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How i can optimize this animation? Canvas. Javascript. Cursor animation

This animation (based on the answer of
Вася Воронцов) loads the computer very much. I do this animation in canvas. Animation loads proccesor very much. Here the light follows the cursor and leaves traces. Animation works correctly but proccesor loads very much.
Deleting and changing the radii of circles is done by saving their coordinates.
The effect is controlled by changing the variables radius (circle radius), period (time for which the circle disappears), color (circle color), blur (blur radius) and cursor radius (pointer circle radius).
How to optimize this animation so that it loads the computer less?
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
var width = document.body.offsetWidth;
var height = document.body.offsetHeight;
var points = [],
cursor = [-10, -10];
var t = 0;
var radius = 100;
var period = 2100;
var color = "rgba(239, 91, 59, .5)";
var blur = 600;
canvas.style.width = canvas.width = width;
canvas.style.height = canvas.height = height;
context.fillStyle = color;
var filter = context.filter = "blur(" + 50 + "px)";
var dr = radius / period;
function draw() {
context.clearRect(0, 0, width, height);
let i = 0;
let deleted = 0;
let dt = -t + (t = window.performance.now());
context.beginPath();
while (i++ < points.length-1) {
let p = points[i];
p[2] += dt;
let r = radius - p[2] * dr;
context.moveTo(p[0], p[1]);
if (p[2] <= period) {
context.arc(p[0], p[1], r, 0, 2*Math.PI, true);
} else deleted = i;
}
context.fill();
points.splice(0, deleted);
context.beginPath();
context.arc(cursor[0], cursor[1], 20, 0, 2*Math.PI, true);
context.filter = "none";
context.fill();
context.filter = filter;
window.requestAnimationFrame(draw);
}
window.onmousemove = function(event) {
let x = event.pageX;
let y = event.pageY;
let backwardX = 0;
let backwardY = 0;
backwardX += (x-backwardX) / 5
backwardY += (y-backwardY) / 5
points.push([x, y, 0]);
cursor = [x, y];
}
t = window.performance.now();
window.requestAnimationFrame(draw);
body {
height: 100%;
width: 100%;
position: absolute;
cursor: none;
margin: 0;
}
<canvas id="canvas"></canvas>
PS: Question in Russian.

It's slow because you have a lot of overdraw. Each frame, a large number of points is being drawn, and each point touches a lot of pixels.
You can achieve something that looks very similar if you realize that the canvas retains its contents between frames. So every frame, you could do something like this:
Fade the canvas towards white by drawing a nearly transparent white rectangle over it.
Draw one new blurred point, at the current cursor location.
The circle that follows the mouse can easily be achieved by overlaying a separate element on top of the canvas, for example a <div>. Use transform: translate(x, y); to move it, which is more performant than using left/top because it's a compositor-only property. Add will-change: transform; for an extra potential performance boost.

Drawing a sine wave from top right to bottom left

I'm trying to design a small project using canvas and I want to design a sine wave in such a way that the wave is generated from the top right corner to the left bottom corner infinitely if possible.
Something like an inverse sine graph.
All I can do is make the sine wave go from left to right but making this work from top right to bottom left is very difficult.
This is my code at the moment
It's looking very sad...
const canvas = document.querySelector(".canvas");
const ctx = canvas.getContext("2d");
canvas.width = window.innerWidth - 5;
canvas.height = window.innerHeight - 5;
window.addEventListener("resize", () => {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
});
const wave = {
y: canvas.height / 2,
length: 0.02,
amplitude: 100,
frequency: 0.01,
yOffSet: canvas.height,
};
let increment = wave.frequency;
function animate() {
requestAnimationFrame(animate);
ctx.fillStyle = "rgb(0,0,0)";
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.beginPath();
ctx.moveTo(0, canvas.height);
for (let i = 0; i < canvas.width; i++) {
ctx.lineTo(Math.sin(i / wave.length + increment) * wave.amplitude + wave.yOffSet, 0);
}
ctx.stroke();
increment += wave.frequency;
}
animate();
body {
margin: 0;
padding: 0;
}
<div>
<canvas class="canvas"></canvas>
</div>
Desired Output

The problem is in this line:
ctx.lineTo(Math.sin(i / wave.length + increment) * wave.amplitude + wave.yOffSet, 0);
You are only moving in the x co-ordinate.
I have added the motion in the y co-ordinate and rewrote on three lines just for clarity.
x=i+wave.amplitude*Math.sin(i/wave.length);
y=canvas.height-(i-(wave. amplitude * Math.sin(i/wave.length)));
ctx.lineTo(x,y);
The result it produces is like what you describe, if I understood correctly. There are many more waves than you show in the drawing, but that can be cahnged by the wave.length parameter.

How to use clearRect to not draw an moving object on canvas

I have a blue circle which is rotating around the red circle and moves on canvas continuously in one direction as long as the button is pressed.
Now I want to draw with the red circle while it is moving when the button is pressed (trace of its path).
Problems:
I have tried to make changes to clearRect() but I didn't succeed. the blue circle starts to draw on the canvas while moving which I don't need.
If its not possible to do with clearRect() function, Is it possible to do this by stacking canvas layers. Please help with example
const canvas = document.getElementById('canvas1');
const ctx = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
let positionX = 100;
let positionY = 100;
let X = 50;
let Y = 50;
let angle = 0;
let mouseButtonDown = false;
document.addEventListener('mousedown', () => mouseButtonDown = true);
document.addEventListener('mouseup', () => mouseButtonDown = false);
function circle(){
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(X, Y, 20, 0, Math.PI*2);
ctx.closePath();
ctx.fill();
}
function direction(){
ctx.fillStyle = 'blue';
ctx.beginPath();
ctx.arc(positionX + X, positionY + Y, 10, 0, Math.PI*2);
ctx.closePath();
positionX = 35 * Math.sin(angle);
positionY = 35 * Math.cos(angle);
ctx.fill();
}
function animate(){
if (mouseButtonDown) {
X += positionX / 10;
Y += positionY / 10;
} else {
angle += 0.1;
}
ctx.clearRect(X-positionX,Y-positionY, 20, 20);
circle();
direction();
requestAnimationFrame(animate);
}
animate();
#canvas1{
position: absolute;
top:0;
left: 0;
width: 100%;
height: 100%;
}
<!DOCTYPE html>
<html lang="en">
<head>
<link rel="stylesheet" href="style.css">
</head>
<body>
<canvas id="canvas1"></canvas>
<script src="script.js"></script>
</body>
</html>

Don`t stack canvas on the page
Each canvas you add to the page increases the amount of work the GPU and page compositor needs to do to render the page.
Use a second canvas that is not on the page and do the compositing by rendering the canvas to the onpage canvas using ctx.drawImage(secondCanvas, 0, 0).
This reduces the workload for the compositor, and in many cases avoid the need to do an addition image render (composite) for the second canvas I.E. onpage can require 3 drawImages (one for each canvas and once for the result) rather than 2 (once in your code and once as the result) if you use only one onpage canvas.
Using second canvas
Create a second canvas to store the drawn red lines.
You can create a copy of a canvas using
function copyCanvas(canvas, copyContent = false) {
const can = Object.assign(document.createElement("canvas"), {
width: canvas.width, height: canvas.height
});
can.ctx = can.getContext("2d");
copyContent && can.ctx.drawImage(canvas, 0, 0);
return can;
}
When you create render functions like circle, and direction pass as an argument the 2D context eg circle(ctx) so that it is easy to direct the rendering to any canvas.
function circle(ctx){
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(X, Y, redSize, 0, Math.PI*2);
ctx.fill();
}
// the background canvas
const bgCan = copyCanvas(canvas);
circle(bgCan.ctx); // will draw to the background canvas
Updating animation
When animating is is easiest to clear the whole canvas rather than mess about clearing only rendered pixels. Clearing rendered pixels gets complicated very quickly and will end up being many times slower than clearing the whole canvas.
After you clear the canvas draw the background canvas to the main canvas
ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
ctx.drawImage(bgCan, 0, 0);
When the mouse button is down draw the circle to the background canvas and while it is up draw to the main canvas.
Example
Adds a function to copy a canvas. copyCanvas
Clears the main canvas, and draws the background canvas onto the main canvas.
Render functions circle and direction have argument ctx to direct rendering to any context.
When mouse is down circle is drawn to background canvas bgCan else to the main canvas.
requestAnimationFrame(animate);
const ctx = canvas1.getContext('2d');
canvas1.width = innerWidth;
canvas1.height = innerHeight;
const bgCan = copyCanvas(canvas1);
const redSize = 10, blueSize = 5; // circle sizes on pixels
const drawSpeed = 2; // when button down draw speed in pixels per frame
var X = 50, Y = 50;
var angle = 0;
var mouseButtonDown = false;
document.addEventListener('mousedown', () => mouseButtonDown = true);
document.addEventListener('mouseup', () => mouseButtonDown = false);
function copyCanvas(canvas) {
const can = Object.assign(document.createElement("canvas"), {
width: canvas.width, height: canvas.height
});
can.ctx = can.getContext("2d");
return can;
}
function circle(ctx){
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(X, Y, redSize, 0, Math.PI*2);
ctx.fill();
}
function direction(ctx){
const d = blueSize + redSize + 5;
ctx.fillStyle = 'blue';
ctx.beginPath();
ctx.arc(d * Math.sin(angle) + X, d * Math.cos(angle) + Y, blueSize, 0, Math.PI*2);
ctx.fill();
}
function animate(){
ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
ctx.drawImage(bgCan, 0, 0);
if (mouseButtonDown) {
circle(bgCan.ctx);
X += Math.sin(angle) * drawSpeed;
Y += Math.cos(angle) * drawSpeed;
} else {
angle += 0.1;
circle(ctx);
}
direction(ctx);
requestAnimationFrame(animate);
}
#canvas1{
position: absolute;
top:0;
left: 0;
width: 100%;
height: 100%;
}
<canvas id="canvas1"></canvas>
BTW ctx.closePath() is like ctx.lineTo it is not the opposite to ctx.beginPath. A full arc or if you are just filling a shape you don't need to use ctx.closePath
BTW window is the default this, you don't need to include it, you dont use it to get at window.documentso why use it forwindow.innerWidth(same asinnerWidth` )

You could alter your code to keep track of the path of the red circle, with an array property, like this:
const canvas = document.getElementById('canvas1');
const ctx = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
let mouseButtonDown = false;
document.addEventListener('mousedown', () => mouseButtonDown = true);
document.addEventListener('mouseup', () => mouseButtonDown = false);
function drawCircle({x, y, radius, color}) {
ctx.fillStyle = color;
ctx.beginPath();
ctx.arc(x, y, radius, 0, Math.PI*2);
ctx.fill();
}
const red = { x: 50, y: 50, radius: 20, color: "red", path: [] };
const blue = { x: 100, y: 100, radius: 10, color: "blue", angle: 0 };
function animate(){
if (mouseButtonDown) {
red.path.push({x: red.x, y: red.y}); // store the old value
red.x += (blue.x - red.x) / 10;
red.y += (blue.y - red.y) / 10;
} else {
blue.angle += 0.1;
}
blue.x = red.x + 35 * Math.sin(blue.angle);
blue.y = red.y + 35 * Math.cos(blue.angle);
ctx.clearRect(0, 0, canvas.width, canvas.height); // clear the whole canvas
for (const {x, y} of red.path) { // draw circle at all the previous positions
drawCircle({...red, x, y});
}
drawCircle(red);
drawCircle(blue);
requestAnimationFrame(animate);
}
animate();
Using 2 canvases also works and may perform better especially when the path of the red circle has gotten long, because the background canvas doesn't need to be cleared and redrawn. Add a 2nd canvas in your html page with the same positioning, and give them ids 'background' and 'foreground'. You can then adjust the code to draw the blue circle to the foreground and red circles to the background (or vice versa).
// Create 2 canvases, set them to full size and get the contexts
const backgroundCanvas = document.getElementById('background');
const foregroundCanvas = document.getElementById('foreground');
const background = backgroundCanvas.getContext("2d");
const foreground = foregroundCanvas.getContext("2d");
backgroundCanvas.width = innerWidth;
backgroundCanvas.height = innerHeight;
foregroundCanvas.width = innerWidth;
foregroundCanvas.height = innerHeight;
let mouseButtonDown = false;
document.addEventListener('mousedown', () => mouseButtonDown = true);
document.addEventListener('mouseup', () => mouseButtonDown = false);
// Create objects to represent the current properties of the red and blue circle
const red = { x: 50, y: 50, radius: 20, color: "red" };
const blue = { x: 100, y: 100, radius: 10, color: "blue", angle: 0};
function drawCircle(ctx, {x, y, radius, color}) {
//--- Draw a circle to the specified canvas context, ctx = foreground or background
ctx.fillStyle = color;
ctx.beginPath();
ctx.arc(x, y, radius, 0, Math.PI*2);
ctx.closePath();
ctx.fill();
}
function animate(){
if (mouseButtonDown) {
red.x += (blue.x - red.x) / 10;
red.y += (blue.y - red.y) / 10;
} else {
blue.angle += 0.1;
}
blue.x = red.x + 35 * Math.sin(blue.angle);
blue.y = red.y + 35 * Math.cos(blue.angle);
drawCircle(background, red); // Draw the red circle in the background (without clearing the existing circles)
foreground.clearRect(0, 0, foregroundCanvas.width, foregroundCanvas.height); // Clear the foreground
drawCircle(foreground, blue); // Draw the blue circle on the foreground
requestAnimationFrame(animate);
}
animate();
Either way, it's convenient to abstract out the circle drawing code into a function or method, and to store the properties of the two circles in objects.
As #Blindman67's answer notes, there may be a performance cost of stacking 2 canvases, and if that is an issue you may want to try drawing the background offscreen then copying it to the on-screen canvas.

If you're not opposed to just building a particle class you can do it using them. In the snippet below I have a Circle class and a Particles class to creat what you are trying to achieve. I currently have the particles max at 500 but you can change it or delete that line all together if you ne er want them gone.
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
let mouseButtonDown = false;
//the array holding particles
let particles = [];
//the counter is only needed it you want to slow down how fast particles are being pushed and dispolayed
let counter = 0;
document.addEventListener("mousedown", () => (mouseButtonDown = true));
document.addEventListener("mouseup", () => (mouseButtonDown = false));
//ES6 constructor class
class Circle {
//sets the basic structor of the object
constructor(r, c) {
this.x = 100;
this.y = 100;
this.x2 = 50;
this.y2 = 50;
this.r = r; //will be assigned the argument passed in through the constructor by each instance created later
this.color = c; //same as above. This allows each instance to have different parameters.
this.angle = 0;
}
//this function creates the red circle
drawRed() {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2);
ctx.fill();
ctx.closePath();
}
//this function creates the blue circle
drawBlue() {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.x + this.x2, this.y + this.y2, this.r, 0, Math.PI * 2);
ctx.fill();
ctx.closePath();
}
//this function is where we'll place parameter that change our object
update() {
//makes the blue circle rotate
this.x2 = 35 * Math.sin(this.angle);
this.y2 = 35 * Math.cos(this.angle);
//mouse action is same as your code
if (mouseButtonDown) {
this.x += this.x2 / 20;
this.y += this.y2 / 20;
} else {
this.angle += 0.1;
}
}
}
//When using this type of constructor class you have to create an instance of it by calling new Object. You can create as money as you want.
let blueCircle = new Circle(10, "blue"); //passing in the radius and color in to the constructor
let redCircle = new Circle(20, "red");
//another class for the particles
class Particles {
constructor() {
this.x = redCircle.x;
this.y = redCircle.y;
this.r = redCircle.r;
this.color = redCircle.color;
}
draw() {
ctx.fillStyle = this.color;
ctx.beginPath();
ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2);
ctx.fill();
ctx.closePath();
}
}
//just wrapping all of the particle stuff into one function
function handleParticles() {
//while the mouse is held it will push particles
if (mouseButtonDown) {
particles.push(new Particles());
}
//this loops through the array and calls the draw() function for each particle
for (let i = 0; i < particles.length; i++) {
particles[i].draw();
}
//this keeps the array from getting too big.
if (particles.length > 500) {
particles.shift();
}
}
//wrap all functions into this one animate one and call requeatAnimationFrame
function animate() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
handleParticles();
//These must be called for each instance created of the object
blueCircle.drawBlue();
blueCircle.update();
redCircle.drawRed();
redCircle.update();
requestAnimationFrame(animate);
}
animate();
#canvas1{
position: absolute;
top:0;
left: 0;
width: 100%;
height: 100%;
}
<canvas id="canvas"></canvas>
I'd also like to add you can change the rate that the particles are drawn by adding a counter variable and then limiting the draw like counter % 10 == 0
EXAMPLE
add global variable let counter = 0;
then in the handleParticles function add this
function handleParticles() {
counter++
if (mouseButtonDown && counter % 10 == 0) {
particles.push(new Particles());
}
for (let i = 0; i < particles.length; i++) {
particles[i].draw();
}
if (particles.length > 500) {
particles.shift();
}
}

Getting relative mouse position on canvas after scaling

Problem: I'm working with an HTML canvas. My canvas has a background image that multiple people can draw over in real-time (via socket.io), but drawing breaks if you've zoomed in.
Cause: To calculate where to start and end a line, I normalize input upon capture to be between 0 and 1 inclusive, like so:
// Pseudocode
line.x = mousePosition.x / canvas.width;
line.y = mousePosition.y / canvas.height;
Because of this, the canvas can be of any size and in any position.
To implement a zoom-on-scroll functionality, I simply translate based on the current mouse position, scale the canvas by a factor of 2, then translate back the negative value of the current mouse position (as recommended here).
Here's where the problem lies
When I zoom, the canvas doesn't seem to have a notion of it's original size.
For instance, let's say I have a 1000px square canvas. Using my normalized x and y above, the upper left corner is 0, 0 and the lower right is 1, 1.
I then zoom into the center through scaling by a factor of 2. I would expect that my new upper left would be 0.5, 0.5 and my lower right would be 0.75, 0.75, but it isn't. Even when I zoom in, the upper left is still 0, 0 and the lower right is 1, 1.
The result is that when I zoom in and draw, the lines appear where they would as if I were not zoomed at all. If I zoomed into the center and "drew" in the upper left, I'd see nothing until I scrolled out to see that the line was actually getting drawn on the original upper left.
What I need to know: When zoomed, is there a way to get a read on what your new origin is relative to the un-zoomed canvas, or what amount of the canvas is hidden? Either of these would let me zoom in and draw and have it track correctly.
If I'm totally off base here and there's a better way to approach this, I'm all ears. If you need additional information, I'll provide what I can.

It's not clear to me what you mean by "zoomed".
Zoomed =
made the canvas a different size?
changed the transform on the canvas
used CSS transform?
used CSS zoom?
I'm going to assume it's transform on the canvas in which case it's something like
function getElementRelativeMousePosition(e) {
return [e.offsetX, e.offsetY];
}
function getCanvasRelativeMousePosition(e) {
const pos = getElementRelativeMousePosition(e);
pos[0] = pos[0] * ctx.canvas.width / ctx.canvas.clientWidth;
pos[1] = pos[1] * ctx.canvas.height / ctx.canvas.clientHeight;
return pos;
}
function getComputedMousePosition(e) {
const pos = getCanvasRelativeMousePosition(e);
const p = new DOMPoint(...pos);
const point = inverseOriginTransform.transformPoint(p);
return [point.x, point.y];
}
Where inverseOriginTransform is the inverse of whatever transform you're using to zoom and scroll the contents of the canvas.
const settings = {
zoom: 1,
xoffset: 0,
yoffset: 0,
};
const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
const lines = [
[[100, 10], [200, 30]],
[[50, 50], [100, 30]],
];
let newStart;
let newEnd;
let originTransform = new DOMMatrix();
let inverseOriginTransform = new DOMMatrix();
function setZoomAndOffsetTransform() {
originTransform = new DOMMatrix();
originTransform.translateSelf(settings.xoffset, settings.yoffset);
originTransform.scaleSelf(settings.zoom, settings.zoom);
inverseOriginTransform = originTransform.inverse();
}
const ui = document.querySelector('#ui')
addSlider(settings, 'zoom', ui, 0.25, 3, draw);
addSlider(settings, 'xoffset', ui, -100, +100, draw);
addSlider(settings, 'yoffset', ui, -100, +100, draw);
draw();
function updateAndDraw() {
draw();
}
function getElementRelativeMousePosition(e) {
return [e.offsetX, e.offsetY];
}
function getCanvasRelativeMousePosition(e) {
const pos = getElementRelativeMousePosition(e);
pos[0] = pos[0] * ctx.canvas.width / ctx.canvas.clientWidth;
pos[1] = pos[1] * ctx.canvas.height / ctx.canvas.clientHeight;
return pos;
}
function getTransformRelativeMousePosition(e) {
const pos = getCanvasRelativeMousePosition(e);
const p = new DOMPoint(...pos);
const point = inverseOriginTransform.transformPoint(p);
return [point.x, point.y];
}
canvas.addEventListener('mousedown', (e) => {
const pos = getTransformRelativeMousePosition(e);
if (newStart) {
} else {
newStart = pos;
newEnd = pos;
}
});
canvas.addEventListener('mousemove', (e) => {
if (newStart) {
newEnd = getTransformRelativeMousePosition(e);
draw();
}
});
canvas.addEventListener('mouseup', (e) => {
if (newStart) {
lines.push([newStart, newEnd]);
newStart = undefined;
}
});
function draw() {
ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
ctx.save();
setZoomAndOffsetTransform();
ctx.setTransform(
originTransform.a,
originTransform.b,
originTransform.c,
originTransform.d,
originTransform.e,
originTransform.f);
ctx.beginPath();
for (const line of lines) {
ctx.moveTo(...line[0]);
ctx.lineTo(...line[1]);
}
if (newStart) {
ctx.moveTo(...newStart);
ctx.lineTo(...newEnd);
}
ctx.stroke();
ctx.restore();
}
function addSlider(obj, prop, parent, min, max, callback) {
const valueRange = max - min;
const sliderRange = 100;
const div = document.createElement('div');
div.class = 'range';
const input = document.createElement('input');
input.type = 'range';
input.min = 0;
input.max = sliderRange;
const label = document.createElement('span');
label.textContent = `${prop}: `;
const valueElem = document.createElement('span');
function setInputValue(v) {
input.value = (v - min) * sliderRange / valueRange;
}
input.addEventListener('input', (e) => {
const v = parseFloat(input.value) * valueRange / sliderRange + min;
valueElem.textContent = v.toFixed(1);
obj[prop] = v;
callback();
});
const v = obj[prop];
valueElem.textContent = v.toFixed(1);
setInputValue(v);
div.appendChild(input);
div.appendChild(label);
div.appendChild(valueElem);
parent.appendChild(div);
}
canvas { border: 1px solid black; }
#app { display: flex; }
<div id="app"><canvas></canvas><div id="ui"></div>
Note: I didn't bother making zoom always zoom from the center. To do so would require adjusting xoffset and yoffset as the zoom changes.

Use HTMLElement.prototype.getBoundingClientRect() to get displayed size and position of canvas in DOM. From the displayed size and origin size, calculates the scale of the canvas.
Example:
canvas.addEventListener("click", function (event) {
var b = canvas.getBoundingClientRect();
var scale = canvas.width / parseFloat(b.width);
var x = (event.clientX - b.left) * scale;
var y = (event.clientY - b.top) * scale;
// Marks mouse position
var ctx = canvas.getContext("2d");
ctx.beginPath();
ctx.arc(x, y, 10, 0, 2 * Math.PI);
ctx.stroke();
});

Rotate individual objects in canvas?

the rotate() function seems to rotate the whole drawing area. Is there a way to rotate paths individually? I want the center for the rotation to be the object, not the drawing area.
Using save() and restore() still makes rotate take into account the whole drawing area.
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
context.save();
context.fillStyle = 'red';
context.rotate(0.35);
context.fillRect(40,40, 100, 100);
context.restore();
context.save();
context.fillStyle = 'blue';
context.rotate(0.35);
context.fillRect(200, 40, 100, 100);
context.restore();
<canvas id="canvas" width="500" height="500"></canvas>

Use local space
Instead of drawing object at the position you want them draw everything around its own origin in its local space. The origin is at (0,0) and is the location that the object rotates around.
So if you have a rectangle that you draw with
function drawRect(){
context.fillRect(200, 40, 100, 100);
}
change it so that it is drawn at its origin
function drawRect(){
context.fillRect(-50,-50 , 100, 100);
}
Now you can easily draw it wherevery you want
Start with the setTransform function as that clears any existing tranforms and is a convenient way to set the location of the center of the object will be
ctx.setTransform(1,0,0,1,posX,posY); // clear transform and set center location
if you want to rotate it then add the rotation
ctx.rotate(ang);
and scale with
ctx.scale(scale,scale);
if you have two different scales you should scale before the rotate.
Now just call the draw function
drawRect();
and it is drawn with its center at posX,posY rotated and scaled.
You can combine it all into a function that has the x,y position, the width and the height, scale and rotation. You can include the scale in the setTransform
function drawRect(x,y,w,h,scale,rotation){
ctx.setTransform(scale,0,0,scale,x,y);
ctx.rotate(rotation);
ctx.strokeRect(-w/2,-h/2,w,h);
}
It also applies to an image as a sprite, and I will include a alpha
function drawImage(img,x,y,w,h,scale,rotation,alpha){
ctx.globalAlpha = alpha;
ctx.setTransform(scale,0,0,scale,x,y);
ctx.rotate(rotation);
ctx.drawImage(img,-img.width/2,-img.height/2,img.width,img.height);
}
On a 6 year old laptop that can draw 2000 sprites on firefox every 1/60th of a second, each rotated, scaled, positioned, and with a alpha fade.
No need to mess about with translating back and forward. Just keep all the objects you draw around there own origins and move that origin via the transform.
Update
Lost the demo so here it is to show how to do it in practice.
Just draws a lot of rotated, scaled translated, alphaed rectangles.
By using setTransform you save a lot of time by avoiding save and restore
// create canvas and add resize
var canvas,ctx;
function createCanvas(){
canvas = document.createElement("canvas");
canvas.style.position = "absolute";
canvas.style.left = "0px";
canvas.style.top = "0px";
canvas.style.zIndex = 1000;
document.body.appendChild(canvas);
}
function resizeCanvas(){
if(canvas === undefined){
createCanvas();
}
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
ctx = canvas.getContext("2d");
}
resizeCanvas();
window.addEventListener("resize",resizeCanvas);
// simple function to draw a rectangle
var drawRect = function(x,y,w,h,scale,rot,alpha,col){
ctx.setTransform(scale,0,0,scale,x,y);
ctx.rotate(rot);
ctx.globalAlpha = alpha;
ctx.strokeStyle = col;
ctx.strokeRect(-w/2,-h/2, w, h);
}
// create some rectangles in unit scale so that they can be scaled to fit
// what ever screen size this is in
var rects = [];
for(var i = 0; i < 200; i ++){
rects[i] = {
x : Math.random(),
y : Math.random(),
w : Math.random() * 0.1,
h : Math.random() * 0.1,
scale : 1,
rotate : 0,
dr : (Math.random() - 0.5)*0.1, // rotation rate
ds : Math.random()*0.01, // scale vary rate
da : Math.random()*0.01, // alpha vary rate
col : "hsl("+Math.floor(Math.random()*360)+",100%,50%)",
};
}
// draw everything once a frame
function update(time){
var w,h;
w = canvas.width; // get canvas size incase there has been a resize
h = canvas.height;
ctx.setTransform(1,0,0,1,0,0); // reset transform
ctx.clearRect(0,0,w,h); // clear the canvas
// update and draw each rect
for(var i = 0; i < rects.length; i ++){
var rec = rects[i];
rec.rotate += rec.dr;
drawRect(rec.x * w, rec.y * h, rec.w * w,rec.h * h,rec.scale + Math.sin(time * rec.ds) * 0.4,rec.rotate,Math.sin(time * rec.da) *0.5 + 0.5,rec.col);
}
requestAnimationFrame(update); // do it all again
}
requestAnimationFrame(update);

All transformations in canvas are for the whole drawing area. If you want to rotate around a point you're going to have to translate that point to the origin, do your rotation and translate it back. Something like this is what you want.

Use a rotate function to rotate all of the shape's points around its center.
<!DOCTYPE html>
<html>
<head>
<style>
body
{
margin: 0px;
padding: 0px;
overflow: hidden;
}
canvas
{
position: absolute;
}
</style>
</head>
<body>
<canvas id="canvas"></canvas>
<script>
var canvas;
var context;
canvas = document.getElementById("canvas");
context = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var degreesToRadians = function(degrees)
{
return degrees*Math.PI/180;
}
var rotate = function(x, y, cx, cy, degrees)
{
var radians = degreesToRadians(degrees);
var cos = Math.cos(radians);
var sin = Math.sin(radians);
var nx = (cos * (x - cx)) + (sin * (y - cy)) + cx;
var ny = (cos * (y - cy)) - (sin * (x - cx)) + cy;
return new Vector2(nx, ny);
}
var Vector2 = function(x, y)
{
return {x:x,y:y};
}
var Shape = function(points, color)
{
this.color = color;
this.points = points;
};
Shape.prototype.rotate = function(degrees)
{
var center = this.getCenter();
for (var i = 0; i < this.points.length; i++)
{
this.points[i] = rotate(this.points[i].x,this.points[i].y,center.x,center.y,degrees);
}
context.beginPath();
context.arc(center.x,center.y,35,0,Math.PI*2);
context.closePath();
context.stroke();
}
Shape.prototype.draw = function()
{
context.fillStyle = this.color;
context.strokeStyle = "#000000";
context.beginPath();
context.moveTo(this.points[0].x, this.points[0].y);
for (var i = 0; i < this.points.length; i++)
{
context.lineTo(this.points[i].x, this.points[i].y);
//context.fillText(i+1, this.points[i].x, this.points[i].y);
}
context.closePath();
context.fill();
context.stroke();
}
Shape.prototype.getCenter = function()
{
var center = {x:0,y:0};
for (var i = 0; i < this.points.length; i++)
{
center.x += this.points[i].x;
center.y += this.points[i].y;
}
center.x /= this.points.length;
center.y /= this.points.length;
return center;
}
Shape.prototype.translate = function(x, y)
{
for (var i = 0; i < this.points.length; i++)
{
this.points[i].x += x;
this.points[i].y += y;
}
}
var Rect = function(x,y,w,h,c)
{
this.color = c;
this.points = [Vector2(x,y),Vector2(x+w,y),Vector2(x+w,y+h),Vector2(x,y+h)];
}
Rect.prototype = Shape.prototype;
var r = new Rect(50, 50, 200, 100, "#ff0000");
r.draw();
r.translate(300,0);
r.rotate(30);
r.draw();
</script>
</body>
</html>