Canvas/JS：计算倾斜斜坡碰撞中物体的新速度矢量？

var W,H; // canvas width and height
// get the canvas context
const ctx = canvas.getContext("2d");
const gravity = 0.19;
function vec(x,y){return {x,y}}

const line = {  
   p1 : { x : 0, y : 0 },
   p2 : { x : 0, y : 0 },
   vnorm : { x : 0, y : 0 }, // normalied vector
   vec : { x : 0, y : 0 }, // line as a vector
   len : 0,   
   update(){
     this.vec.x = this.p2.x - this.p1.x;
     this.vec.y = this.p2.y - this.p1.y;
     this.len = Math.sqrt(this.vec.x * this.vec.x + this.vec.y * this.vec.y);
     this.vnorm.x = this.vec.x / this.len;
     this.vnorm.y = this.vec.y / this.len;
   },
   draw(){
      ctx.beginPath();
      ctx.moveTo(this.p1.x,this.p1.y);
      ctx.lineTo(this.p2.x,this.p2.y);
      ctx.stroke();
   },
   testBall(ball){  // line must be updated befor this call
       // line is one sided and ball can only approch from the right
       // find line radius distance from line
       var x = this.p1.x + this.vnorm.y * ball.radius;
       var y = this.p1.y - this.vnorm.x * ball.radius;
       x = ball.pos.x - x;
       y = ball.pos.y - y;
       // get cross product to see if ball hits line
       var c = x * this.vec.y - y * this.vec.x;
       if(c <= 0){ // ball has hit the line

            // get perpendicular line away from line
            var ox = this.vnorm.y * ball.radius
            var oy = -this.vnorm.x * ball.radius
            // get relative ball pos
            var px = ball.pos.x - (this.p1.x + ox);
            var py = ball.pos.y - (this.p1.y + oy);
            // find ball position that contacts the line
            var ld = (px * this.vec.x + py * this.vec.y)/(this.len * this.len);
            ball.pos.x  = this.vec.x * ld + (this.p1.x+ox);
            ball.pos.y = this.vec.y * ld + (this.p1.y+oy);            
            // find the reflection delta (bounce direction)
            var dd = (ball.dnorm.x * this.vnorm.x + ball.dnorm.y * this.vnorm.y) * 2;
            ball.delta.x = this.vnorm.x * dd - ball.dnorm.x;
            ball.delta.y = this.vnorm.y * dd - ball.dnorm.y;
            
            // the ball has lost some speed (should not have but this is a fix)
            
           var m = Math.sqrt(ball.delta.x * ball.delta.x + ball.delta.y * ball.delta.y);
            ball.delta.x = (ball.delta.x / m) * ball.speed;
            ball.delta.y = (ball.delta.y / m) * ball.speed;
            
            ball.updateContact();
            ball.col = "red";
       
       }
   }
}



// create a ball
function createLine(x,y,x1,y1){
    var l = Object.assign({},line,{
       p1 : vec(x,y),
       p2 : vec(x1,y1),
       vec : vec(0,0),
       vnorm : vec(0,0),
    
    });
    l.update();
    return l;
}
const lines = {
   items : [],
   add(line){ lines.items.push(line); return line },
   update(){
        var i;
        for(i = 0; i < lines.items.length; i++){
            lines.items[i].update();
        }
        return lines;
    },
   draw(){
        var i;
        for(i = 0; i < lines.items.length; i++){
            lines.items[i].draw();
        }
        return lines;
    },
    testBall(ball){
        var i;
        for(i = 0; i < lines.items.length; i++){
            lines.items[i].testBall(ball);
        }
        return lines;
    }
    

};

var ball = {
   pos : { x : 0, y: 0},
   radius : 0,
   speed : 0,
   col : "black",
   delta : { x : 0, y : 0},  // movement as vector
   dnorm : { x : 0, y: 0},  // normalised delta
   updateContact(){ // update after ball hits wall
      this.speed = Math.sqrt(this.delta.x * this.delta.x + this.delta.y * this.delta.y);
      this.dnorm.x = this.delta.x / this.speed;
      this.dnorm.y = this.delta.y / this.speed;   
   },
   update(){
      this.col = "black";
      this.delta.y += gravity;
      this.pos.x += this.delta.x;
      this.pos.y += this.delta.y;
      this.speed = Math.sqrt(this.delta.x * this.delta.x + this.delta.y * this.delta.y);
      this.dnorm.x = this.delta.x / this.speed;
      this.dnorm.y = this.delta.y / this.speed;   
   },
   draw(){
      ctx.strokeStyle = this.col;
      ctx.beginPath();
      ctx.arc(this.pos.x,this.pos.y,this.radius,0,Math.PI*2);
      ctx.stroke();
   },
}
// create a ball
function createBall(x,y,r){
    var b = Object.assign({},ball,{
      pos : vec(x,y),
      radius : r,
      delta : vec(0,0),
      dnorm : vec(0,0),
    });
    return b;
}

const balls = {
    items : [],
    add(b){
        balls.items.push(b);
    },
    update(){
        var i;
        for(i = 0; i < balls.items.length; i++){
            balls.items[i].update();
            lines.testBall(balls.items[i]);
            if(balls.items[i].pos.y - balls.items[i].radius > canvas.height ||
              balls.items[i].pos.x < -50 || balls.items[i].pos.x - 50  > W){
                balls.items.splice(i--,1);
            }
        }
        return balls;
    },
    draw(){
        var i;
        for(i = 0; i < balls.items.length; i++){
            balls.items[i].draw();
        }
        return balls;
    }
}
    




const l1 = lines.add(createLine(0,0,100,100)); 
const l2 = lines.add(createLine(0,0,100,100)); 
const mouse = { x : 0, y : 0};
canvas.addEventListener("mousemove",(e)=>{
   mouse.x = e.pageX;
   mouse.y = e.pageY;
})


function mainLoop(time){
    // resize if needed
    if(canvas.width !== innerWidth || canvas.height !== innerHeight){ // resize canvas if window size has changed
        W = canvas.width = innerWidth;
        H = canvas.height = innerHeight;
    }
    // clear canvas 
    ctx.setTransform(1,0,0,1,0,0); // set default transform
    ctx.clearRect(0,0,W,H); // clear the canvas
    if(balls.items.length < 10){
        balls.add(createBall(Math.random() * W , Math.random() * 30, Math.random() * 20 + 10));

    }

    time /= 5;
    l1.p1.x = 0;
    l1.p2.x = W ;
    l2.p1.x = 0;
    l2.p2.x = W ;
    l1.p1.y = Math.sin(time / 1000) * H * 0.4 + H * 0.6;
    l1.p2.y = Math.cos(time / 1000) * H * 0.4 + H * 0.6;
    l2.p1.y = Math.sin(time / 500) * H * 0.4 + H * 0.6;
    l2.p2.y = Math.cos(time / 500) * H * 0.4 + H * 0.6;
    lines.update().draw(); 
    balls.update().draw();    


    // get next animation loop
    requestAnimationFrame(mainLoop);
}
requestAnimationFrame(mainLoop);

canvas {
    position : absolute;
    top : 0px;
    left : 0px;
    z-index : -10;
}

<canvas id=canvas></canvas>