【canvas】四角光阑

代码：
<!DOCTYPE html>
<html lang="utf-8">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<head>
     <title>四角光阑 2018年4月7日</title>
    </head>

     <body onload="draw()">
        <canvas id="myCanvus" width="400px" height="400px" style="border:1px dashed black;">
            出现文字表示你的浏览器不支持HTML5
        </canvas>
     </body>
</html>
<script type="text/javascript">
<!--
function draw(){
    var canvas=document.getElementById('myCanvus');
    canvas.width=400;
    canvas.height=400;    

    context=canvas.getContext('2d');
    context.translate(200,200);

    slot=new Slot();
    animate();
};

var delta=0;// 旋转角
var radius=0;// 旋转半径
var outerRad=200;// 外径
var context;
var slot;
var angleCount=4;

function animate(){
    context.clearRect(-200,-200,400,400);// 清屏

    slot.update(radius,delta,outerRad);
    slot.paintBg(context);
    slot.paint(context);
    slot.paintBase(context);

    delta+=1;
    radius+=1;

    if(radius<outerRad){
        // 让浏览器自行决定帧速率
        window.requestAnimationFrame(animate);
    }
}

function Slot(){
    var obj=new Object;

    obj.bx=0;
    obj.by=0;
    obj.cx=0;
    obj.cy=0;
    obj.dx=0;
    obj.dy=0;
    obj.angleC=0;
    obj.angleD=0;
    obj.radius=0;
    obj.outerRad=0;
    obj.img;

    // 计算
    obj.update=function(radius,theta,outerRad){
        this.img=new Image();
        this.img.src="earth.jpg";
        this.radius=radius;
        this.outerRad=outerRad;        

        this.bx=radius*Math.cos(getRad(theta+360/angleCount));
        this.by=radius*Math.sin(getRad(theta+360/angleCount));

        var ra=Math.PI/angleCount;
        var x=Math.sqrt(outerRad*outerRad-radius*radius*Math.sin(ra)*Math.sin(ra))-radius*Math.cos(ra);
        var alpha=Math.asin(x*Math.sin(ra)/this.outerRad);
        this.angleD=getRad(theta+360/angleCount)+alpha;
        this.dx=outerRad*Math.cos(this.angleD);
        this.dy=outerRad*Math.sin(this.angleD);

        this.angleC=this.angleD+2*Math.PI/angleCount;
        this.cx=outerRad*Math.cos(this.angleC);
        this.cy=outerRad*Math.sin(this.angleC);
    };

    // 画背景
    obj.paintBg=function(ctx){
        context.drawImage(this.img,0,0,800,800,-200,-200,400,400);
    };

    // 描光阑
    obj.paint=function(ctx){
        ctx.strokeStyle = "black";

        for(var i=0;i<angleCount;i++){
            ctx.save();
            ctx.fillStyle = getColor(i+5);
            ctx.rotate(2*Math.PI/angleCount*i);

            ctx.beginPath();

            ctx.lineTo(this.bx,this.by);
            ctx.lineTo(this.dx,this.dy);
            ctx.arc(0,0,this.outerRad,this.angleD,this.angleC,false);
            ctx.lineTo(this.bx,this.by);

            ctx.closePath();
            ctx.stroke();
            ctx.fill();

            ctx.restore();
        }
    };

    // 描基座
    obj.paintBase=function(ctx){
        ctx.strokeStyle = "black";

        for(var i=0;i<4;i++){
            ctx.save();
            ctx.fillStyle = getColor(13);
            ctx.rotate(Math.PI/2*i);

            ctx.beginPath();

            ctx.arc(0,0,this.outerRad,0,Math.PI/2,false);
            ctx.lineTo(this.outerRad,this.outerRad);
            ctx.lineTo(this.outerRad,0);

            ctx.closePath();
            ctx.stroke();
            ctx.fill();

            ctx.restore();
        }
    };

    return obj;
}

// 角度得到弧度
function getRad(degree){
    return degree/180*Math.PI;
}

// 得到颜色
function getColor(index){
    if(index==0){
        return "green";
    }else if(index==1){
        return "silver";
    }else if(index==2){
        return "lime";
    }else if(index==3){
        return "gray";
    }else if(index==4){
        return "white";
    }else if(index==5){
        return "yellow";
    }else if(index==6){
        return "maroon";
    }else if(index==7){
        return "navy";
    }else if(index==8){
        return "red";
    }else if(index==9){
        return "blue";
    }else if(index==10){
        return "purple";
    }else if(index==11){
        return "teal";
    }else if(index==12){
        return "fuchsia";
    }else if(index==13){
        return "aqua";
    }else if(index==14){
        return "black";
    }
}

//-->
</script>
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【canvas】四角光阑

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