先上图:
代码:
<!DOCTYPE html> <html lang="utf-8"> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/> <head> <title>狗撵兔子与贝塞尔曲线</title> <style type="text/css"> .centerlize{ margin:0 auto; border:0px solid red; width:1200px;height:550px; } </style> </head> <body onl oad="draw();"> <div class="centerlize"> <canvas id="myCanvas" width="1200px" height="550px" style="border:1px dashed black;"> 出现文字表示您的浏览器尚不支持HTML5 Canvas </canvas> </div> </body> </html> <script type="text/javascript"> <!-- // 画布宽度 const WIDTH=1200; // 画布高度 const HEIGHT=550; // 画布环境 var context=0; // 时间 var t=0; // 画图前初始化 function draw(){ var canvas=document.getElementById('myCanvas'); canvas.width=WIDTH; canvas.height=HEIGHT; context=canvas.getContext('2d'); // 进行屏幕坐标系到笛卡尔坐标系的变换,原点移动到画布*,右方为X正向,上方为Y的正向 context.translate(WIDTH/2,HEIGHT/2); context.rotate(getRad(180)); context.scale(-1,1); slot=new Slot(); animate(); }; //------------------------------- // 画图 //------------------------------- function animate(){ t=0.5; slot.update(t); slot.paintBg(context); slot.paint(context); if(slot.getDistance()>1){ window.requestAnimationFrame(animate); } } //------------------------------- // Slot对象定义处 //------------------------------- function Slot(){ var obj=new Object; obj.rabbit={"x":0,"y":200,"speed":0.5,"cds":[]}; obj.dog={"x":0,"y":0,"speed":1,"cds":[]}; // 随时间更新狗兔位置 obj.update=function(t){ let d=this.getDistance(); let sin=(this.rabbit.y-this.dog.y)/d; this.dog.y+=this.dog.speed*sin*t; let cos=(this.rabbit.x-this.dog.x)/d; this.dog.x+=this.dog.speed*cos*t; let arr2={"x":this.dog.x,"y":this.dog.y}; this.dog.cds.push(arr2); this.rabbit.x+=this.rabbit.speed*t; let arr={"x":this.rabbit.x,"y":this.rabbit.y}; this.rabbit.cds.push(arr); }; // 求狗兔距离 obj.getDistance=function(){ var x=this.dog.x-this.rabbit.x; x=x*x; var y=this.dog.y-this.rabbit.y; y=y*y; return Math.sqrt(x+y); } // 画背景 obj.paintBg=function(ctx){ // 清屏 ctx.clearRect(-WIDTH/2,-HEIGHT/2,WIDTH,HEIGHT); drawAxisX(ctx,-WIDTH/2,WIDTH/2,100); drawAxisY(ctx,-HEIGHT/2,HEIGHT/2,100); ctx.strokeStyle="yellow"; ctx.moveTo(0,0); ctx.quadraticCurveTo(0,200,132.5,200); ctx.stroke(); drawText(ctx,"绿线:兔子轨迹 红线:狗轨迹 黄线:贝塞尔曲线",500,200); drawText(ctx,"绘制:逆火狂飙",500,-200); }; // 画前景 obj.paint=function(ctx){ paintCurve(ctx,"green",this.rabbit.cds); paintCurve(ctx,"red",this.dog.cds); let d=this.getDistance(); if(d<=1){ drawText(ctx," 结束点 x:"+this.rabbit.x+" y:"+this.rabbit.y,-500,200); } }; return obj; } // 连点成线画曲线 function paintCurve(ctx,color,cds){ var SU=1;// Scale Unit ctx.strokeStyle = color; ctx.beginPath(); for(var i=0; i<cds.length; i++){ ctx.lineTo(cds[i].x*SU,cds[i].y*SU); } ctx.stroke(); ctx.closePath(); } // 画横轴 function drawAxisX(ctx,start,end,step){ ctx.save(); ctx.lineWidth=0.5; ctx.strokeStyle='navy'; ctx.fillStyle='navy'; // 画轴 ctx.beginPath(); ctx.moveTo(start, 0); ctx.lineTo(end, 0); ctx.stroke(); ctx.closePath(); // 画箭头 ctx.beginPath(); ctx.moveTo(end-Math.cos(getRad(15))*10, Math.sin(getRad(15))*10); ctx.lineTo(end, 0); ctx.lineTo(end-Math.cos(getRad(15))*10, -Math.sin(getRad(15))*10); ctx.stroke(); ctx.closePath(); // 画刻度 var x,y; y=5; for(x=start;x<end;x+=step){ ctx.beginPath(); ctx.moveTo(x, 0); ctx.lineTo(x, y); ctx.stroke(); ctx.closePath(); drawText(ctx,x+"",x,y-20); } ctx.restore(); } // 画纵轴 function drawAxisY(ctx,start,end,step){ ctx.save(); ctx.lineWidth=0.5; ctx.strokeStyle='navy'; ctx.fillStyle='navy'; var start=-300; var end=300; // 画轴 ctx.beginPath(); ctx.moveTo(0, start); ctx.lineTo(0, end); ctx.stroke(); ctx.closePath(); // 画箭头 ctx.beginPath(); ctx.moveTo(Math.sin(getRad(15))*10, end-Math.cos(getRad(15))*10); ctx.lineTo(0, end); ctx.lineTo(-Math.sin(getRad(15))*10, end-Math.cos(getRad(15))*10); ctx.stroke(); ctx.closePath(); // 画刻度 var x,y; x=5; for(y=start;y<end;y+=step){ ctx.beginPath(); ctx.moveTo(x, y); ctx.lineTo(0, y); drawText(ctx,y+"",x-15,y); ctx.stroke(); ctx.closePath(); } } //------------------------------- // 角度得到弧度 //------------------------------- function getRad(degree){ return degree/180*Math.PI; } //------------------------------- // 得到颜色 //------------------------------- function getColor(index){ var arr=["green","skyblue","purple","#aa0000", "orange","yellow","maroon","navy", "red","blue","lime","teal","fuchsia", "aqua","black"]; if(index>arr.length){ index=index % arr.length; } return arr[index]; } //------------------------------- // 在笛卡尔坐标系中绘制文字 //------------------------------- function drawText(ctx,text,x,y){ ctx.save(); ctx.translate(x,y) ctx.rotate(getRad(180)) ctx.scale(-1,1) ctx.textBaseline="bottom"; ctx.textAlign="center"; ctx.fillText(text,0,0); ctx.restore(); } //--> </script>
小记:
原以为狗撵兔子的曲线会和控制点为(0,200),结束点为(132.5,200)的贝塞尔二次曲线重合,结果却与预期不同。
看来,狗撵兔子的曲线应该是摆线(最优降速线),而贝塞尔曲线不是摆线。这个结论有待日后证明。
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