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Canvas bezierCurveTo heavy flicker just in Chrome. Nice in Firefox and Edge

I adopted a Javascript 1k demo of swinging grass (http://labs.hyperandroid.com/js1k) for my website. Unfortunately, the grass flickers very strong just in Chrome. Not in Firefox nor in MS Edge.
I already changed the code to use requestAnimationFrame, which did not help.
I have no idea what to look for any further. Any help to avoid the problem in Google Chrome is much appreciated.
Comment right after initial post: Here on stackoverflow is much less flicker using Chrome as on JSFiddle (https://jsfiddle.net/4sqpL1b9/) or my website. I do not unterstand this...
Finally the solution for my situation: 1) add 2 pixel to the end of the blade of grass (see variable 'tuneWidth') and 2) up-scale canvas by 2 and down-scale again with CSS to the original underlying object size. This reduces the flicker in Google Chrome almost complete. At least good enough for me :-). I updated the code snippet below.
// Original version http://labs.hyperandroid.com/js1k
// 2019 MD: Modified for being an transparent overlay on top of HTML id "thisOverlay"
// w, d, thisOverlay, thisDay, thisMonth, thisDoy are set from the initiating HTML/PHP file
var grassId = "grass";
var callbackId = null;
var canvas = null;
var ctx = null;
var garden = null;
var gradient;
var grassBaseColor;
var grassSizeFactor;
var allColors = {
60:"00FF00", 64:"07FD01", 69:"0EFB02", 73:"15F903", 77:"1CF704", 82:"22F506", 86:"29F307", 91:"2FF208",
95:"35F009", 99:"3BEE0A",104:"41EC0B",108:"47EA0C",112:"4CE80D",117:"52E70E",121:"57E50F",125:"5CE310",
130:"61E111",134:"66DF12",138:"6ADD13",143:"6FDC14",147:"73DA15",152:"77D816",156:"7CD617",160:"80D418",
165:"83D218",169:"87D019",173:"8BCF1A",178:"8ECD1B",182:"91CB1B",186:"94C91C",191:"97C71D",195:"9AC51E",
200:"9DC41E",204:"A0C21F",208:"A2C020",213:"A5BE20",217:"A7BC21",221:"A9BA21",226:"ABB922",230:"ADB723",
234:"AFB523",239:"B1B324",243:"B1B024",247:"AFAB25",252:"ADA625",256:"ACA126",261:"AA9C26",265:"A89826",
269:"A69327",274:"A48E27",278:"A28A28",282:"A18628",287:"9F8128",291:"9D7D28",295:"9B7929",300:"997529",
304:"977229",309:"966E29",313:"946A2A",317:"92672A",322:"90632A",326:"8E602A",330:"8C5D2A",335:"8B5A2B"
};
(function() {
Grass = function() {
return this;
};
Grass.prototype = {
alto_hierba: 0, // grass height
maxAngle: 0, // maximum grass rotation angle (wind movement)
angle: 0, // construction angle. thus, every grass is different to others
coords: null, // quadric bezier curves coordinates
color: null, // grass color. modified by ambient component
offset_control_point: 3, // grass base width. greater values, wider at the basement
initialize : function(canvasWidth,canvasHeight,minHeight,maxHeight,angleMax,initialMaxAngle) {
// grass start position
var sx = Math.floor(Math.random()*canvasWidth);
var sy = canvasHeight;
// quadric curve middle control point. higher values means wider grass from base to peak
// try offset_control_x = 10 for thicker grass. default = 1.5
var offset_control_x = 1.5;
this.alto_hierba = minHeight + Math.random() * maxHeight;
this.maxAngle = 10 + Math.random() * angleMax;
this.angle = Math.random() * initialMaxAngle * (Math.random() < 0.5 ? 1 : -1) * Math.PI / 180;
// hand crafted value. modify offset_control_x to play with grass curvature slope
var csx = sx-offset_control_x ;
// grass curvature. greater values make grass bender. try with:
// var csy = sy-this.alto_hierba; -> much more bended grass
// var csy = sy-1; -> totally unbended grass
// var csy = sy-this.alto_hierba/2; -> original, good looking grass
var csy = Math.random() < 0.1 ? sy - this.alto_hierba : sy - this.alto_hierba / 2;
// both bezier curves that conform each grass should have the same middle control point to be parallel
// play with psx/psy by adding or removing values to slightly modify grass geometry
var psx = csx;
// changed var psy = csy; to
var psy = csy - offset_control_x;
// the bigger offset_control_point, the wider on its basement. default is 1.5
this.offset_control_point = 1.5;
var dx = sx + this.offset_control_point;
var dy = sy;
this.coords = [sx,sy,csx,csy,psx,psy,dx,dy];
// make random grass color
this.color = [
parseInt(grassBaseColor.slice(0,2),16) + Math.random()*20,
parseInt(grassBaseColor.slice(2,4),16) + Math.random()*50,
parseInt(grassBaseColor.slice(4,6),16) + Math.random()*20
];
},
// paint every grass
// ctx is the canvas2drendering context
// time for grass animation
// ambient to dim or brighten every grass
// returns nothing
paint : function(ctx,time,ambient) {
// grass peak position. how much to rotate the peak
// less values, will make as if there were a softer wind. default is 0.0005
var inc_punta_hierba = Math.sin(time*0.0005);
// rotate the point, so grass curves are modified accordingly. if just moved
// horizontally, the curbe would end by being unstable with undesired visuals
var ang = this.angle + Math.PI/2 + inc_punta_hierba * Math.PI/180 * (this.maxAngle * Math.cos(time*0.0002));
var px = this.coords[0] + this.offset_control_point + this.alto_hierba * Math.cos(ang);
var py = this.coords[1] - this.alto_hierba * Math.sin(ang);
var c = this.coords;
ctx.beginPath();
ctx.moveTo(c[0],c[1]);
// add some pixel to the end of the blade of grass to make
// it thicker and therefore less flicker. default is 1
var tuneWidth = 1;
// draw it
ctx.bezierCurveTo(c[0],c[1],c[2],c[3],px-tuneWidth,py);
ctx.bezierCurveTo(px+tuneWidth,py,c[4],c[5],c[6],c[7]);
ctx.fillStyle ='rgb(' +
Math.floor(this.color[0]*ambient) + ',' +
Math.floor(this.color[1]*ambient) + ',' +
Math.floor(this.color[2]*ambient) + ')';
ctx.fill();
}
};
})();
function getGrassBaseColor() {
var doy = thisDoy; // 1..366 (from PHP)
// get max doy (=max key in color array)
var maxDoy = Math.max.apply(null,Object.keys(allColors));
// loop until valid or max color index (=day of year)
while (!allColors[doy] && doy < maxDoy) { doy++; }
// just in case...
doy = Math.min(doy,maxDoy);
return allColors[doy];
}
function getGrassSizeFactor() {
return 1;
var day = thisDay; // 1..30 (from PHP)
var month = thisMonth; // 1..12 (from PHP)
if (month == 3) { return day/60; } // March: 0.0 - 0,5
else if (month == 4) { return 0.5 + day/60; } // April: 0.5 - 1.0
else if (month >= 5 && month <= 9) { return 1; } // May-September: 1.0
else if (month == 10) { return 1 - day/60; } // October: 1.0 - 0.5
else if (month == 11) { return 0.5 - day/60; } // November: 0.5 - 0.0
return 1; // default: 1.0
}
function clearCanvas() {
ctx.clearRect(0,0,canvas.width,canvas.height);
}
(function() {
Garden = function() {
return this;
};
Garden.prototype = {
grass: null,
ambient: 1,
width: 0,
height: 0,
initialize : function(width,height,size) {
this.width = width;
this.height = height;
this.grass = [];
for(var i = 0; i < size; i++) {
var thisGrass = new Grass();
thisGrass.initialize(
width,
height,
5, // min grass height. default 5
height*grassSizeFactor, // max grass height
20, // grass max initial random angle. default 20
45 // max random angle for animation. default 45
);
this.grass.push(thisGrass);
}
},
paint : function(ctx,time) {
clearCanvas();
for(var i = 0; i < this.grass.length; i++) {
this.grass[i].paint(ctx,time,this.ambient);
}
}
};
})();
function paintGarden(timeStamp) {
garden.paint(ctx,timeStamp);
callbackId = requestAnimationFrame(paintGarden);
}
function initGrass() {
var container = d.getElementById(thisOverlay);
var thisWidth = container.clientWidth;
var thisHeight = container.clientHeight;
// clear current canvas area
if (ctx) { clearCanvas(); }
// create canvas only if first run of script, not on resize
if (!canvas) {
canvas = d.createElement("canvas");
canvas.id = grassId;
canvas.title = "Title";
container.appendChild(canvas);
}
if (canvas) {
// 2 seems to avoid flickering in Chrome best
var thisScale = 2;
ctx = canvas.getContext("2d");
// up-scale canvas and down-scale again with CSS to underlying object size
// this - together with the above grass thickness tune - avoids almost complete flicker in Google Chrome
ctx.canvas.width = thisScale * thisWidth;
ctx.canvas.height = thisScale * thisHeight;
ctx.scale(thisScale,thisScale);
canvas.style.width = thisWidth + "px";
canvas.style.height = thisHeight + "px";
garden = new Garden();
// 3rd parameter is grass density. default is 300
garden.initialize(thisWidth,thisHeight,300);
requestAnimationFrame(paintGarden);
}
}
function resetGrass() {
cancelAnimationFrame(callbackId);
initGrass();
}
grassBaseColor = getGrassBaseColor();
grassSizeFactor = getGrassSizeFactor();
w.onresize = resetGrass;
w.onload = initGrass;
<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.0 Transitional//EN'
'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd'>
<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=utf-8' />
<style>
#header {
height: 100px;
margin: 0;
padding: 0;
background-color: black;
position: relative; /* to allow canvas overlay */
}
#grass { /* to allow canvas overlay */
left: 0;
position: absolute;
z-index: 50;
}
</style>
</head>
<body>
<div id='header'></div>
<script type='text/javascript'>var w = window, d = document, thisOverlay = 'header', thisDay = 7, thisMonth = 3, thisDoy = 66;</script>
</body>
</html>

Detect a mouse click on an overlapping grid of boxes - JavaScript Canvas

I am currently trying to detect a mouse click on two grids of boxes simultaneously. One grid is easy, and I've just been using:
var gridPosX = Math.floor(mouseClickX/BoxWidth);
var gridPosY = Math.floor(mouseClickY/BoxHeight);
Now I also want to detect a mouse click on a secondary grid of boxes, located at the corners of the first grid of boxes. This could be achieved in a similar way to the first grid. The problem comes in because I want to detect a click on either the first grid, or the second one, at the same time. What is the best way to differentiate a click on the first grid verses a click on the second grid? I've tried to remove the Math.floor and used the greater than and less than operators (> <) to see if the click was closer to one grid spot than the other, but I've had no luck with that so far.
This is an image example of the grid. The black being the main one, the red being the second one
var WIDTH = 1280, HEIGHT = 1280;
var canvas, context;
var grid = [];
var grid2 = [];
var gridWidth = 10, gridHeight = 10;
var boxWidth = WIDTH/gridWidth, boxHeight = HEIGHT/gridHeight;
function main(){
canvas = document.createElement("canvas");
canvas.width = WIDTH;
canvas.height = HEIGHT;
context = canvas.getContext("2d");
document.body.appendChild(canvas);
canvas.onmousedown = function(e){
if(e.which == 1){
var gridPosX = Math.floor(e.offsetX/boxWidth);
var gridPosY = Math.floor(e.offsetY/boxHeight);
grid[gridPosX][gridPosY] = 0;
}
}
init();
setInterval(draw, 30);
}
function init(){
for(var x = 0; x < gridWidth; x++){
grid[x] = [];
grid2[x] = [];
for(var y = 0; y < gridHeight; y++){
grid[x][y] = 1;
grid2[x][y] = 1;
}
}
}
function draw(){
for(var x = 0; x < gridWidth; x++){
for(var y = 0; y < gridHeight; y++){
if(grid[x][y] == 1){
context.fillStyle = 'gray';
context.fillRect(x*boxWidth, y*boxHeight, boxWidth, boxHeight);
context.strokeRect(x*boxWidth, y*boxHeight, boxWidth, boxHeight);
}
}
}
for(var x = 0; x < gridWidth; x++){
for(var y = 0; y < gridHeight; y++){
if(grid2[x][y] == 1){
context.fillStyle = 'red';
context.fillRect((x*boxWidth)+(boxWidth)-(boxWidth/4), (y*boxHeight)+(boxHeight)-(boxHeight/4), boxWidth/2, boxHeight/2);
context.strokeRect((x*boxWidth)+(boxWidth)-(boxWidth/4), (y*boxHeight)+(boxHeight)-(boxHeight/4), boxWidth/2, boxHeight/2);
}
}
}
}
main();

Since the red grids show on top of the gray ones, I think you can first decide whether a mouse event is on a red grid or not. If not, then it must be on gray grids.
Based on the calculations below to check if a red grid is clicked:
var xRedIndex = Math.floor((e.offsetX - 3 / 4 * boxWidth) / (boxWidth / 2));
var yRedIndex = Math.floor((e.offsetY - 3 / 4 * boxHeight) / (boxHeight / 2));
if (xRedIndex % 2 === 0 && yRedIndex % 2 === 0) {
console.log("red");
console.log("Red grid x: " + (xRedIndex / 2));
console.log("Red grid y: " + (yRedIndex / 2));
} else {
console.log("gray");
var gridPosX = Math.floor(e.offsetX / boxWidth);
var gridPosY = Math.floor(e.offsetY / boxHeight);
grid[gridPosX][gridPosY] = 0;
}
Basically, you first subtract the initial gray area in the first column/row from the offsetX/Y, then see if the rest of the offsetX/Y contains an odd or even number of boxSize/2 (side length of red grid). An even number means the click is on red grids, otherwise it falls on the uncovered gray area.
Working fiddle: https://jsfiddle.net/mwxzgth6/1/

Bad zooming with pixi.js / WebGL

If you put some text on a pixi.js / WebGL canvas, and zoom on it, here is what happens : http://jsbin.com/qeqoneselelo/1/.
The result is bad : blurred / pixelized like if we were zooming on a bitmap.
Instead I would like to be able to zoom on this text, as if it was vector graphics (text actually is !), ie no blur at all, like here for example (you can zoom infinitely many times, no blur !) : http://s419743653.onlinehome.fr/things/test2.htm
How to do a proper zooming on text with pixi.js ? (or, if impossible with pixi.js, with another WebGL canvas javascript toolkit ?)
Here is the code I used (available only on http://jsbin.com/qeqoneselelo/1/) :
var text = new PIXI.Text("Hello World", {font:"50px Arial", fill:"black"});
var scrollArea = new PIXI.DisplayObjectContainer();
scrollArea.scale.x = 10;
scrollArea.scale.y = 10;
scrollArea.addChild(text);
stage.addChild(scrollArea);

As far as I know there is no way.
That's the whole point of pixi.js. It gets its speed by using bitmap sprites. The consequence of which you get the effect you see when you scale but you get super speed.
If you want smooth text you don't need pixi.js. Just you use the canvas API. Of course you'll give up some of the speed and other features of pixi.js but you'll get smooth text.
If you want to keep using pixi.js, one solution is to use LODs. Make multiple sprites with progressively larger text on them and as you zoom in use a progressively larger sprite with higher res text but with its individual scale so it keeps the same size. Unfortunately because font sizes are slightly unpredictable getting the sprites to transition flawlessly might require some trial and error.
var texts = [];
for (var ii = 0; ii < 15; ++ii) {
var text = new PIXI.Text("Hello World", {font: (ii * 10) +"px Arial", fill:"black"});
text.scale.x = 1 / (1 + ii);
text.scale.y = 1 / (1 + ii);
texts.push(text);
}
...
text = undefined;
function animate() {
var t = Date.now() * 0.001;
var scale = 1 + 14 * (Math.sin(t) * 0.5 + 0.5);
if (text) {
scrollArea.removeChild(text);
}
text = texts[Math.floor(scale)];
scrollArea.addChild(text);
scrollArea.scale.x = scale;
scrollArea.scale.y = scale;
renderer.render(stage);
requestAnimFrame(animate);
}
Here's an example
var stage = new PIXI.Stage(0xFFFFFF);
var renderer = PIXI.autoDetectRenderer(800, 600);
document.body.appendChild(renderer.view);
var texts = [];
for (var ii = 0; ii < 15; ++ii) {
var text = new PIXI.Text("Hello World", {font: (ii * 10) +"px Arial", fill:"black"});
text.scale.x = 1 / (1 + ii);
text.scale.y = 1 / (1 + ii);
texts.push(text);
}
var scrollArea = new PIXI.DisplayObjectContainer();
scrollArea.interactive = true;
scrollArea.buttonMode = true;
// scrollArea.addChild(text);
stage.addChild(scrollArea);
scrollArea.mousedown = function(data) {
data.originalEvent.preventDefault();
this.data = data;
this.dragging = true;
}
scrollArea.mouseup = scrollArea.mouseupoutside = function(data) {
this.dragging = false;
this.data = null;
}
scrollArea.mousemove = function(data) {
if (this.dragging) {
var newPos = this.data.getLocalPosition(this.parent);
this.position.x = newPos.x;
this.position.y = newPos.y;
}
}
text = undefined;
function animate() {
var t = Date.now() * 0.001;
var scale = 1 + 14 * (Math.sin(t) * 0.5 + 0.5);
if (text) {
scrollArea.removeChild(text);
}
text = texts[Math.floor(scale)];
scrollArea.addChild(text);
scrollArea.scale.x = scale;
scrollArea.scale.y = scale;
renderer.render(stage);
requestAnimFrame(animate);
}
animate();
<script src="//cdnjs.cloudflare.com/ajax/libs/pixi.js/1.6.1/pixi.js"></script>

Override Pixi's default renderer resolution of the Text instance by setting the resolution property:
var text = new PIXI.Text("Hello World", {font:"50px Arial", fill:"black"});
text.resolution = 8;

Grass like smoothing animation on beziercurve?

This is what I am trying to achieve--GRASS Animation(Desired animation)
This is where the project is standing currently --My hair animation
This is a more structurised code of the above code --My hair animation(by markE)--markE`s code of hair animation
PROBLEM:--
I am able to give movements to hairs but animation should be more like wavy grass like freeflowing.Its not very smooth now.What can be done to make the hairs flow in more natural manner.
Please provide me with a small sample if possible!!!
<canvas id="myCanvas" width="500" height="500" style="background-color: antiquewhite" ></canvas>
JAVASCRIPT
//mouse position
var x2=0;
var y2=0;
window.addEventListener("mousemove",function(){moving(event);init()},false)
//these variables define the bend in our bezier curve
var bend9=0;
var bend8=0;
var bend7=0;
var bend6=0;
var bend5=0;
var bend4=0;
var bend3=0;
var bend2=0;
var bend1=0;
//function to get the mouse cordinates
function moving(event) {
bend_value();//this function is defined below
try
{
x2 = event.touches[0].pageX;
y2 = event.touches[0].pageY;
}
catch (error)
{
try
{
x2 = event.clientX;
y2 = event.clientY;
}
catch (e)
{
}
}
try
{
event.preventDefault();
}
catch (e)
{
}
if(between(y2,204,237) && between(x2,115,272))
{
console.log("Xmove="+x2,"Ymove="+y2)
}
}
//function for declaring range of bezier curve
function between(val, min, max)
{
return val >= min && val <= max;
}
(function() {
hair = function() {
return this;
};
hair.prototype={
draw_hair:function(a,b,c,d,e,f,g,h){
var sx =136+a;//start position of curve.used in moveTo(sx,sy)
var sy =235+b;
var cp1x=136+c;//control point 1
var cp1y=222+d;
var cp2x=136+e;//control point 2
var cp2y=222+f;
var endx=136+g;//end points
var endy=210+h;
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
// context.clearRect(0, 0,500,500);
context.strokeStyle="grey";
context.lineWidth="8";
context.beginPath();
context.moveTo(sx,sy);
context.bezierCurveTo(cp1x,cp1y,cp2x,cp2y,endx,endy);
context.lineCap = 'round';
context.stroke();
// context.restore();
// context.save();
}
};
})();
//this function provides and calculate the bend on mousemove
function bend_value(){
var ref1=135;//this is ref point for hair or curve no 1
var ref2=150;//hair no 2 and so on
var ref3=165;
var ref4=180;
var ref5=195;
var ref6=210;
var ref7=225;
var ref8=240;
var ref9=255;
if(between(x2,115,270) && between(y2,205,236))
{
if(x2>=135 && x2<=145){bend1=(x2-ref1)*(2.2);}
if(x2<=135 && x2>=125){bend1=(x2-ref1)*(2.2);}
if(x2>=150 && x2<=160){bend2=(x2-ref2)*(2.2);}
if(x2<=150 && x2>=140){bend2=(x2-ref2)*(2.2);}
if(x2>=165 && x2<=175){bend3=(x2-ref3)*(2.2);}
if(x2<=165 && x2>=155){bend3=(x2-ref3)*(2.2);}
if(x2>=180 && x2<=190){bend4=(x2-ref4)*(2.2);}
if(x2<=180 && x2>=170){bend4=(x2-ref4)*(2.2);}
if(x2>=195 && x2<=205){bend5=(x2-ref5)*(2.2);}
if(x2<=195 && x2>=185){bend5=(x2-ref5)*(2.2);}
if(x2>=210 && x2<=220){bend6=(x2-ref6)*(2.2);}
if(x2<=210 && x2>=200){bend6=(x2-ref6)*(2.2);}
if(x2>=225 && x2<=235){bend7=(x2-ref7)*(2.2);}
if(x2<=225 && x2>=215){bend7=(x2-ref7)*(2.2);}
if(x2>=240 && x2<=250){bend8=(x2-ref8)*(2.2);}
if(x2<=240 && x2>=230){bend8=(x2-ref8)*(2.2);}
if(x2>=255 && x2<=265){bend9=(x2-ref9)*(2.2);}
if(x2<=255 && x2>=245){bend9=(x2-ref9)*(2.2);}
}
}
function init(){//this function draws each hair/curve
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var clear=context.clearRect(0, 0,500,500);
var save=context.save();
// /* console.log("bend2="+bend2)
// console.log("bend3="+bend3)
// console.log("bend4="+bend4)
// console.log("bend5="+bend5)
// console.log("bend6="+bend6)
// console.log("bend7="+bend7)
// console.log("bend8="+bend8)
// console.log("bend9="+bend9)*/
hd1 = new hair();//hd1 stands for hair draw 1.this is an instance created for drawing hair no 1
clear;
hd1.draw_hair(0,0,0,0,0,0,0+bend1/2,0);//these parameters passed to function drawhair and bend is beint retrieved from function bend_value()
save;
hd2 = new hair();
clear;
hd2.draw_hair(15,0,15,0,15,0,15+bend2/2,0);
save;
hd3 = new hair();
clear;
hd3.draw_hair(30,0,30,0,30,0,30+bend3/2,0);
save;
hd4 = new hair();
clear;
hd4.draw_hair(45,0,45,0,45,0,45+bend4/2,0);
save;
hd5 = new hair();
clear;
hd5.draw_hair(60,0,60,0,60,0,60+bend5/2,0);
save;
}
window.onload = function() {
init();
disableSelection(document.body)
}
function disableSelection(target){
if (typeof target.onselectstart!="undefined") //IE
target.onselectstart=function(){return false}
else if (typeof target.style.MozUserSelect!="undefined") //Firefox
target.style.MozUserSelect="none"
else //All other ie: Opera
target.onmousedown=function(){return false}
target.style.cursor = "default"
}

Update: I'm currently adjusting the code to produce the requested result and commenting it.
(function() { // The code is encapsulated in a self invoking function to isolate the scope
"use strict";
// The following lines creates shortcuts to the constructors of the Box2D types used
var B2Vec2 = Box2D.Common.Math.b2Vec2,
B2BodyDef = Box2D.Dynamics.b2BodyDef,
B2Body = Box2D.Dynamics.b2Body,
B2FixtureDef = Box2D.Dynamics.b2FixtureDef,
B2Fixture = Box2D.Dynamics.b2Fixture,
B2World = Box2D.Dynamics.b2World,
B2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape,
B2RevoluteJoint = Box2D.Dynamics.Joints.b2RevoluteJoint,
B2RevoluteJointDef = Box2D.Dynamics.Joints.b2RevoluteJointDef;
// This makes sure that there is a method to request a callback to update the graphics for next frame
window.requestAnimationFrame =
window.requestAnimationFrame || // According to the standard
window.mozRequestAnimationFrame || // For mozilla
window.webkitRequestAnimationFrame || // For webkit
window.msRequestAnimationFrame || // For ie
function (f) { window.setTimeout(function () { f(Date.now()); }, 1000/60); }; // If everthing else fails
var world = new B2World(new B2Vec2(0, -10), true), // Create a world with gravity
physicalObjects = [], // Maintain a list of the simulated objects
windInput = 0, // The input for the wind in the current frame
wind = 0, // The current wind (smoothing the input values + randomness)
STRAW_COUNT = 10, // Number of straws
GRASS_RESET_SPEED = 2, // How quick should the straw reset to its target angle
POWER_MOUSE_WIND = 120, // How much does the mouse affect the wind
POWER_RANDOM_WIND = 180; // How much does the randomness affect the wind
// GrassPart is a prototype for a piece of a straw. It has the following properties
// position: the position of the piece
// density: the density of the piece
// target: the target angle of the piece
// statik: a boolean stating if the piece is static (i.e. does not move)
function GrassPart(position, density, target, statik) {
this.width = 0.05;
this.height = 0.5;
this.target = target;
// To create a physical body in Box2D you have to setup a body definition
// and create at least one fixture.
var bdef = new B2BodyDef(), fdef = new B2FixtureDef();
// In this example we specify if the body is static or not (the grass roots
// has to be static to keep the straw in its position), and its original
// position.
bdef.type = statik? B2Body.b2_staticBody : B2Body.b2_dynamicBody;
bdef.position.SetV(position);
// The fixture of the piece is a box with a given density. The negative group index
// makes sure that the straws does not collide.
fdef.shape = new B2PolygonShape();
fdef.shape.SetAsBox(this.width/2, this.height/2);
fdef.density = density;
fdef.filter.groupIndex = -1;
// The body and fixture is created and added to the world
this.body = world.CreateBody(bdef);
this.body.CreateFixture(fdef);
}
// This method is called for every frame of animation. It strives to reset the original
// angle of the straw (the joint). The time parameter is unused here but contains the
// current time.
GrassPart.prototype.update = function (time) {
if (this.joint) {
this.joint.SetMotorSpeed(GRASS_RESET_SPEED*(this.target - this.joint.GetJointAngle()));
}
};
// The link method is used to link the pieces of the straw together using a joint
// other: the piece to link to
// torque: the strength of the joint (stiffness)
GrassPart.prototype.link = function(other, torque) {
// This is all Box2D specific. Look it up in the manual.
var jdef = new B2RevoluteJointDef();
var p = this.body.GetWorldPoint(new B2Vec2(0, 0.5)); // Get the world coordinates of where the joint
jdef.Initialize(this.body, other.body, p);
jdef.maxMotorTorque = torque;
jdef.motorSpeed = 0;
jdef.enableMotor = true;
// Add the joint to the world
this.joint = world.CreateJoint(jdef);
};
// A prototype for a straw of grass
// position: the position of the bottom of the root of the straw
function Grass(position) {
var pos = new B2Vec2(position.x, position.y);
var angle = 1.2*Math.random() - 0.6; // Randomize the target angle
// Create three pieces, the static root and to more, and place them in line.
// The second parameter is the stiffness of the joints. It controls how the straw bends.
// The third is the target angle and different angles are specified for the pieces.
this.g1 = new GrassPart(pos, 1, angle/4, true); // This is the static root
pos.Add(new B2Vec2(0, 1));
this.g2 = new GrassPart(pos, 0.75, angle);
pos.Add(new B2Vec2(0, 1));
this.g3 = new GrassPart(pos, 0.5);
// Link the pieces into a straw
this.g1.link(this.g2, 20);
this.g2.link(this.g3, 3);
// Add the pieces to the list of simulate objects
physicalObjects.push(this.g1);
physicalObjects.push(this.g2);
physicalObjects.push(this.g3);
}
Grass.prototype.draw = function (context) {
var p = new B2Vec2(0, 0.5);
var p1 = this.g1.body.GetWorldPoint(p);
var p2 = this.g2.body.GetWorldPoint(p);
var p3 = this.g3.body.GetWorldPoint(p);
context.strokeStyle = 'grey';
context.lineWidth = 0.4;
context.lineCap = 'round';
context.beginPath();
context.moveTo(p1.x, p1.y);
context.quadraticCurveTo(p2.x, p2.y, p3.x, p3.y);
context.stroke();
};
var lastX, grass = [], context = document.getElementById('canvas').getContext('2d');
function updateGraphics(time) {
window.requestAnimationFrame(updateGraphics);
wind = 0.95*wind + 0.05*(POWER_MOUSE_WIND*windInput + POWER_RANDOM_WIND*Math.random() - POWER_RANDOM_WIND/2);
windInput = 0;
world.SetGravity(new B2Vec2(wind, -10));
physicalObjects.forEach(function(obj) { if (obj.update) obj.update(time); });
world.Step(1/60, 8, 3);
world.ClearForces();
context.clearRect(0, 0, context.canvas.width, context.canvas.height);
context.save();
context.translate(context.canvas.width/2, context.canvas.height/2);
context.scale(context.canvas.width/20, -context.canvas.width/20);
grass.forEach(function (o) { o.draw(context); });
context.restore();
}
document.getElementsByTagName('body')[0].addEventListener("mousemove", function (e) {
windInput = Math.abs(lastX - e.x) < 200? 0.2*(e.x - lastX) : 0;
lastX = e.x;
});
var W = 8;
for (var i = 0; i < STRAW_COUNT; i++) {
grass.push(new Grass(new B2Vec2(W*(i/(STRAW_COUNT-1))-W/2, -1)));
}
window.requestAnimationFrame(updateGraphics);
})();

Waving grass algorithm
UPDATE
I made a reduced update to better meet what I believe is your requirements. To use mouse you just calculate the angle between the mouse point and the strain root and use that for new angle in the update.
I have incorporated a simple mouse-move sensitive approach which makes the strains "point" towards the mouse, but you can add random angles to this as deltas and so forth. Everything you need is as said in the code - adjust as needed.
New fiddle (based on previous with a few modifications):
http://jsfiddle.net/AbdiasSoftware/yEwGc/
Image showing 150 strains being simulated.
Grass simulation demo:
http://jsfiddle.net/AbdiasSoftware/5z89V/
This will generate a nice realistic looking grass field. The demo has 70 grass rendered (works best in Chrome or just lower the number for Firefox).
The code is rather simple. It consists of a main object (grassObj) which contains its geometry as well as functions to calculate the angles, segments, movements and so forth. I'll show this in detail below.
First some inits that are accessed globally by the functions:
var numOfGrass = 70, /// number of grass strains
grass,
/// get canvas context
ctx = canvas.getContext('2d'),
w = canvas.width,
h = canvas.height,
/// we use an animated image for the background
/// The image also clears the canvas for each loop call
/// I rendered the clouds in a 3D software.
img = document.createElement('img'),
ix = 0, /// background image position
iw = -1; /// used for with and initial for flag
/// load background image, use it whenever it's ready
img.onload = function() {iw = this.width}
img.src = 'http://i.imgur.com/zzjtzG7.jpg';
The heart - grassObj
The main object as mentioned above is the grassObj:
function grassObj(x, y, seg1, seg2, maxAngle) {
/// exposed properties we need for rendering
this.x = x; /// bottom position of grass
this.y = y;
this.seg1 = seg1; /// segments of grass
this.seg2 = seg2;
this.gradient = getGradient(Math.random() * 50 + 50, 100 * Math.random() + 170);
this.currentAngle; ///current angle that will be rendered
/// internals used for calculating new angle, goal, difference and speed
var counter, /// counter between 0-1 for ease-in/out
delta, /// random steps in the direction goal rel. c.angle.
angle, /// current angle, does not change until goal is reached
diff, /// diff between goal and angle
goal = getAngle();
/// internal: returns an angel between 0 and maxAngle
function getAngle() {
return maxAngle * Math.random();
}
/// ease in-out function
function easeInOut(t) {
return t < 0.5 ? 4 * t * t * t : (t-1) * (2 * t - 2) * (2 * t - 2) + 1;
}
/// sets a new goal for grass to move to. Does the main calculations
function newGoal() {
angle = goal; /// set goal as new angle when reached
this.currentAngle = angle;
goal = getAngle(); /// get new goal
diff = goal - angle; /// calc diff
counter = 0; /// reset counter
delta = (4 * Math.random() + 1) / 100;
}
/// creates a gradient for this grass to increase realism
function getGradient(min, max) {
var g = ctx.createLinearGradient(0, 0, 0, h);
g.addColorStop(1, 'rgb(0,' + parseInt(min) + ', 0)');
g.addColorStop(0, 'rgb(0,' + parseInt(max) + ', 0)');
return g;
}
/// this is called from animation loop. Counts and keeps tracks of
/// current position and calls new goal when current goal is reached
this.update = function() {
/// count from 0 to 1 with random delta value
counter += delta;
/// if counter passes 1 then goal is reached -> get new goal
if (counter > 1) {
newGoal();
return;
}
/// ease in/out function
var t = easeInOut(counter);
/// update current angle for render
this.currentAngle = angle + t * diff;
}
/// init
newGoal();
return this;
}
Grass generator
We call makeGrass to generate grass at random positions, random heights and with random segments. The function is called with number of grass to render, width and height of canvas to fill and a variation variable in percent (0 - 1 float).
The single grass consist only of four points in total. The two middle points are spread about 1/3 and 2/3 of the total height with a little variation to break pattern. The points when rendered, are smoother using a cardinal spline with full tension to make the grass look smooth.
function makeGrass(numOfGrass, width, height, hVariation) {
/// setup variables
var x, y, seg1, seg2, angle,
hf = height * hVariation, /// get variation
i = 0,
grass = []; /// array to hold the grass
/// generate grass
for(; i < numOfGrass; i++) {
x = width * Math.random(); /// random x position
y = height - hf * Math.random(); /// random height
/// break grass into 3 segments with random variation
seg1 = y / 3 + y * hVariation * Math.random() * 0.1;
seg2 = (y / 3 * 2) + y * hVariation * Math.random() * 0.1;
grass.push(new grassObj(x, y, seg1, seg2, 15 * Math.random() + 50));
}
return grass;
}
Render
The render function just loops through the objects and updates the current geometry:
function renderGrass(ctx, grass) {
/// local vars for animation
var len = grass.length,
i = 0,
gr, pos, diff, pts, x, y;
/// renders background when loaded
if (iw > -1) {
ctx.drawImage(img, ix--, 0);
if (ix < -w) {
ctx.drawImage(img, ix + iw, 0);
}
if (ix <= -iw) ix = 0;
} else {
ctx.clearRect(0, 0, w, h);
}
/// loops through the grass object and renders current state
for(; gr = grass[i]; i++) {
x = gr.x;
y = gr.y;
ctx.beginPath();
/// calculates the end-point based on length and angle
/// Angle is limited [0, 60] which we add 225 deg. to get
/// it upwards. Alter 225 to make grass lean more to a side.
pos = lineToAngle(ctx, x, h, y, gr.currentAngle + 225);
/// diff between end point and root point
diff = (pos[0] - x)
pts = [];
/// starts at bottom, goes to top middle and then back
/// down with a slight offset to make the grass
pts.push(x); /// first couple at bottom
pts.push(h);
/// first segment 1/4 of the difference
pts.push(x + (diff / 4));
pts.push(h - gr.seg1);
/// second segment 2/3 of the difference
pts.push(x + (diff / 3 * 2));
pts.push(h - gr.seg2);
pts.push(pos[0]); /// top point
pts.push(pos[1]);
/// re-use previous data, but go backward down to root again
/// with a slight offset
pts.push(x + (diff / 3 * 2) + 10);
pts.push(h - gr.seg2);
pts.push(x + (diff / 4) + 12);
pts.push(h - gr.seg1 + 10);
pts.push(x + 15); /// end couple at bottom
pts.push(h);
/// smooth points (extended context function, see demo)
ctx.curve(pts, 0.8, 5);
ctx.closePath();
/// fill grass with its gradient
ctx.fillStyle = gr.gradient;
ctx.fill();
}
}
Animate
The main loop where we animate everything:
function animate() {
/// update each grass objects
for(var i = 0;i < grass.length; i++) grass[i].update();
/// render them
renderGrass(ctx, grass);
/// loop
requestAnimationFrame(animate);
}
And that's all there is to it for this version.

Darn! Late to the party...
But LOTS of neat answers here -- I'm upvoting all !
Anyway, here's my idea:
Here's code and a Fiddle: http://jsfiddle.net/m1erickson/MJjHz/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.1/jquery-ui.js"></script>
<style>
body { font-family: arial; padding:15px; }
canvas { border: 1px solid red;}
input[type="text"]{width:35px;}
</style>
</head>
<body>
<p>Move mouse across hairs</p>
<canvas height="100" width="250" id="canvas"></canvas>
<script>
$(function() {
var canvas=document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var cHeight=canvas.height;
var showControls=false;
var lastMouseX=0;
// preset styling CONSTANTS
var SWAY=.55; // max endpoint sway from center
var C1Y=.40; // fixed Y of cp#1
var C2SWAY=.20 // max cp#2 sway from center
var C2Y=.75; // fixed Y of cp#2
var YY=20; // max height of ellipse at top of hair
var PIPERCENT=Math.PI/100;
var hairs=[];
// create hairs
var newHairX=40;
var hairCount=20;
for(var i=0;i<hairCount;i++){
var randomLength=50+parseInt(Math.random()*5);
addHair(newHairX+(i*8),randomLength);
}
function addHair(x,length){
hairs.push({
x:x,
length:length,
left:0,
right:0,
top:0,
s:{x:0,y:0},
c1:{x:0,y:0},
c2:{x:0,y:0},
e:{x:0,y:0},
isInMotion:false,
currentX:0
});
}
for(var i=0;i<hairs.length;i++){
var h=hairs[i];
setHairPointsFixed(h);
setHairPointsPct(h,50);
draw(h);
}
function setHairPointsFixed(h){
h.s.x = h.x;
h.s.y = cHeight;
h.c1.x = h.x;
h.c1.y = cHeight-h.length*C1Y;
h.c2.y = cHeight-h.length*C2Y;
h.top = cHeight-h.length;
h.left = h.x-h.length*SWAY;
h.right = h.x+h.length*SWAY;
}
function setHairPointsPct(h,pct){
// endpoint
var a=Math.PI+PIPERCENT*pct;
h.e.x = h.x - ((h.length*SWAY)*Math.cos(a));
h.e.y = h.top + (YY*Math.sin(a));
// controlpoint#2
h.c2.x = h.x + h.length*(C2SWAY*2*pct/100-C2SWAY);
}
//////////////////////////////
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// draw this frame based on mouse moves
ctx.clearRect(0,0,canvas.width,canvas.height);
for(var i=0;i<hairs.length;i++){
hairMoves(hairs[i],mouseX,mouseY);
}
lastMouseX=mouseX;
}
$("#canvas").mousemove(function(e){handleMouseMove(e);});
function hairMoves(h,mouseX,mouseY){
// No hair movement if not touching hair
if(mouseY<cHeight-h.length-YY){
if(h.isInMotion){
h.isInMotion=false;
setHairPointsPct(h,50);
}
draw(h);
return;
}
// No hair movement if too deep in hair
if(mouseY>h.c1.y){
draw(h);
return;
}
//
var pct=50;
if(mouseX>=h.left && mouseX<=h.right){
if(h.isInMotion){
var pct=-(mouseX-h.right)/(h.right-h.left)*100;
setHairPointsPct(h,pct);
draw(h);
}else{
// if hair is at rest
// but mouse has just contacted hair
// set hair in motion
if( (lastMouseX<=h.x && mouseX>=h.x )
||(lastMouseX>=h.x && mouseX<=h.x )
){
h.isInMotion=true;
var pct=-(mouseX-h.right)/(h.right-h.left)*100;
}
setHairPointsPct(h,pct);
draw(h);
}
}else{
if(h.isInMotion){
h.isInMotion=false;
setHairPointsPct(h,50);
};
draw(h);
}
}
function dot(pt,color){
ctx.beginPath();
ctx.arc(pt.x,pt.y,5,0,Math.PI*2,false);
ctx.closePath();
ctx.fillStyle=color;
ctx.fill();
}
function draw(h){
ctx.beginPath();
ctx.moveTo(h.s.x,h.s.y);
ctx.bezierCurveTo(h.c1.x,h.c1.y,h.c2.x,h.c2.y,h.e.x,h.e.y);
ctx.strokeStyle="orange";
ctx.lineWidth=3;
ctx.stroke();
if(showControls){
dot(h.s,"green");
dot(h.c1,"red");
dot(h.c2,"blue");
dot(h.e,"purple");
ctx.beginPath();
ctx.rect(h.left,h.top-YY,(h.right-h.left),h.length*(1-C1Y)+YY)
ctx.lineWidth=1;
ctx.strokeStyle="lightgray";
ctx.stroke();
}
}
});
</script>
</body>
</html>

Here is a simple hair simulation that seems to be what you are looking for. The basic idea is to draw a bezier curve (in this case I use two curves to provide thickness for the hair). The curve will have a base, a bending point, and a tip. I set the bending point halfway up the hair. The tip of the hair will rotate about the axis of the base of the hair in response to mouse movement.
Place this code in a script tag below the canvas element declaration.
function Point(x, y) {
this.x = x;
this.y = y;
}
function Hair( ) {
this.height = 100; // hair height
this.baseWidth = 3; // hair base width.
this.thickness = 1.5; // hair thickness
this.points = {};
this.points.base1 = new Point(Math.random()*canvas.width, canvas.height);
// The point at which the hair will bend. I set it to the middle of the hair, but you can adjust this.
this.points.bendPoint1 = new Point(this.points.base1.x-this.thickness, this.points.base1.y - this.height / 2)
this.points.bendPoint2 = new Point(this.points.bendPoint1.x, this.points.bendPoint1.y-this.thickness); // complement of bendPoint1 - we use this because the hair has thickness
this.points.base2 = new Point(this.points.base1.x + this.baseWidth, this.points.base1.y) // complement of base1 - we use this because the hair has thickness
}
Hair.prototype.paint = function(mouseX, mouseY, direction) {
ctx.save();
// rotate the the tip of the hair
var tipRotationAngle = Math.atan(Math.abs(this.points.base1.y - mouseY)/Math.abs(this.points.base1.x - mouseX));
// if the mouse is on the other side of the hair, adjust the angle
if (mouseX < this.points.base1.x) {
tipRotationAngle = Math.PI - tipRotationAngle;
}
// if the mouse isn't close enough to the hair, it shouldn't affect the hair
if (mouseX < this.points.base1.x - this.height/2 || mouseX > this.points.base1.x + this.height/2 || mouseY < this.points.base1.y - this.height || mouseY > this.points.base1.y) {
tipRotationAngle = Math.PI/2; // 90 degrees, which means the hair is straight
}
// Use the direction of the mouse to as a lazy way to simulate the direction the hair should bend.
// Note that in real life, the direction that the hair should bend has nothing to do with the direction of motion. It actually depends on which side of the hair the force is being applied.
// Figuring out which side of the hair the force is being applied is a little tricky, so I took this shortcut.
// If you run your finger along a comb quickly, this approximation will work. However if you are in the middle of the comb and slowly change direction, you will notice that the force is still applied in the opposite direction of motion as you slowly back off the set of tines.
if ((mouseX < this.points.base1.x && direction == 'right') || (mouseX > this.points.base1.x && direction == 'left')) {
tipRotationAngle = Math.PI/2; // 90 degrees, which means the hair is straight
}
var tipPoint = new Point(this.points.base1.x + this.baseWidth + this.height*Math.cos(tipRotationAngle), this.points.base1.y - this.height*Math.sin(tipRotationAngle));
ctx.beginPath();
ctx.moveTo(this.points.base1.x, this.points.base1.y); // start at the base
ctx.bezierCurveTo(this.points.base1.x, this.points.base1.y, this.points.bendPoint1.x, this.points.bendPoint1.y, tipPoint.x, tipPoint.y); // draw a curve to the tip of the hair
ctx.bezierCurveTo(tipPoint.x, tipPoint.y, this.points.bendPoint2.x, this.points.bendPoint2.y, this.points.base2.x, this.points.base2.y); // draw a curve back down to the base using the complement points since the hair has thickness.
ctx.closePath(); // complete the path so we have a shape that we can fill with color
ctx.fillStyle='rgb(0,0,0)';
ctx.fill();
ctx.restore();
}
// I used global variables to keep the example simple, but it is generally best to avoid using global variables
window.canvas = document.getElementById('myCanvas');
window.ctx = canvas.getContext('2d');
ctx.fillStyle = 'rgb(200,255,255)'; // background color
window.hair = [];
window.prevClientX = 0;
for (var i = 0; i < 100; i++) {
hair.push(new Hair());
}
// initial draw
ctx.fillRect(0,0,canvas.width,canvas.height); // clear canvas
for (var i = 0; i < hair.length; i++) {
hair[i].paint(0, 0, 'right');
}
window.onmousemove = function(e) {
ctx.fillRect(0,0,canvas.width,canvas.height); // clear canvas
for (var i = 0; i < hair.length; i++) {
hair[i].paint(e.clientX, e.clientY, e.clientX > window.prevClientX ? 'right' : 'left');
}
window.prevClientX = e.clientX;
}

Made this some time ago, might be useful to some people. Just adjust the variables at the beginning of the code with the values that fits your wishes:
...
Mheight = 1;
height = 33;
width = 17;
distance = 10;
randomness = 14;
angle = Math.PI / 2;
...
Also on http://lucasm0ta.github.io/JsGrass/

bounce check function for HTML5 canvas fails at corners

I have a HTML5 canvas that generates a bouncing box every time you click on it. The box array stores the x-value, y-value, x-velocity, and y-velocity of each box created. The box will travel in a random direction at first and will bounce of the sides of the canvas but if it hits a corner the box dissappears instead of bouncing back. EDIT: I answered my own question noticing that the soundY and soundX functions were causing the problem.
var box = new Array();
var width = window.innerWidth;
var height = window.innerHeight;
var field = document.getElementById('canvas');
field.width = width;
field.height = height;
field.ctx = field.getContext('2d');
field.ctx.strokeStyle = 'rgba(255,255,255,1)';
setInterval('redraw()', 200);
addEventListener('click', createBox, false);
function createBox(e) { // this box will always fail collision detection at the upper-left corner
box.push(100); // x-value is normally mouse position
box.push(100); // y-value is normally mouse position
box.push(-5); // x-speed is normally random
box.push(-5); // y-speed is normally random
}
function redraw() {
field.ctx.clearRect(0,0,width,height);
for(var i = 0; i < box.length; i+=4) {
if(box[i] < 0) { box[i+2] *= -1; soundY(box[i+1]); } // parameter of soundY is less than 0
else if(box[i] > width) { box[i+2] *= -1; soundY(box[i+1]); } // which is invalid and causes this to break
if(box[i+1] < 0) { box[i+3] *= -1; soundX(box[i]); }
else if(box[i+1] > height) { box[i+3] *= -1; soundX(box[i]); }
box[i] += box[i+2];
box[i+1] += box[i+3];
field.ctx.strokeRect(box[i], box[i+1], 4, 4);
}
}
function soundX(num) {
// play a sound file based on a number between 0 and width
}
function soundY(num) {
// play a sound file based on a number between 0 and height
}

The only way I could recreate the problem was by generating the box in one of the corners so that with the right x and y velocity the box was initially created outside the bounds of the canvas. When that happens, the inversion of the velocity isn't enough to bring the item back in bounds and so on the next frame the velocity is inverted again (and so on).
I think this might solve your problem:
var boxes = [];
var boxSize = 4;
var width = window.innerWidth;
var height = window.innerHeight;
var field = document.getElementById('canvas');
function redraw() {
field.ctx.clearRect(0, 0, width, height);
var box;
for (var i = 0; i < boxes.length; i++) {
box = boxes[i];
field.ctx.strokeRect(box.x, box.y, boxSize, boxSize);
if (box.x < 0) {
box.x = 0;
box.dx *= -1;
} else if (box.x > width - boxSize) {
box.x = width - boxSize;
box.dx *= -1;
}
if (box.y < 0) {
box.y = 0;
box.dy *= -1;
} else if (box.y > height - boxSize) {
box.y = height - boxSize;
box.dy *= -1;
}
box.x += box.dx;
box.y += box.dy;
}
}
field.width = width;
field.height = height;
field.ctx = field.getContext('2d');
field.ctx.strokeStyle = 'rgba(0,0,0,1)';
setInterval(redraw, 200);
addEventListener('click', createBox, false);
function createBox(e) {
boxes.push({
x: e.clientX - 10,
y: e.clientY - 10, // arbitrary offset to place the new box under the mouse
dx: Math.floor(Math.random() * 8 - boxSize),
dy: Math.floor(Math.random() * 8 - boxSize)
});
}
I fixed a few errors in your code and made some changes to make it a bit more readable (I hope). Most importantly, I extended your collision detection so that it resets the coordinates of the box to the bounds of your canvas should the velocity take it outside.
Created a jsfiddle which might be handy if further discussion is needed.

It was additional code (see edit) that I left out assuming it was unrelated to the issue, but removing the code solved the problem as it appears this use-case would cause an invalid input in this part of the code.