S2JS - How to fall and bounce convincingly : Rebound problem 3 fixed

Here's the bounce code with the adjustment made.

You don't need to worry about the details unless you particularly want to, but the upshot is:

Calculate how much overrun there was at the bottom
Adjust the y-velocity using simple scaling -- if we only overran a little, we only need to tweak the velocity a little, etc.
Make use of the overrun to reposition the object at the exact bottom

Now it really does bounce perfectly -- to the exact bottom every time, and never rebounding past its starting height.

<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 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 ; } var gravity = 0.25; var FPS = 40; // How many frames per second 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}; // bounce off sides if (this.x + this.MyImg.width > myCanvas.width) {this.velocity_x= -this.velocity_x}; if (this.y + this.MyImg.height > myCanvas.height) { var overrun = this.y + this.MyImg.height - myCanvas.height; // calculate overrun this.velocity_y = this.velocity_y - (overrun / this.velocity_y * gravity); //adjust this.y = this.y - overrun; // be exactly at the bottom this.velocity_y= -this.velocity_y; // bounce off the bottom } else { // accelerate by gravity, but only if we're not actually bouncing at this moment this.velocity_y = this.velocity_y + gravity; } } var MyBall= new MySprite("http://s2js.com/img/etc/tennis-ball.png"); MyBall.velocity_x= 0; MyBall.velocity_y= 0; MyBall.y= 80; MyBall.x= myCanvas.width / 2; 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>