如何在javascript

Question

我正在尝试使用P5.js库在javascript中制作简单的动画。我想让一个球出现在某个高度，然后让它掉下来使其弹跳直到停止。

我不是在寻找外部库，只是在寻找P5。

我的代码是这样的：

function Ball() {
    this.diameter = 50;
    this.v_speed = 5;
    this.ypos = height/2 - 100;
    this.xpos = width/2;

    this.update = function(){
        this.ypos = this.ypos + this.v_speed;
        this.ypos = constrain(this.ypos, this.diameter/2, height-this.diameter/2);
    }

    this.show = function(){
        ellipse(this.xpos, this.ypos, this.diameter);
        fill(255);
    }
}

var ball;

function setup() {
    createCanvas(600, 600);
    ball = new Ball();
}

function draw() {
    background(0);
    ball.update();
    ball.show();
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.6.1/p5.js"></script>

有人可以帮助我吗？非常感谢

Answer 1

首先，您必须将起始速度设置为0并定义重力：

this.v_speed = 0;
this.gravity = 0.2;

一种有效的update方法，可以直接应用于您的示例，如下所示：

this.starty = height/2 - 100;
this.endy = height-this.diameter/2;

this.update = function(){

    this.v_speed  = this.v_speed + this.gravity; 
    this.ypos = this.ypos + this.v_speed;

    if (this.ypos >= this.endy){
    this.ypos = this.endy;
        this.v_speed *= -1.0; // change direction
        this.v_speed = this.v_speed*0.9; 
        if ( Math.abs(this.v_speed) < 0.5 ) {
            this.ypos = this.starty;
        }
    }
}

关键是当球在地面上反弹时降低速度并改变方向：

this.v_speed *= -1.0;
this.v_speed = this.v_speed*0.9;

另请参阅Bouncing Balls, struggling with getting more than 1 (processing)。

请参见示例，其中我将建议应用于您的原始代码：

function Ball() {
    
  this.diameter = 50;
      this.v_speed = 0;
      this.gravity = 0.2;
      this.starty = height/2 - 100;
      this.endy = height-this.diameter/2;
      this.ypos = this.starty;
      this.xpos = width/2;

      this.update = function(){

          this.v_speed  = this.v_speed + this.gravity; 
          this.ypos = this.ypos + this.v_speed;
          
          if (this.ypos >= this.endy){
            this.ypos = this.endy;
              this.v_speed *= -1.0; // change direction
              this.v_speed = this.v_speed*0.9; 
              if ( Math.abs(this.v_speed) < 0.5 ) {
                  this.ypos = this.starty;
              }
          }
      }

      this.show = function(){
          ellipse(this.xpos, this.ypos, this.diameter);
          fill(255);
      }
}

var ball;

function setup() {
    createCanvas(600, 600);
    ball = new Ball();
}

function draw() {
    background(0);
    ball.update();
    ball.show();
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/0.7.3/p5.js"></script>

Answer 2

这很简单，这是一种可行的（但很粗糙）的方法：

var velY = 0;
var yPos = 0;
draw = function() {
velY += 0.1;//Make this value higher for stronger gravity.
yPos += velY;

if (yPos > 400) {
velY = -velY;
yPos = 400;
}//This may need a little tweaking.

ellipse(300, yPos, this.diameter);
};