html5 canvas 特效（JavaScript canvas实现流星特效）

function easeInQuad(curtime,begin,end,duration){
            let x = curtime/duration; //x值
            let y = x*x; //y值
            return begin+(end-begin)*y; //套入最初的公式
        }
        //用平方根构建的缓慢减速运动  核心函数:x*x + 2*x
function easeOutQuad(curtime,begin,end,duration){
            let x = curtime/duration;         //x值
            let y = -x*x + 2*x;  //y值
            return begin+(end-begin)*y;        //套入最初的公式
        }
function easeInoutQuad(curtime,begin,end,duration){
            if(curtime<duration/2){ //前半段时间
                return easeInQuad(curtime,begin,(begin+end)/2,duration/2);//改变量和时间都除以2
            }else{
                let curtime1 = curtime-duration/2; //注意时间要减去前半段时间
                let begin1 = (begin+end)/2;//初始量要加上前半段已经完成的
                return easeOutQuad(curtime1,begin1,end,duration/2);//改变量和时间都除以2
            }
}

function intervalOpcity (cur=0,start=0.3,end=0.8,dur=3) {
            if(fadeOpcity(cur,start,end,dur)===end){
                cur = 0
                let temp = start
                start = end 
                end = temp
            }
            let opcity = fadeOpcity(cur,start,end,dur)
            let color = `rgba(0,0,0,${opcity})`
            clearBgcolor(wrap)
            newParticle (wrap,[x,y],r,color)
            cur=parseFloat(cur+0.1)
            setTimeout(()=>{intervalOpcity (cur,start,end,dur)},33)
}

function intervalMove (speed,g=0) {
            if(g===720){
                g = 360
            }
            g = g+speed
            let nextPosition = movePosition (a,g,startPosition)
            x = nextPosition[0]
            y = nextPosition[1]
            clearBgcolor(wrap1)
            for(let i =1;i<=360;i++){
                let g1 = g-i/2
                if(g1<0&&g<=360){
                    break
                }
                let g1Position = movePosition (a,g1,startPosition)
                newParticle (wrap1,[g1Position[0],g1Position[1]],r/(1+i/5),`rgba(255,200,200,0.5)`)
            }
            setTimeout(()=>{intervalMove (speed,g)},33)
        }

function easeInQuad(curtime,begin,end,duration){
            let x = curtime/duration; //x值
            let y = x*x; //y值
            return begin+(end-begin)*y; //套入最初的公式
        }
        //用平方根构建的缓慢减速运动  核心函数:x*x + 2*x
        function easeOutQuad(curtime,begin,end,duration){
            let x = curtime/duration;         //x值
            let y = -x*x + 2*x;  //y值
            return begin+(end-begin)*y;        //套入最初的公式
        }
        function easeInoutQuad(curtime,begin,end,duration){
            if(curtime<duration/2){ //前半段时间
                return easeInQuad(curtime,begin,(begin+end)/2,duration/2);//改变量和时间都除以2
            }else{
                let curtime1 = curtime-duration/2; //注意时间要减去前半段时间
                let begin1 = (begin+end)/2;//初始量要加上前半段已经完成的
                return easeOutQuad(curtime1,begin1,end,duration/2);//改变量和时间都除以2
            }
        }
        function newCanvas (width,height) {
            let bodyEl = document.body
            let canvasEl = document.createElement("canvas")
            canvasEl.width = width
            canvasEl.height = height
            canvasEl.style.position = "absolute"
            bodyEl.append(canvasEl)
            let wrap = canvasEl.getContext("2d")
            return wrap
        }
        function setBgcolor (wrap,color) {
            wrap.fillStyle=color;
            wrap.fillRect(0,0,wrap.canvas.width,wrap.canvas.height);
        }
        function clearBgcolor(wrap){
            wrap.clearRect(0,0,wrap.canvas.width,wrap.canvas.height)
        } 
        function newParticle (wrap,position,r,color) {
            let x = position[0]
            let y = position[1]
            wrap.fillStyle=color;
            wrap.beginPath();
            wrap.arc(x,y,r,0,2*Math.PI);
            wrap.shadowBlur=20;
            wrap.shadowColor=`rgba(255,255,255,0.8)`;
            wrap.fill();
        }
        function fadeOpcity(cur,start,end,dur){
            let opcity = parseFloat(easeInoutQuad(cur,start,end,dur).toFixed(2)) 
            return opcity
        }
       let wrap0 = newCanvas (1000,800)
       let wrap2 = newCanvas (1000,800)
       let wrap = newCanvas (1000,800)
       let wrap1 = newCanvas (1000,800)
       
        setBgcolor (wrap0,"black")
        setBgcolor (wrap,"rgba(0,0,0,0)")
        setBgcolor (wrap1,"rgba(0,0,0,0)")
        setBgcolor (wrap2,"rgba(0,0,0,0)")
        let startPosition = [500,200]
        let r = 10
        let a = -200
        let x =  a*(1-Math.cos(0))*Math.sin(0)+startPosition[0]
        let y = a*(1-Math.cos(0))*Math.cos(0)+startPosition[1]
        function movePosition (a,g,startPosition) {
            let t = Math.PI*2/360
            let x = a*(1-Math.cos(g*t))*Math.sin(g*t)+startPosition[0]
            let y = a*(1-Math.cos(g*t))*Math.cos(g*t)+startPosition[1]
            return [x,y]
        }
        function intervalMove (speed,g=0) {
            if(g===720){
                g = 360
            }
            g = g+speed
            let nextPosition = movePosition (a,g,startPosition)
            x = nextPosition[0]
            y = nextPosition[1]
            clearBgcolor(wrap1)
            for(let i =1;i<=360;i++){
                let g1 = g-i/2
                if(g1<0&&g<=360){
                    break
                }
                let g1Position = movePosition (a,g1,startPosition)
                newParticle (wrap1,[g1Position[0],g1Position[1]],r/(1+i/5),`rgba(255,200,200,0.5)`)
            }
            setTimeout(()=>{intervalMove (speed,g)},33)
        }

        function intervalOpcity (cur=0,start=0.3,end=0.8,dur=3) {
            if(fadeOpcity(cur,start,end,dur)===end){
                cur = 0
                let temp = start
                start = end 
                end = temp
            }
            let opcity = fadeOpcity(cur,start,end,dur)
            let color = `rgba(0,0,0,${opcity})`
            clearBgcolor(wrap)
            newParticle (wrap,[x,y],r,color)
            cur=parseFloat(cur+0.1)
            setTimeout(()=>{intervalOpcity (cur,start,end,dur)},33)
        }
intervalOpcity ()
intervalMove (1)