The following question is based on an adaption I wish to make of this codepen:
Here is a Pen
I have the following html for displaying the canvas (I want it to be only in a section on the html page and not take up the whole page)
<div class="container">
<canvas id="c"></canvas>
</div>
The javascript for the canvas which shows an animation is below.
<script>
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
var H = window.innerHeight;
var W = window.innerWidth;
canvas.height = H;
canvas.width = W;
var NBR_PARTICLES = 100;
var INTENSITY = 55;
var BLUE_RATIO = 5;
particles = [];
for (var i = 0; i < NBR_PARTICLES; i++) {
particles.push( new particle(i) );
};
function particle(i){
this.size = rand(0, 1.4);
this.x = W / 2;
this.y = H / 2;
this.vx = rand(-1, 1);
this.vy = rand(-1, 1);
this.decay = rand(0.9, 1);
this.c = 0;
}
function draw(){
for (var i = 0; i < NBR_PARTICLES; i++) {
p = particles[i];
ctx.fillStyle = color(p.size);
ctx.beginPath();
ctx.arc(p.x, p.y, p.size, Math.PI*2, false);
ctx.fill();
p.size *= p.decay;
p.x += p.vx;
p.y += p.vy;
p.vx += rand(-.2, .2);
p.vy += rand(-.2, .2);
p.c++;
if(p.size < .2){
particles[i] = new particle(i);
}
};
}
function color(i){
value = 255 - Math.round( (i * (255 / NBR_PARTICLES)) * INTENSITY);
return "rgba(" + value + ", 0, " + Math.round((NBR_PARTICLES - i) / BLUE_RATIO) + ", .75)";
}
setInterval(draw, 33);
/*************************
CONSTRUCT FUNCTIONS
**************************/
function rand(min, max){
value = min + Math.random() * ( max - min );
return value;
}
function cd(args){ // FOR DEBUG
console.dir(args);
}
</script>
I want the canvas size to be a rectangular banner across the page rather than the whole page as it is now.
I have tried changing these variables
var H = window.innerHeight;
var W = window.innerWidth;
canvas.height = H;
canvas.width = W;
to
canvas.height = 200;
canvas.width = 800;
but that doesn't render the animation at all but does appear to resize the canvas.
The CSS here appears to override the existing body as the whole animation takes over the page and my existing content is no longer displayed.
body, html{
margin: 0;
padding: 0;
overflow: hidden;
background-color: #ddd;
}
I tried removing the body from the css but that didn't work at all.
I also, as you can see above, added a div container, hoping that would isolate the canvas but that hasn't worked either.
<div class="container">
<canvas id="c"></canvas>
</div>
How do I adapt this code to make the canvas only render on a portion (width and height of the canvas decided by me) on the screen.
Setting directly the height H and width W , will show canvas correctly at center :
below snippet you can see result centred annilation correctly
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
// set here canvas width and height directly
var H = 200;
var W = 200;
canvas.height = H;
canvas.width = W;
var NBR_PARTICLES = 100;
var INTENSITY = 55;
var BLUE_RATIO = 5;
particles = [];
for (var i = 0; i < NBR_PARTICLES; i++) {
particles.push( new particle(i) );
};
function particle(i){
this.size = rand(0, 1.4);
this.x = W / 2;
this.y = H / 2;
this.vx = rand(-1, 1);
this.vy = rand(-1, 1);
this.decay = rand(0.9, 1);
this.c = 0;
}
function draw(){
for (var i = 0; i < NBR_PARTICLES; i++) {
p = particles[i];
ctx.fillStyle = color(p.size);
ctx.beginPath();
ctx.arc(p.x, p.y, p.size, Math.PI*2, false);
ctx.fill();
p.size *= p.decay;
p.x += p.vx;
p.y += p.vy;
p.vx += rand(-.2, .2);
p.vy += rand(-.2, .2);
p.c++;
if(p.size < .2){
particles[i] = new particle(i);
}
};
}
function color(i){
value = 255 - Math.round( (i * (255 / NBR_PARTICLES)) * INTENSITY);
return "rgba(" + value + ", 0, " + Math.round((NBR_PARTICLES - i) / BLUE_RATIO) + ", .75)";
}
setInterval(draw, 33);
/*************************
CONSTRUCT FUNCTIONS
**************************/
function rand(min, max){
value = min + Math.random() * ( max - min );
return value;
}
function cd(args){ // FOR DEBUG
console.dir(args);
}
body, html{
margin: 0;
padding: 0;
overflow: hidden;
background-color: #ddd;
text-align:center
}
canvas {
border : 1px solid gray;
}
<canvas id="c"></canvas>
Related
After having tried sine and cosine wave animations, I am facing issues while animating a hexagonal wave with rotating radius. I am unable to get the diagonal wave lines to be straight in the wave. They come out slightly curled.
var hexStep = 0;
var hexID;
function drawAxes(context) {
var width = context.canvas.width;
var height = context.canvas.height;
context.beginPath();
context.strokeStyle = "rgb(128,128,128)";
context.lineWidth = 1;
// X-axis
context.moveTo(0, height / 2);
context.lineTo(width, height / 2);
// Sine Wave starting wall
context.moveTo(width / 4, 0);
context.lineTo(width / 4, height);
// Sine Wave Y-axis
context.moveTo((2.5 * width) / 4, 0);
context.lineTo((2.5 * width) / 4, height);
// Shape Y-axis
context.moveTo(width / 8, 0);
context.lineTo(width / 8, height);
context.stroke();
}
// Hexagon
function drawHexagon(context, x, y, r, side) {
var width = context.canvas.width;
var height = context.canvas.height;
var angle = (2 * Math.PI) / side;
context.beginPath();
context.strokeStyle = "midnightblue";
context.lineWidth = 2;
context.moveTo(x + r, y);
var hx = 0;
var hy = 0;
for (var i = 1; i <= side; i++) {
hx = x + r * Math.cos(angle * i);
hy = y + r * Math.sin(angle * i);
context.lineTo(hx, hy);
}
context.stroke();
}
function getHexagonalWave(context, x, y, r, side, step, freq) {
var width = context.canvas.width;
var height = context.canvas.height;
var angle = (2 * Math.PI) / side;
var inr = r * Math.sqrt(0.75);
context.beginPath();
context.strokeStyle = "midnightblue";
context.lineWidth = 2;
context.moveTo(x, y);
var sf = step / freq;
var hr = inr / Math.cos(((sf + angle / 2) % angle) - angle / 2);
var hx = x + hr * Math.sin(sf);
var hy = y - hr * Math.cos(sf);
context.lineTo(hx, hy);
var wx = width / 4;
context.moveTo(hx, hy);
context.lineTo(wx, hy);
context.moveTo(wx, hy);
var hvr = 0;
var wvx = 0;
var wvy = 0;
var id = 0;
for (var i = 0; i < width; i++) {
hvr = inr / Math.cos((((i + step) / freq + angle / 2) % angle) - angle / 2);
wvy = y - hvr * Math.cos((i + step) / freq);
wvx = wx + hvr * Math.sin((i + step) / freq);
context.lineTo(wx + i, wvy);
}
context.stroke();
}
function doGenerateHexagonalWave() {
var canvas = document.getElementById("canvas3");
var context = canvas.getContext("2d");
var width = canvas.width;
var height = canvas.height;
var x = width / 8;
var y = height / 2;
var r = (1.5 * height) / 4;
var side = 6;
var freq = 60;
context.clearRect(0, 0, width, height); // Check
drawAxes(context);
drawHexagon(context, x, y, r, side);
getHexagonalWave(context, x, y, r, side, hexStep, freq);
hexStep++;
hexID = window.requestAnimationFrame(doGenerateHexagonalWave);
}
function doStopHexagonalWave() {
window.cancelAnimationFrame(hexID);
}
function doClearHexagonalWave() {
window.cancelAnimationFrame(hexID);
var canvas = document.getElementById("canvas3");
var context = canvas.getContext("2d");
var width = canvas.width;
var height = canvas.height;
context.clearRect(0, 0, width, height);
}
body {
background-color:darkgray;
margin-left: 10px;
}
h1 {
margin: 5px;
}
hr {
border: 1px solid;
}
canvas {
margin: 5px;
margin-bottom: -8px;
background-color:lightgrey;
/*
width: 500px;
height: 100px;
*/
}
p {
margin-left: 5px;
}
<h1>Hexagonal Wave</h1>
<hr>
<div>
</p>
<canvas id="canvas3" width="800" height="200">
</canvas>
<p>
<input type="button" value="Generate" onclick="doGenerateHexagonalWave()"/>
<input type="button" value="Stop" onclick="doStopHexagonalWave()" />
<input type="button" value="Clear" onclick="doClearHexagonalWave()" />
</p>
</div>
How do I get the angles to form straight edges between vertices in the wave by using the variable 'wvx' in the for loop ? Can't seem to figure it out!
Been having issues with this for a couple days, not sure why the text I'm rendering on the canvas is flashing so much. I'm using the requestAnimationFrame() function, but its still flickering.
What I want to have happen is for the text to move smoothly and they remove themselves from the array when they move completely off screen.
var canvas = document.getElementById("myCanvas");
var c = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var w = canvas.width;
var h = canvas.height;
var texts = [];
window.addEventListener("load", init);
function init() {
draw();
mainLoop();
}
function mainLoop() {
texts.push(createText("wow", "blue"));
texts.push(createText("wow", "green"));
texts.push(createText("wow", "red"));
setTimeout(function() { mainLoop(); }, 100);
}
function Text(x, y, vx, vy, varText, theColor){
this.x = x;
this.y = y;
this.vx = vx;
this.vy = vy;
this.color = theColor;
this.draw = function() {
drawStroked(varText, this.x, this.y, this.color);
}
}
function drawStroked(text, x, y, color) {
c.font = "30px bold Comic Sans MS";
c.strokeStyle = 'black';
c.lineWidth = 8;
c.strokeText(text, x, y);
c.fillStyle = color;
c.fillText(text, x, y);
}
function createText(varText, color) {
var x = (Math.random() * w / 2 ) + w/4;
var y = (Math.random() * h / 2) + h/2;
var vx = (Math.random() * .5) - .25
var vy = -(Math.random() * 3) - 1
return new Text(x, y, vx, vy, varText, color);
}
function draw() {
c.clearRect(0, 0, c.canvas.width, c.canvas.height);
for(var i = 0;i < texts.length; i++) {
var currentText = texts[i];
currentText.x += currentText.vx;
currentText.y += currentText.vy;
currentText.draw();
if(currentText.x>w||currentText.x<0||currentText.y<10){
texts.splice(i, 1);
}
}
requestAnimationFrame(draw);
}
body {
margin: 0;
padding: 0;
overflow: hidden;
}
<!DOCTYPE html>
<html>
<head>
<title>Game Screen</title>
</head>
<body>
<canvas id="myCanvas"></canvas>
</body>
</html>
The flickering you are seeing results from your looping code, where you skip elements of the array when deleting elements (Element 3 needs to be deleted? You call splice(3, 1) and then continue with the loop at index 4. Since the array shifts when you call splice, you should process element 3 again).
The imho easiest way to fix this is to iterate backwards over the array. Please note that iterating backwards is less CPU cache efficient (because every array access leads to a cache miss), so another fix would be
if(currentText.x>w||currentText.x<0||currentText.y<10){
texts.splice(i--, 1); // decrement i after accessing and deleting
}
var canvas = document.getElementById("myCanvas");
var c = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var w = canvas.width;
var h = canvas.height;
var texts = [];
window.addEventListener("load", init);
function init() {
draw();
mainLoop();
}
function mainLoop() {
texts.push(createText("wow", "blue"));
texts.push(createText("wow", "green"));
texts.push(createText("wow", "red"));
setTimeout(function() { mainLoop(); }, 100);
}
function Text(x, y, vx, vy, varText, theColor){
this.x = x;
this.y = y;
this.vx = vx;
this.vy = vy;
this.color = theColor;
this.draw = function() {
drawStroked(varText, this.x, this.y, this.color);
}
}
function drawStroked(text, x, y, color) {
c.font = "30px bold Comic Sans MS";
c.strokeStyle = 'black';
c.lineWidth = 8;
c.strokeText(text, x, y);
c.fillStyle = color;
c.fillText(text, x, y);
}
function createText(varText, color) {
var x = (Math.random() * w / 2 ) + w/4;
var y = (Math.random() * h / 2) + h/2;
var vx = (Math.random() * .5) - .25
var vy = -(Math.random() * 3) - 1
return new Text(x, y, vx, vy, varText, color);
}
function draw() {
c.clearRect(0, 0, c.canvas.width, c.canvas.height);
for(var i = texts.length - 1;i >= 0; i--) {
var currentText = texts[i];
currentText.x += currentText.vx;
currentText.y += currentText.vy;
currentText.draw();
if(currentText.x>w||currentText.x<0||currentText.y<10){
texts.splice(i, 1);
}
}
requestAnimationFrame(draw);
}
body {
margin: 0;
padding: 0;
overflow: hidden;
}
<!DOCTYPE html>
<html>
<head>
<title>Game Screen</title>
</head>
<body>
<canvas id="myCanvas"></canvas>
</body>
</html>
I have the following canvas. I'm trying to make it smaller using the width and height attributes, but it doesn't work. This is my code:
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
</style>
</head>
<body>
<canvas id="canvas" width="200" height="50"></canvas>
<script>
//set the variables
var a = document.getElementById('canvas'),
c = a.getContext('2d'),
w = a.width = window.innerWidth,
h = a.height = window.innerHeight,
area = w * h,
particleNum = 300,
ANIMATION;
var particles = [];
//create the particles
function Particle(i) {
this.id = i;
this.hue = rand(50, 0, 1);
this.active = false;
}
Particle.prototype.build = function() {
this.x = w / 2;
this.y = h / 2;
this.r = rand(7, 2, 1);
this.vx = Math.random() * 10 - 5;
this.vy = Math.random() * 10 - 5;
this.gravity = .01;
this.opacity = Math.random() + .5;
this.active = true;
c.beginPath();
c.arc(this.x, this.y, this.r, 0, 2 * Math.PI, false);
c.fillStyle = "hsla(" + this.hue + ",100%,50%,1)";
c.fill();
};
Particle.prototype.draw = function() {
this.active = true;
this.x += this.vx;
this.y += this.vy;
this.vy += this.gravity;
this.hue -= 0.5;
this.r = Math.abs(this.r - .05);
c.beginPath();
c.arc(this.x, this.y, this.r, 0, 2 * Math.PI, false);
c.fillStyle = "hsla(" + this.hue + ",100%,50%,1)";
c.fill();
// reset particle
if(this.r <= .05) {
this.active = false;
}
};
//functionality
function drawScene() {
c.fillStyle = "black";
c.fillRect(0,0,w,h);
for(var i = 0; i < particles.length; i++) {
if(particles[i].active === true) {
particles[i].draw();
} else {
particles[i].build();
}
}
ANIMATION = requestAnimationFrame(drawScene);
}
function initCanvas() {
var s = getComputedStyle(a);
if(particles.length) {
particles = [];
cancelAnimationFrame(ANIMATION);
ANIMATION;
console.log(ANIMATION);
}
w = a.width = window.innerWidth;
h = a.height = window.innerHeight;
for(var i = 0; i < particleNum; i++) {
particles.push(new Particle(i));
}
drawScene();
console.log(ANIMATION);
}
//init
(function() {
initCanvas();
addEventListener('resize', initCanvas, false);
})();
//helper functions
function rand(max, min, _int) {
var max = (max === 0 || max)?max:1,
min = min || 0,
gen = min + (max - min) * Math.random();
return (_int) ? Math.round(gen) : gen;
};
</script>
</body>
</html>
Pay more attention here:
var a = document.getElementById('canvas'),
c = a.getContext('2d'),
w = a.width = window.innerWidth,
h = a.height = window.innerHeight,
area = w * h,
As you can see, w is the width of the window and h is the height of the window. Try to edit w and h variable with custom values and tell me if it worked ;)
EDIT:
Simply replace h and w with this in your script tag:
a.style.width = '500px'; //random value, insert custom here
a.style.height = '500px';
I'm trying to make a blackhole simulation, and although I'm almost done, I wish to make disappear the dots that are drawn on the blackhole progressively, here is my code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>test trou noir</title>
<script>
var canvas, ctx;
var blackhole;
var circle;
var circles = new Array();
var G = 6.67e-11,//gravitational constant
pixel_G = G/1e-11,
c = 3e8, //speed of light (m/s)
M = 12e31,// masseof the blackhole in kg (60 solar masses)
pixel_M = M/1e32
Rs = (2 * G * M) / 9e16, //Schwarzchild radius
pixel_Rs = Rs/1e3, // scaled radius
ccolor=128;
function update() {
var pos, i, distance, somethingMoved = false;
for (i = 0; i < circles.length; i++) {
pos = circles[i].position;
distance = Math.sqrt(((pos.x - 700) * (pos.x - 700)) + ((pos.y - 400) * (pos.y - 400)));
if (distance > pixel_Rs && visible(circles[i])) {
var delta = new Vector2D(0, 0);
var forceDirection = Math.atan2(pos.y - 400, pos.x - 700);
var evelocity = Math.sqrt( (2*pixel_G*pixel_M)/(distance*1e-2));
delta.x += Math.cos(forceDirection) * evelocity;
delta.y += Math.sin(forceDirection) * evelocity;
pos.x += delta.x;
pos.y += delta.y;
somethingMoved = true;
}
}
if (somethingMoved) {
drawEverything();
requestAnimationFrame(update);
} else {
ccolor -=10;
};
}
function visible(ball) {
return ball.position.x > ball.radius && ball.position.x < canvas.width - ball.radius &&
ball.position.y > ball.radius && ball.position.y < canvas.height - ball.radius;
}
function drawEverything() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
blackhole.draw(ctx);
for (var i = 0; i < circles.length; i++) {
if (visible(circles[i])) {
circles[i].draw(ctx);
}
}
}
function init() {
canvas = document.getElementById("space");
ctx = canvas.getContext('2d');
blackhole = new Ball(pixel_Rs, {
x: 700,
y: 400
}, "black");
for (var i = 0; i < 200; i++) {
var vec2D = new Vector2D(Math.floor(Math.random() * 1400), Math.floor(Math.random() * 800));
circle = new Ball(5, vec2D, 'rgba('+ccolor+','+ccolor+','+ccolor+',1)');
circles.push(circle);
}
drawEverything();
requestAnimationFrame(update);
}
function Ball(radius, position, color) {
this.radius = radius;
this.position = position;
this.color = color;
}
Ball.prototype.draw = function(ctx) {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.position.x, this.position.y, this.radius, 0, 2 * Math.PI);
ctx.closePath();
ctx.fill();
};
function Vector2D(x, y) {
this.x = x;
this.y = y;
}
window.onload = init;
</script>
<style>
body {
background-color: #021c36;
margin: 0px;
}
</style>
</head>
<body>
<canvas id="space" , width="1400" , height="800">
</canvas>
</body>
</html>
now as you can see, I created a variable called ccolor which is integrated in the rgba code, but I don't know why the colors don't tend to zero, so the circles that are inside the blackhole gradually disappear, if someone could lend me a hand it'd be great
In update, if any circles[i] is captured by the Blackhole you decrement its this.color. It might be easier if you change this.color to an integer that you use to create a fillStyle:
ctx.fillStyle='rgba(' + this.color + ',' + this.color + ',' + this.color + ',1)'.
Here's a quick demo:
View this demo Full Page or the black hole is off-screen
var canvas, ctx;
var blackhole;
var circle;
var circles = new Array();
var G = 6.67e-11, //gravitational constant
pixel_G = G / 1e-11,
c = 3e8, //speed of light (m/s)
M = 12e31, // masseof the blackhole in kg (60 solar masses)
pixel_M = M / 1e32
Rs = (2 * G * M) / 9e16, //Schwarzchild radius
pixel_Rs = Rs / 1e3, // scaled radius
ccolor = 128;
function update() {
var pos, i, distance, somethingMoved = false;
for (i = 0; i < circles.length; i++) {
pos = circles[i].position;
distance = Math.sqrt(((pos.x - 700) * (pos.x - 700)) + ((pos.y - 400) * (pos.y - 400)));
if (distance > pixel_Rs && visible(circles[i])) {
var delta = new Vector2D(0, 0);
var forceDirection = Math.atan2(pos.y - 400, pos.x - 700);
var evelocity = Math.sqrt((2 * pixel_G * pixel_M) / (distance * 1e-2));
delta.x += Math.cos(forceDirection) * evelocity;
delta.y += Math.sin(forceDirection) * evelocity;
pos.x += delta.x;
pos.y += delta.y;
somethingMoved = true;
} else {
circles[i].color -= 0.50;
}
}
if (somethingMoved) {
drawEverything();
requestAnimationFrame(update);
};
}
function visible(ball) {
return ball.position.x > ball.radius && ball.position.x < canvas.width - ball.radius &&
ball.position.y > ball.radius && ball.position.y < canvas.height - ball.radius;
}
function drawEverything() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
blackhole.draw(ctx);
for (var i = 0; i < circles.length; i++) {
if (visible(circles[i])) {
circles[i].draw(ctx);
}
}
}
function init() {
canvas = document.getElementById("space");
ctx = canvas.getContext('2d');
blackhole = new Ball(pixel_Rs, {
x: 700,
y: 400
}, 0);
for (var i = 0; i < 200; i++) {
var vec2D = new Vector2D(Math.floor(Math.random() * 1400), Math.floor(Math.random() * 800));
circle = new Ball(5, vec2D, ccolor);
circles.push(circle);
}
drawEverything();
requestAnimationFrame(update);
}
function Ball(radius, position, color) {
this.radius = radius;
this.position = position;
this.color = color;
}
//
Ball.prototype.draw = function(ctx) {
var c=parseInt(this.color);
ctx.fillStyle = 'rgba(' + c + ',' + c + ',' + c + ',1)';
ctx.beginPath();
ctx.arc(this.position.x, this.position.y, this.radius, 0, 2 * Math.PI);
ctx.closePath();
ctx.fill();
};
function Vector2D(x, y) {
this.x = x;
this.y = y;
}
init();
body{ background-color: #021c36; margin: 0px; }
<canvas id="space" width=1400 height=800></canvas>
Simple solution:
In drawEverything() function move blackhole.draw(ctx) to be the last step
Not so simple: Use one of the many JS particle systems
I am a newbie on using html5 canvas.
And I'd like to use canvas generate a smoke animation and place it on top of a cigarette image.
The follow code is a sample code of a smoke generator but i don't know how to make the background of the canvas transparent.
I have try different globalCompositeOperation but it seem that i go into a wrong direction.(I don't know much about canvas
I need some help.
Sorry for my poor English.
The following is my code.
And the link is the sample code i used.
http://codepen.io/CucuIonel/pen/hFJlr
function start_smoke( smoke_id ) {
var canvas = document.getElementById(smoke_id);
var w = canvas.width = 200,
h = canvas.height = 150;
var c = canvas.getContext('2d');
var img = new Image();
img.src = 'http://oi41.tinypic.com/4i2aso.jpg';
var position = {x: w / 2, y: h};
var particles = [];
var random = function (min, max) {
return Math.random() * (max - min) * min;
};
function Particle(x, y) {
this.x = x;
this.y = y;
this.velY = -2;
this.velX = (random(1, 10) - 5) / 10;
this.size = random(3, 5) / 10;
this.alpha = 0.3;
this.update = function () {
this.y += this.velY;
this.x += this.velX;
this.velY *= 0.99;
if (this.alpha < 0)
this.alpha = 0;
c.globalAlpha = this.alpha;
c.save();
c.translate(this.x, this.y);
c.scale(this.size, this.size);
c.drawImage(img, -img.width / 2, -img.height / 2);
c.restore();
this.alpha *= 0.96;
this.size += 0.02;//
};
}
var draw = function () {
var p = new Particle(position.x, position.y);
particles.push(p);
while (particles.length > 500) particles.shift();
c.globalAlpha = 1;
c.fillRect(0, 0, w, h);
for (var i = 0; i < particles.length; i++) {
particles[i].update();
}
};
setInterval(draw, 1000 / 20);
}
$(function(){
start_smoke("main_censer_smoke");
});
Canvas transparent by default.
But anyway this question could have a pretty easy solution, which not using globalAlpha, and not using a rgba() color. The simple, effective answer is:
context.clearRect(0,0,width,height);