margin:0px auto;}

</style>

<script src="js/dot.js"></script>

<script src="js/main.js"></script>

</head>

<body>

<div id="info"></div>

<canvas id="canvas" width="800" height="600"></canvas>

</body>

</html>

JS代码

dot.js

// JavaScript Document

var dot = function ( x , y ){

    this.x = x;

    this.y = y;

    this.r = dotR;

}

dot.prototype.draw = function (ctx){

    ctx.beginPath();

    ctx.arc( this.x , this.y , this.r , 0 , Math.PI*2 );

    ctx.fill();

    ctx.closePath();

}

main.js

// JavaScript Document

var CANVERS_WIDTH = 800;

var CANVERS_HEIGHT = 600;

var dotR = 10;

var dotArr = [];

window.onload = function(){

    var oCanvas = document.querySelector("#canvas");

    var oInfo = document.querySelector("#info");

    var ctx = oCanvas.getContext("2d");

    var dotA = new dot( 100 , 400 );

    dotArr.push( dotA );

    var dotB = new dot( 200 , 200 );

    dotArr.push( dotB );

    var dotC = new dot( 400 , 400 );

    dotArr.push( dotC );

    creatGuides();

    createBezier();

    function createBezier(){

        ctx.beginPath();

        ctx.moveTo( dotA.x , dotA.y );

        ctx.quadraticCurveTo( dotB.x , dotB.y , dotC.x , dotC.y );

        ctx.stroke();

        ctx.closePath();

    }

    //绘制辅助线

    ctx.lineWidth = 2;

    function creatGuides(){

        dotA.draw( ctx );

        dotB.draw( ctx );

        dotC.draw( ctx );

        ctx.beginPath();

        ctx.moveTo( dotA.x, dotA.y );

        ctx.lineTo( dotB.x , dotB.y );

        ctx.lineTo( dotC.x , dotC.y );

        ctx.stroke();

        ctx.closePath();

    }

    oCanvas.onmousedown = function (e){

        var disX = e.clientX - this.offsetLeft;

        var disY = e.clientY - this.offsetTop;

        //判断鼠标放下是是否在控制点上

        var curDot = checkDot(disX,disY)

        if( curDot ){

            if(oCanvas.setCapture){ oCanvas.setCapture(); }

            document.onmousemove = function (e){

                oInfo.textContent ="ctx.quadraticCurveTo("+ dotB.x + "," + dotB.y + "," + dotC.x + "," + dotC.y + ")";

                disX = e.clientX - oCanvas.offsetLeft;

                disY = e.clientY - oCanvas.offsetTop;

                curDot.x = disX;

                curDot.y = disY;

                console.log(disX)

                ctx.clearRect(0,0,CANVERS_WIDTH,CANVERS_HEIGHT);

                creatGuides();

                createBezier();

            }

            document.onmouseup = function (){

                creatGuides();

                createBezier();

                document.onmousemove = null;

                document.onmouseup = null;

                if(oCanvas.setCapture){ oCanvas.releasesCaptrue(); }

            }

            return false;

        }

    }

    function checkDot(x,y){

        for( var i=0; i<dotArr.length; i++ ){

            if( Math.abs( dotArr[i].x - x ) < dotR && Math.abs( dotArr[i].y - y ) < dotR ){

                return dotArr[i];

            }

        }

        return false;

    }

}