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JavaScript Canvas how to stroke special text [duplicate]

I'm drawing an image onto a canvas using drawImage. It's a PNG that is surrounded by transparent pixels, like this:
How can I add a solid-colored border to the visible part of that image on the canvas? To clarify: I don't want a rectangle that surrounds the image's bounding box. The border should go around the grass patch.
I did consider using shadows, but I don't really want a glowing border, I want a solid one.

A bit late, but just draw the image offset which is much faster than analyzing the edges:
var ctx = canvas.getContext('2d'),
img = new Image;
img.onload = draw;
img.src = "http://i.stack.imgur.com/UFBxY.png";
function draw() {
var dArr = [-1,-1, 0,-1, 1,-1, -1,0, 1,0, -1,1, 0,1, 1,1], // offset array
s = 2, // thickness scale
i = 0, // iterator
x = 5, // final position
y = 5;
// draw images at offsets from the array scaled by s
for(; i < dArr.length; i += 2)
ctx.drawImage(img, x + dArr[i]*s, y + dArr[i+1]*s);
// fill with color
ctx.globalCompositeOperation = "source-in";
ctx.fillStyle = "red";
ctx.fillRect(0,0,canvas.width, canvas.height);
// draw original image in normal mode
ctx.globalCompositeOperation = "source-over";
ctx.drawImage(img, x, y);
}
<canvas id=canvas width=500 height=500></canvas>

==> ==>
First, attributions:
As #Philipp says, you'll need to analyze pixel data to get your outline border.
You can use the "Marching Squares" algorithm to determine which transparent pixels border the non-transparent grass pixels. You can read more about the Marching Squares algorithm here: http://en.wikipedia.org/wiki/Marching_squares
Michael Bostock has a very nice plugin version of Marching Squares in his d3 data visualization application (IMHO, d3 is the best open-source data visualization program available). Here's a link to the plugin: https://github.com/d3/d3-plugins/tree/master/geom/contour
You can outline the border of your grass image like this:
Draw your image on the canvas
Grab the image's pixel data using .getImageData
Configure the plug-in to look for transparent pixels bordering opaque pixels
// This is used by the marching ants algorithm
// to determine the outline of the non-transparent
// pixels on the image using pixel data
var defineNonTransparent=function(x,y){
var a=data[(y*cw+x)*4+3];
return(a>20);
}
Call the plugin which returns a set of points which outline the border of your image.
// call the marching ants algorithm
// to get the outline path of the image
// (outline=outside path of transparent pixels
points=geom.contour(defineNonTransparent);
Use the set of points to draw a path around your image.
Here's annotated code and a Demo:
// Marching Squares Edge Detection
// this is a "marching ants" algorithm used to calc the outline path
(function() {
// d3-plugin for calculating outline paths
// License: https://github.com/d3/d3-plugins/blob/master/LICENSE
//
// Copyright (c) 2012-2014, Michael Bostock
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//* Redistributions of source code must retain the above copyright notice, this
// list of conditions and the following disclaimer.
//* Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
//* The name Michael Bostock may not be used to endorse or promote products
// derived from this software without specific prior written permission.
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL MICHAEL BOSTOCK BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
geom = {};
geom.contour = function(grid, start) {
var s = start || d3_geom_contourStart(grid), // starting point
c = [], // contour polygon
x = s[0], // current x position
y = s[1], // current y position
dx = 0, // next x direction
dy = 0, // next y direction
pdx = NaN, // previous x direction
pdy = NaN, // previous y direction
i = 0;
do {
// determine marching squares index
i = 0;
if (grid(x-1, y-1)) i += 1;
if (grid(x, y-1)) i += 2;
if (grid(x-1, y )) i += 4;
if (grid(x, y )) i += 8;
// determine next direction
if (i === 6) {
dx = pdy === -1 ? -1 : 1;
dy = 0;
} else if (i === 9) {
dx = 0;
dy = pdx === 1 ? -1 : 1;
} else {
dx = d3_geom_contourDx[i];
dy = d3_geom_contourDy[i];
}
// update contour polygon
if (dx != pdx && dy != pdy) {
c.push([x, y]);
pdx = dx;
pdy = dy;
}
x += dx;
y += dy;
} while (s[0] != x || s[1] != y);
return c;
};
// lookup tables for marching directions
var d3_geom_contourDx = [1, 0, 1, 1,-1, 0,-1, 1,0, 0,0,0,-1, 0,-1,NaN],
d3_geom_contourDy = [0,-1, 0, 0, 0,-1, 0, 0,1,-1,1,1, 0,-1, 0,NaN];
function d3_geom_contourStart(grid) {
var x = 0,
y = 0;
// search for a starting point; begin at origin
// and proceed along outward-expanding diagonals
while (true) {
if (grid(x,y)) {
return [x,y];
}
if (x === 0) {
x = y + 1;
y = 0;
} else {
x = x - 1;
y = y + 1;
}
}
}
})();
//////////////////////////////////////////
// canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
// checkbox to show/hide the original image
var $showImage=$("#showImage");
$showImage.prop('checked', true);
// checkbox to show/hide the path outline
var $showOutline=$("#showOutline");
$showOutline.prop('checked', true);
// an array of points that defines the outline path
var points;
// pixel data of this image for the defineNonTransparent
// function to use
var imgData,data;
// This is used by the marching ants algorithm
// to determine the outline of the non-transparent
// pixels on the image
var defineNonTransparent=function(x,y){
var a=data[(y*cw+x)*4+3];
return(a>20);
}
// load the image
var img=new Image();
img.crossOrigin="anonymous";
img.onload=function(){
// draw the image
// (this time to grab the image's pixel data
ctx.drawImage(img,canvas.width/2-img.width/2,canvas.height/2-img.height/2);
// grab the image's pixel data
imgData=ctx.getImageData(0,0,canvas.width,canvas.height);
data=imgData.data;
// call the marching ants algorithm
// to get the outline path of the image
// (outline=outside path of transparent pixels
points=geom.contour(defineNonTransparent);
ctx.strokeStyle="red";
ctx.lineWidth=2;
$showImage.change(function(){ redraw(); });
$showOutline.change(function(){ redraw(); });
redraw();
}
img.src="http://i.imgur.com/QcxIJxa.png";
// redraw the canvas
// user determines if original-image or outline path or both are visible
function redraw(){
// clear the canvas
ctx.clearRect(0,0,canvas.width,canvas.height);
// draw the image
if($showImage.is(':checked')){
ctx.drawImage(img,canvas.width/2-img.width/2,canvas.height/2-img.height/2);
}
// draw the path (consisting of connected points)
if($showOutline.is(':checked')){
// draw outline path
ctx.beginPath();
ctx.moveTo(points[0][0],points[0][4]);
for(var i=1;i<points.length;i++){
var point=points[i];
ctx.lineTo(point[0],point[1]);
}
ctx.closePath();
ctx.stroke();
}
}
body{ background-color: ivory; }
canvas{border:1px solid red;}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="checkbox" id="showImage" />Show Image<br>
<input type="checkbox" id="showOutline" />Show Outline Path<br>
<canvas id="canvas" width=300 height=450></canvas>

I was looking for a way to do this and it seems there are only laborious solutions.
I came up with a little workaround using shadows and a loop to display them all around the image:
// Shadow color and blur
// To get a blurry effect use rgba() with a low opacity as it will be overlaid
context.shadowColor = "red";
context.shadowBlur = 0;
// X offset loop
for(var x = -2; x <= 2; x++){
// Y offset loop
for(var y = -2; y <= 2; y++){
// Set shadow offset
context.shadowOffsetX = x;
context.shadowOffsetY = y;
// Draw image with shadow
context.drawImage(img, left, top, width, height);
}
}

Animated footsteps in html svg

Is it possible to create small animated footsteps in HTML SVG or Canvas. I am just starting out with these technologies, and it is very much necessary for a small project i am working on.
My current idea was to create and use a "gif" of animated footsteps. But i would like to know if it can be achieved in any other way through HTML/CSS/JS
PS : The footsteps i keep mentioning should be similar to the footsteps that appear on the "Marauders Map" in harry potter Movies.
Thanks for any help

Walk about.
I have never seen the movie you talk about so I am guessing what you are after.
To do it on the canvas is easy, and I am more than happy to write an example of how it's done for you.
Draw a foot
You need an image of a foot, or some way to draw a foot. I used a set of shapes I happened to have to draw a foot.
Then create a function that draws the foot at a location and in a direction. You also need to specify if its a left or right step.
If you have a foot image you want to use just replace the code in the drawFoot function after the comment // draw the foot... with ctx.drawImage(footImage,-footImage.width / 2, -footImage.height / 2);
You may have to scale the image, and the foot image should be of the left foot toes pointing to the right of screen
Path
The path to walk along is just an array of points, [x,y,x,y...] in order that define the path to travel along, They are just a guide as each foot step is a fixed distance apart and the points can be any distance appart.
Animate
Each step is some time apart (demo is 500ms or half a second). I use setTimeout to call the function to draw the next step. When the function is called I find the next position on the path and draw the next foot step.
When the walk has gone past the end of the path I clear the canvas and restart.
Demo
It's self explanatory in the demo. I track two positions on the path, one is where the foot step is, and the other is just ahead and used to get the direction the foot should be drawn.
The function that gets the distance along the path is recursive because the path points are not uniform in distance apart, so may have to skip line segments if the distance to travel is greater than the current or next or few line segments.
// create a canvas and add it to the DOM
var createImage=function(w,h){var i=document.createElement("canvas");i.width=w;i.height=h;i.ctx=i.getContext("2d");return i;}
var canvas = createImage(1024,512);
var ctx = canvas.ctx;
document.body.appendChild(canvas);
// shapes for the foot. Left foot
const foot = [
[50.86759744022923,-21.798383338076917,54.16000854997335,-23.917474843549847,57.778065334829385,-25.310771525314685,61.579706305344985,-24.823762596975733,65.0168431653022,-22.69100336700319,65.22736925598322,-19.777045647294724,63.09649708656406,-16.826669881834157,59.66512409715465,-15.356252875428147,56.12216018899968,-14.92259970211097,52.06404407417057,-16.231839553378,50.2945579442406,-18.938589556263246],
[60.12039562389232,-12.45714817900668,63.92394094034509,-14.994440399059425,68.75013312287521,-15.737202635924493,73.10937504616481,-14.878459739068003,76.36064492186433,-12.559833524837757,77.6863729180915,-9.181208344485064,75.4625672565435,-5.673231626251427,71.79886053681197,-3.7381471592011817,66.8618942467243,-3.4903609701416993,62.29264392518654,-5.1248680438596885,58.98975517513061,-8.760952968033395],
[65.57414109270752,1.1797411270282658,69.37768640916029,-1.3575510930244814,74.20387859169041,-2.1003133298895467,78.56312051498001,-1.241570433033059,81.81439039067953,1.0770557811971881,83.14011838690669,4.455680961549877,80.9163127253587,7.963657679783514,77.25260600562717,9.898742146833756,72.3156397155395,10.146528335893239,67.74638939400174,8.512021262175251,64.44350064394581,4.875936338001546],
[65.89915812212375,15.917267748549033,69.97688522977245,12.635767809535894,76.25232402290966,11.736330263416008,81.47328014710077,12.477566678331382,86.09545897877996,15.579569438835726,86.99032987455637,21.425830739951824,83.82647235747945,24.97297628549917,79.18353918133074,27.064789354098487,73.69821507617947,27.23418460503707,68.46309469655478,24.972976285499172,64.88602415530316,20.55351481505123],
[58.48881251239855,36.09589759380796,65.7080603859302,29.82038065752831,74.19222753183148,28.331948884004674,82.75081761131048,31.085582242549528,88.10923922724437,34.28575070762116,91.45825273720305,41.65358042953028,87.067323913035,47.45853718012533,79.77391671356942,50.28659303297933,71.73628428966856,50.06332546564875,64.14518700042888,47.45853718012533,60.12637078847845,42.69549574373964],
[-73.48953442580058,20.579088801900756,-80.48690958722098,13.959950657259256,-81.93681598574045,6.269142804242765,-81.49554012532147,-1.6107832746678348,-77.90207999991593,-9.181527272091415,-71.6611785393426,-14.98115303708649,-64.60076477263831,-17.880965834138024,-57.35123278004056,-19.078714598132443,-50.09111381970131,-19.902008890566687,-42.96765884171789,-19.08249722231963,-35.655087439697766,-18.51514254492067,-28.90987071615029,-18.578181953551955,-21.74774447703016,-19.60773669210723,-14.309090001741001,-20.364210136314323,-6.933479190821032,-21.688037717705875,0.33383104043200396,-22.888118253462963,7.772483543580581,-23.77067027395373,15.274173171058239,-24.338024951681817,22.460665755024706,-24.590182586206954,29.710197747622452,-23.707630865368966,36.8557533915613,-22.565778766908277,44.16832856198768,-19.28772830000901,51.48089996424725,-14.370654426435763,56.713170880643965,-7.499358885625703,60.24016927073608,-0.41616112008138906,61.75311234056134,6.518193267525415,62.38350642684393,13.515567625813127,61.67231220934209,20.50500542283382,59.08769637136125,27.56541945045329,54.35974072401175,34.87799085169213,48.686193947196124,39.41682827314464,41.87793781501736,42.379680478815025,34.313208779263185,43.26223219965301,27.063676786665432,42.25360166155246,19.625026568173826,38.28211891778152,13.457927624759604,31.720792179781256,9.486440924356895,25.290772320002496,6.019273449215143,18.7346738223298,0.21964785513691965,13.565442314564446,-5.832135373466421,9.467880753530935,-12.632472285211591,8.882365666622222,-19.188577231399584,11.277861070017005,-26.31203040678842,14.49287091019486,-32.99420772170462,17.833959567652954,-39.2981485848331,20.670732956060768,-45.854247082486715,23.192309301312157,-52.47338498877162,24.89437333435685,-59.5968381641068,25.020452151619416,-67.33461307855538,23.697386555044208],
]
const footScale = 0.2; // how big the foot is
const stepLen = 60; // distance between steps
var stepCount = 0; // current set num so left and right can be known
var stepTime = 500; // time ms between steps
// path to walk
const path = [56.20191403126277,162.4451015086347,83.38688266344644,172.80127602121507,107.98280549274708,192.86637096042372,121.57528916149568,221.34586055208607,124.81159479691195,256.2979614145819,141.64038410107662,293.8391067854107,168.82535143857336,311.9624183437419,203.77745230106916,336.5583411729056,238.0822920364817,344.9727358249879,278.2124819156436,341.0891690624884,316.40088841355566,329.43846877498976,343.58585575105235,310.6678960895754,370.77082308854904,275.7157952270795,359.12012280105046,244.64726112708325,344.23311687813566,207.10611575625444,355.23655603855093,168.9177092583423,394.0722236635463,137.2019140312628,438.0859803052077,137.84917515834604,487.27782596353507,174.0957982750084,507.9901820301992,221.9931216791693,513.1682710468652,269.243183956247,500.87030963228347,318.43502961457443,480.1579535656192,354.68165273123674,453.62744426338134,396.86543776550707,414.1445200788371,427.9340428271046,372.7198079555102,447.3518767949864,320.2916566617712,442.173787778395,272.39433325761024,427.9340429825634,218.02439858261675,441.5265266513118,185.66134222845398,472.59506075130815,160.418158272207,514.6670340117198,168.2291881671332,557.5405924870362,200.59229872785795,598.9654951914354,232.9553551615553,627.4449850627141,273.08554504131894,651.3936467669101,320.3356073183967,663.0443470544095,368.2329307225576,663.6916081814927,417.4247763808851,649.4518633856611,460.7912718954633,626.150462810664,509.33585642670744,593.1401453294179,530.6954736204549,556.8935222127556,559.9273870166451,517.9197870310509,582.4287517306153,484.11964343037323,597.1560832290169,459.03222473087396,621.0274949086466,438.11039022321154,651.3689440081158,429.43667093843567,686.3731150817684,432.05029606733103,726.1878666750503,421.6902139845064,748.5744620042316,397.8927935245363,778.6337708564557,367.5111094723503,792.6287871481064,335.0802046803193,795.4641381478963,294.8623601925252,790.3177294792127,255.26933447013639,776.3228370821212,225.344431214269,746.3711518226298,192.73203550406103,713.7991149596966,199.06094085265394,674.3068624609349,207.5062077919911,638.4763261746227,190.31310645331482,613.6509940547375,146.74931837304698,621.5992452450397,103.454341485492,665.5124383180124,60.96567428151931,716.1845355322713,48.49595708249788,763.6383682758693,51.23726133320403,810.1045243122669,71.53440096982465,842.407749982487,97.97907893142005,879.5993779794437,131.14717279570036,903.6790094213126,175.24174017377706,915.9471803279671,219.31612086267396,902.1335310600084,270.1561321514687,880.1365756762476,315.0232456643523,884.5103070340778,370.89556334366307,909.7723644212043,407.9691345807976,947.0675376346722,439.8492118274288,990.6429384281869,439.26727537005956,1036.8675099917996,414.23364852545194,1070.3264506272462,387.0500494883014,1100.6074853525497,351.4546920217324,1119.943854180156,306.7958514306488,1128.5371035594999,259.67124425611814,1122.6651029017848,208.79760059460207,1106.8898009575623,162.16340658911932,1082.4004208812885,108.81054339506417,1050.2046949092428,81.72759371897288,1016.627194271211,46.42875173061529];
const pLen = path.length;
// fix the path so it starts at zero
for(var i = 2; i < pLen; i += 2){
path[i] -= path[0];
path[i+1] -= path[1];
}
path[0] = path[1] = 0;
// tracks the foot pos
var pos = {
x : path[0],
y : path[1],
index : 0,
};
// tracks the foot pointing to pos
var pos1 = {
x : path[0],
y : path[1],
index : 0,
};
// get a distance alone the path
function getDistOnPath(dist,pos){
var nx = path[(pos.index + 2) % pLen] - pos.x;
var ny = path[(pos.index + 3) % pLen] - pos.y;
var d = Math.hypot(nx, ny);
if(d > dist){
pos.x += (nx / d) * dist;
pos.y += (ny / d) * dist;
return pos;
}
dist -= d;
pos.index += 2;
pos.x = path[pos.index % pLen];
pos.y = path[(pos.index + 1) % pLen];
return getDistOnPath(dist, pos);
}
function drawFoot(x,y,dir,left){
var i,j,shape;
var xdx = Math.cos(dir) * footScale;
var xdy = Math.sin(dir) * footScale;
if(left){
ctx.setTransform(xdx, xdy, -xdy, xdx, x + xdy * 50, y - xdx * 50);
ctx.rotate(-0.1); // make the foot turn out a bit
}else{
ctx.setTransform(xdx, xdy, -xdy, xdx, x - xdy * 50, y + xdx * 50);
ctx.rotate(-0.1); // make the foot turn out a bit
ctx.scale(1,-1); // right foot needs to be mirrored
}
// draw the foot as a set of paths points
ctx.beginPath();
for(j = 0; j < foot.length; j ++){
shape = foot[j];
i = 0;
ctx.moveTo(shape[i++],shape[i++]);
while(i < shape.length){
ctx.lineTo(shape[i++],shape[i++]);
}
ctx.closePath();
}
ctx.fill();
}
ctx.setTransform(1,0,0,1,0,0);
ctx.clearRect(0,0,canvas.width,canvas.height);
pos1 = getDistOnPath(stepLen/10,pos1); // put the second pos infront so that a direction can be found
function drawStep(){
if(pos.index > pLen){ // if past end of path clear and restart
ctx.setTransform(1,0,0,1,0,0);
ctx.clearRect(0,0,canvas.width,canvas.height);
pos.index = 0;
pos1.index = 0;
pos1.x = pos.x = path[0];
pos1.y = pos.y = path[1];
pos1 = getDistOnPath(stepLen/10,pos1);
}
pos = getDistOnPath(stepLen,pos);
pos1 = getDistOnPath(stepLen,pos1);
drawFoot(pos.x,pos.y,Math.atan2(pos1.y - pos.y, pos1.x - pos.x),(stepCount++) % 2 === 0);
setTimeout(drawStep,stepTime);
}
drawStep();
Note some of the code is ES6 and will require babel (or equivilent) to run on legacy browsers.

Rotating One Object Around Another In createJS/easelJS

In easelJS, what is the best way to rotate an object around another? What I'm trying to accomplish is a method to rotate the crosshair around the circle pictured below, just like a planet orbits the sun:
I've been able to rotate objects around their own center point, but am having a difficult time devising a way to rotate one object around the center point of a second object. Any ideas?

Might make sense to wrap content in a Container. Translate the coordinates so the center point is where you want it, and then rotate the container.

To build on what Lanny is suggesting, there may be cases where you don't want to rotate the entire container. An alternative would be to use trigonometric functions and an incrementing angle to calculate the x/y position of the crosshair. You can find the x/y by using an angle (converted to radians) and Math.cos(angleInRadians) for x and Math.sin(angleInRadians) for y, the multiply by the radius of the orbit.
See this working example for reference.
Here's a complete snippet.
var stage = new createjs.Stage("stage");
var angle = 0;
var circle = new createjs.Shape();
circle.graphics.beginFill("#FF0000").drawEllipse(-25, -25, 50, 50).endFill();
circle.x = 100;
circle.y = 100;
var crosshair = new createjs.Shape();
crosshair.graphics.setStrokeStyle(2).beginStroke("#FF0000").moveTo(5, 0).lineTo(5, 10).moveTo(0, 5).lineTo(10, 5).endStroke();
stage.addChild(circle);
stage.addChild(crosshair);
createjs.Ticker.addEventListener("tick", function(){
angle++;
if(angle > 360)
angle = 1;
var rads = angle * Math.PI / 180;
var x = 100 * Math.cos(rads);
var y = 100 * Math.sin(rads);
crosshair.x = x + 100;
crosshair.y = y + 100;
stage.update();
});

Put another point respect to origin point with the same direction
var one_meter = 1 / map_resolution;
// get one meter distance from pointed points
var extra_x = one_meter * Math.cos(temp_rotation);
var extra_y = one_meter * Math.sin(-temp_rotation);
var new_x = mapXY.x + extra_x;
var new_y = mapXY.y + extra_y;
var home_point = new createjs.Shape().set({ x: new_x, y: new_y });
home_point.graphics.beginFill("Blue").drawCircle(0, 0, 10);
stage.addChild(home_point);
stage.update();

wall collision detection with html5 canvas [closed]

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I am making a game and there is a wall that I don't want the player to pass. I am using html5 canvas and have a player object to hold the x and y values. The wall is at x: 650 and y: 0. Since the player is 20x20 pixels when its x coordinate is 630, it touches the wall.
if(player.x > 630 && player.y <= 500) {
player.x = 630;
}
What is wrong with this code? I appreciate any help!

Answer
The code you have give is OK, there is nothing wrong with it. So I suspect the problem is elsewhere in the code, most likely in the movement code. If you are moving the player after the wall test and then display it, the player may start to creep into the wall, but without the rest of the code it is hard to know what is wrong with your code.
I have included more details on the correct way to do collision tests as there are two answers showing only a partial solution. It is there as a general guide to collision testing and may not be directly applicable to the question.
Inter frame movement
The correct way to reflect an object from a surface.
You must take into account that the ball is moving between frames and that the collision may have happened at any time during the previous frame. The ball's distance from the wall after the collision is dependent on when during the previous frame it hit the wall. This is important if the ball moves slowly or quickly.
var dx = 10; // delta x velocity of object in pixels
var wx = 10; // width of object in pixels
var px = 90; // position of object in pixels
var wallX = 105; // position of wall
px += dx; // move the ball. Its position is now 100.
// its right side is at px + wx = 110.
// test if it has it the wall
if(px+wx > wallX){
dx = -dx; // reflect delta x
// The object is 5 pixel into the wall.
// The object has hit the wall some time during the last frame
// We need to adjust the position as the ball may have been
// traveling away from the wall for some time during the last frame.
var dist = (px+wx)-wallX; // get the distance into the wall
px -= dist*2; // the object hit the wall at position 95 and has been
// traveling away since then so it is easy to just
// subtract 2 times the distance the ball entered the wall
// the above two lines can be done in one
// px -= ((px+wx)-wallX)*2;
}
Why it matters
Below is a simulation of a ball bouncing inside the canvas.
To illustrate that the ball is moving between frames it has been motion blurred to show its motion between frames. Please note this is not the perfect solution as the bounce is assumed to occur while the ball is in linear motion while infact it is in freefall and under constant acceleration. But it still conserves energy.
In the correct test the height the ball bounces back to, stays around the same over time. No energy is lost or gained.
Right click to turn off the inter frame adjustment and you will notice that the ball begins to decrease its height each frame. This is because at each collision the ball loses a little energy because it motion during the previous frame is not taken into account when positioning it after the collision test. It will settle down to a constant rate when the collision occurres at precisely the frame time. When that will be is very hard to determine in advance.
Left click to slow the simulation frame rate, left click again to return to normal.
The code below is not really part of the answer, it is there to demonstrate the effect of not correctly adjusting the position during collision test on the overall accuracy of the simulation.
// helper functions. NOT part of the answer
var canvas = document.getElementById("canV");
var ctx = canvas.getContext("2d");
var mouseButton = 0;
canvas.addEventListener('mousedown',function(event){mouseButton = event.which;});
canvas.addEventListener('mouseup' ,function(){mouseButton = 0;});
canvas.addEventListener("contextmenu", function(e){ e.preventDefault();}, false);
var currentSurface = ctx;
var createImage = function (w, h) {// create an canvas image of size w,h and attach context 2d
var image = document.createElement("canvas");
image.width = w;
image.height = h !== undefined?h:w;
currentSurface = image.ctx = image.getContext("2d");
return image;
}
var setColour = function (fillC, strokeC, lineW) {
currentSurface.fillStyle = fillC !== undefined ? fillC : currentSurface.fillStyle;
currentSurface.strokeStyle = strokeC !== undefined ? strokeC : currentSurface.strokeStyle;
currentSurface.lineWidth = lineW !== undefined ? lineW : currentSurface.lineWidth;
}
var circle = function(x,y,r,how){
currentSurface.beginPath();
currentSurface.arc(x,y,r,0,Math.PI*2);
how = how.toLowerCase().replace(/[os]/g,"l"); // how to draw
switch(how){
case "f": // fill
currentSurface.fill();
break;
case "l":
currentSurface.stroke();
break;
case "lf":
currentSurface.stroke();
currentSurface.fill();
break;
case "fl":
currentSurface.fill();
currentSurface.stroke();
break;
}
}
function createGradImage(size,col1,col2){
var image = createImage(size);
var g = currentSurface.createLinearGradient(0,0,0,currentSurface.canvas.height);
g.addColorStop(0,col1);
g.addColorStop(1,col2);
currentSurface.fillStyle = g;
currentSurface.fillRect(0,0,currentSurface.canvas.width,currentSurface.canvas.height);
return image;
}
function createColouredBall (ballR,col) {
var ball = createImage(ballR*2);
var unit = ballR/100;
setColour("black");
circle(ballR,ballR,ballR,"f");
setColour("hsl("+col+",100%,30%)");
circle(ballR-unit*3,ballR-unit*3,ballR-unit*7,"f");
setColour("hsl("+col+",100%,50%)");
circle(ballR-unit*10,ballR-unit*10,ballR-unit*16,"f");
setColour("White");
circle(ballR-unit*50,ballR-unit*50,unit*16,"f");
return ball;
}
//===================================
// _
// /_\ _ _ ____ __ _____ _ _
// / _ \| ' \(_-< V V / -_) '_|
// /_/ \_\_||_/__/\_/\_/\___|_|
//
// ==================================
// Answer code
// lazy coder variables
var w = canvas.width;
var h = canvas.height;
// ball is simulated 5cm
var pixSize = 0.24; // in millimeters for simulation
// Gravity is 9.8 ms^2 so convert to pixels per frame squared
// Assuming constant 60 frames per second. ()
var gravity = 9800*pixSize/60;
gravity *= 0.101; // because Earth's gravity is stupidly large let's move to Pluto
// ball 5cm
var ballR = (25/pixSize)/2; // radius is 2.5cm for 5cm diamiter ball
var ballX = w/2; // get center of canvas
var ballY = ballR+3; // start at the top
var ballDX = (Math.random()-0.5)*15; // start with random x speed
ballDX += ballDX < 0 ? -5 : 5; // make sure it's not too slow
var ballDY = 0; // star with no downward speed;
var ballLastX = ballX;
var ballLastY = ballY;
//create an image of the Ball
var ball = createColouredBall(ballR,Math.floor(Math.random()*360)); // create an image of ball
// create a background. Image is small as it does not have much detail in it
var background = createGradImage(16,"#5af","#08C");
// time to run for
var runFor = 10*60; // ten secons yimes 60 frames per second
// draws the ball motion blured. This introduces extra complexity
var drawMotionBlur = function(image,px,py,dx,dy,steps){
var i,sx,sy;
sx = dx / steps;
sy = dy / steps;
px -= dx; // move back to start position
py -= dy;
ctx.globalAlpha = 1/(steps*0.8); // set alpha to slightly higher for each step
for(i = 0; i < steps; i+= 1){
ctx.drawImage(image,px+i*sx,py+i*sy);
}
ctx.globalAlpha = 1; // reset alpha
}
// style for text
ctx.fillStyle = "white";
ctx.strokeStyle = "black";
ctx.textAlign = "center";
ctx.lineJoin = "round"; // stop some letters getting ears.
ctx.lineWidth = 3;
ctx.textBaseline = "bottom";
var textCenterX = w/2;
var maxHeight = Infinity;
var lastMaxHeight = ballY;
var slowMotion = false; // slow motion flag
var frameTravel = true; // use frame travel in collision test
var update = function(){
var blurSteps = 10; // motion blur ball render steps
const bSteps = 10;
if(mouseButton === 1){
slowMotion = ! slowMotion;
mouseButton = 0;
}
if(mouseButton === 3){
frameTravel = ! frameTravel;
ballX = w/2; // get center of canvas
ballY = ballR+3; // start at the top
ballDY = 0; // start at 0 y speed
mouseButton = 0;
}
// clear the canvas with background canvas image
ctx.drawImage(background,0,0,w,h);
ballDY += gravity; // accelrate due to grav
// add deltas to ball position
ballX += ballDX;
ballY += ballDY;
// test for collison on left and right walls. Need to
// ajust for motion blur
if (ballX < ballR) {
ballDX = -ballDX; // refect delta x
if (frameTravel) { // if using frame travel time
// blur the outward traveling ball only for the time it has been traveling away
blurSteps = Math.ceil(10 * ((ballX - ballR) / -ballDX));
// get position it should have traveled since
ballX -= (ballX - ballR) * 2;
}else{
ballX = ballR; // move ball to touching wall
blurSteps = 1; // there is no outward motion
}
} else
if (ballX > w - ballR) {
ballDX = -ballDX;
if (frameTravel) { // if using frame travel time
// blur the outward traveling ball only for the time it has been traveling away
blurSteps = Math.ceil(10 * ((ballX - (w - ballR)) / -ballDX));
ballX -= (ballX - (w - ballR)) * 2;
}else{
ballX = w - ballR; // move ball to touching wall
blurSteps = 1; // there is no outward motion
}
}
if (ballY > h - ballR) {
ballDY = -ballDY;
// to show max height
lastMaxHeight = maxHeight;
maxHeight = Infinity;
if (frameTravel) { // if using frame travel time
// blur the outward traveling ball only for the time it has been traveling away
blurSteps = Math.ceil(10 * ((ballY - (h - ballR)) / -ballDY));
ballY -= (ballY - (h - ballR)) * 2;
}else{
ballY = h - ballR; // move ball to touching wall
blurSteps = 1; // there is no outward motion
}
}
// draw the ball motion blured
drawMotionBlur(
ball, // image to draw
ballX - ballR, // offset radius
ballY - ballR,
ballDX * (blurSteps / bSteps), // speed and adjust for bounced
ballDY * (blurSteps / bSteps),
blurSteps // number of blurs
);
// show max height. Yes it is min but everything is upside down.
maxHeight = Math.min(maxHeight,ballY);
lastMaxHeight = Math.min(ballY,lastMaxHeight);
// show max height
ctx.font = "12px arial black";
ctx.beginPath();
ctx.moveTo(0,lastMaxHeight - ballR);
ctx.lineTo(w,lastMaxHeight - ballR);
ctx.stroke();
ctx.fillText("Max height.",40,lastMaxHeight - ballR + 6);
var str = ""; // display status string
if(slowMotion){ // show left click help
str += "10fps."
ctx.fillText("click for 60fps.",textCenterX,43);
}else{
str += "60fps."
ctx.fillText("click for 10fps.",textCenterX,43);
}
if(frameTravel){ // show mode and right click help
str += " Mid frame collision.";
ctx.fillText("Right click for Simple collision",textCenterX,55);
}else{
str += " Simple collision.";
ctx.fillText("Right click for mid frame collision",textCenterX,55);
}
// display help text
ctx.font = "18px arial black";
ctx.strokeText(str,textCenterX,30);
ctx.fillText(str,textCenterX,28);
if(slowMotion){
setTimeout(update,100); // show in slow motion
}else{
requestAnimationFrame(update); // request next frame (1/60) seconds from now
}
// all done
}
update(); // to start the ball rolling
.canC { width:500px; height:500px;}
<canvas class="canC" id="canV" width=500 height=500></canvas>

For a wall running along the X axis at the bottom (Y = 0) of a 650 x 650 field, we would want:
if (player.y <= 20) {
player.y = 20;
}
For a wall running along the Y axis at the left side (X = 0) of a 650 x 650 field, we would want:
if (player.x <= 20) {
player.x = 20;
}
For a wall running along the Y axis at the right side (X = 650) of a 650 x 650 field, we would want:
if (player.x >= 630) {
player.x = 630;
}
For a wall running along the X axis at the top (Y = 650) of a 650 x 650 field, we would want:
if (player.y >= 630) {
player.y = 630;
}

This code is similar to the code I use, if we attach horizontal (h) and vertical (v) velocity attributes to the player object we can multiply them by negative one to get them to bounce off of the wall if the player is going to go beyond the bounds. Or if you want it to stop, set them equal to zero at the wall.
//player.x+player.h gives us the future position of the player
if (player.x+player.h>630||player.x+player.h<0)
{
player.h*=-1;//bounce
//stop player.h=0;
}
if (player.y+player.v>500||player.y+player.v<0)
{
player.v*=-1;
//stop player.v=0;
}
//new player coordinates
player.x+=player.h;
player.y+=player.v;
Hope this helps.

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/