S2JS - How to fall and bounce convincingly : Bouncing off the bottom

Bouncing off the bottom is similar to bouncing off the right hand side, because the canvas x-positions increase to the right, and the y-positions increase going down.

So we need to compare the bottom of the ball (given by its y-position plus its height) to the bottom of the canvas (given by its height). And we reverse the y-velocity.

if (this.y + this.MyImg.height > myCanvas.height) {this.velocity_y= -this.velocity_y};

<html> <head> <title> Bouncing objects</title> </head> <body> <canvas id="myCanvas" width=350 height=300 style="background-color:powderblue"> </canvas> <script> var ctx = myCanvas.getContext("2d"); // Get the drawing context for the canvas var FPS = 40; // How many frames per second function MySprite (img_url) { this.x = 0; this.y = 0; this.visible= true; this.velocity_x = 0; this.velocity_y = 0; this.MyImg = new Image(); this.MyImg.src = img_url ; } MySprite.prototype.Do_Frame_Things = function() { if (this.visible) ctx.drawImage(this.MyImg, this.x, this.y); // draw the thing this.x = this.x + this.velocity_x; this.y = this.y + this.velocity_y; // move the thing if (this.x < 0) {this.velocity_x= -this.velocity_x}; if (this.x + this.MyImg.width > myCanvas.width) {this.velocity_x= -this.velocity_x}; if (this.y + this.MyImg.height > myCanvas.height) {this.velocity_y= -this.velocity_y}; } var MyBall= new MySprite("http://s2js.com/img/etc/tennis-ball.png"); MyBall.velocity_x= 3; MyBall.velocity_y= 1; function Do_a_Frame () { ctx.clearRect(0, 0, myCanvas.width, myCanvas.height); MyBall.Do_Frame_Things(); } setInterval(Do_a_Frame, 1000/FPS); // set my frame renderer </script> </body> </html>