I need to apply several matrix transformations before drawing a shape, however (if on somewhere) I use rotate() the coordinates are inverted and/or reversed and cannot continue without knowing if the matrix was previously rotated.
How can solve this problem?
Example:
<canvas width="300" height="300"></canvas>
<script>
let canvas = document.querySelector("canvas");
let ctx = canvas.getContext("2d");
ctx.fillStyle = "silver";
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.strokeStyle = "black";
ctx.lineWidth = 1;
ctx.beginPath();
ctx.moveTo(0, canvas.height/2);
ctx.lineTo(canvas.width, canvas.height/2);
ctx.stroke();
ctx.beginPath();
ctx.moveTo(canvas.width/2, 0);
ctx.lineTo(canvas.width/2, canvas.height);
ctx.stroke();
ctx.translate(150, 150);
ctx.rotate(-90 * 0.017453292519943295);
ctx.translate(-150, -150);
// move the red rectangle 100px to the left (top-left)
// but instead is moved on y axis (right-bottom)
ctx.translate(-100, 0);
// more matrix transformations
// ....
// ....
// now finally draw the shape
ctx.fillStyle = "red";
ctx.fillRect(150, 150, 100, 50);
</script>
Can be this Translation after rotation the solution?
It looks like you aren't resetting the canvas matrix each time you make a new transformation.
The Canvas API has the save() and restore() methods. Canvas states are stored on a stack. Every time the save() method is called, the current drawing state is pushed onto the stack. A drawing state consists of transformations that have been applied along with the attributes of things like the fillStyle. When you call restore(), the previous settings are restored.
// ...
ctx.save(); // save the current canvas state
ctx.translate(150, 150);
ctx.rotate(-90 * 0.017453292519943295);
ctx.translate(-150, -150);
ctx.restore(); // restore the last saved state
// now the rectangle should move the correct direction
ctx.translate(-100, 0);
Check out this link for more information on the save and restore methods.
OK finally, i solved the problem by rotating the translation point before applying it. This function does the trick:
function helperRotatePoint(point, angle) {
let s = Math.sin(angle);
let c = Math.cos(angle);
return { x: point.x * c - point.y * s, y: point.x * s + point.y * c};
}
rotating the translation point using the inverted angle I obtain the corrected translation
helperRotatePoint(translation_point, -rotation_angle);
working code:
let canvas = document.querySelector("canvas");
// proper size on HiDPI displays
canvas.style.width = canvas.width;
canvas.style.height = canvas.height;
canvas.width = Math.floor(canvas.width * window.devicePixelRatio);
canvas.height = Math.floor(canvas.height * window.devicePixelRatio);
let ctx = canvas.getContext("2d");
ctx.scale(window.devicePixelRatio, window.devicePixelRatio);
ctx.fillStyle = "whitesmoke";
ctx.fillRect(0, 0, canvas.width, canvas.height);
class UIElement {
constructor(x, y, width, height, color) {
// PoC
this.draw_pos = {x, y};
this.draw_size = {width, height};
this.color = color;
this.rotate = 0;
this.scale = {x: 1, y: 1};
this.translate = {x: 0, y: 0};
this.skew = {x: 0, y: 0};
this.childs = [];
}
addChild(uielement) {
this.childs.push(uielement);
}
helperRotatePoint(point, angle) {
let s = Math.sin(angle);
let c = Math.cos(angle);
return {
x: point.x * c - point.y * s,
y: point.x * s + point.y * c
};
}
draw(cnvs_ctx, parent_x, parent_y) {
// backup current state
cnvs_ctx.save();
let elements_drawn = 1;// "this" UIElement
// step 1: calc absolute coordinates
let absolute_x = parent_x + this.draw_pos.x;
let absolute_y = parent_y + this.draw_pos.y;
// step 2: apply all transforms
if (this.rotate != 0) {
cnvs_ctx.translate(absolute_x, absolute_y)
cnvs_ctx.rotate(this.rotate);
cnvs_ctx.translate(-absolute_x, -absolute_y);
// rotate translate point before continue
let tmp = this.helperRotatePoint(this.translate, -this.rotate);
// apply rotated translate
cnvs_ctx.translate(tmp.x, tmp.y);
} else {
cnvs_ctx.translate(this.translate.x, this.translate.y);
}
cnvs_ctx.scale(this.scale.x, this.scale.y);
cnvs_ctx.transform(1, this.skew.y, this.skew.x, 1, 0, 0);
// step 3: self draw (aka parent element)
cnvs_ctx.fillStyle = this.color;
cnvs_ctx.fillRect(absolute_x, absolute_y, this.draw_size.width, this.draw_size.height);
// step 4: draw childs elements
for (let i = 0; i < this.childs.length ; i++) {
elements_drawn += this.childs[i].draw(
cnvs_ctx, absolute_x, absolute_y
);
}
// done, restore state
cnvs_ctx.restore();
return elements_drawn;
}
}
// spawn some ui elements
var ui_panel = new UIElement(120, 50, 240, 140, "#9b9a9e");
var ui_textlabel = new UIElement(10, 10, 130, 18, "#FFF");
var ui_image = new UIElement(165, 25, 90, 60, "#ea9e22");
var ui_textdesc = new UIElement(17, 46, 117, 56, "#ff2100");
var ui_icon = new UIElement(5, 5, 10, 10, "#800000");
ui_panel.addChild(ui_textlabel);
ui_panel.addChild(ui_image);
ui_panel.addChild(ui_textdesc);
ui_textdesc.addChild(ui_icon);
// add some matrix transformations
ui_textdesc.skew.x = -0.13;
ui_textdesc.translate.x = 13;
ui_image.rotate = -90 * 0.017453292519943295;
ui_image.translate.y = ui_image.draw_size.width;
ui_panel.rotate = 15 * 0.017453292519943295;
ui_panel.translate.x = -84;
ui_panel.translate.y = -50;
// all ui element elements
ui_panel.draw(ctx, 0, 0);
<canvas width="480" height="360"></canvas>
Related
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();
}
}
<!DOCTYPE html>
<html>
<body>
<canvas id="myCanvas" width="350" height="300"
style="border:6px solid black;">
</canvas>
<script>
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");
ctx.strokeStyle = 'gold';
ctx.strokeRect(20, 10, 160, 100);
</script>
</body>
</html>
Now, I want to go ahead and turn this drawn rectangle:
ctx.strokeStyle = 'gold';
ctx.strokeRect(20, 10, 160, 100);
Into a variable that I can just simply name "Rectangle" I can easily modify and call it out throughout my project. How can I do that? thank you!
You could use Path2D
Using Path2D to create paths and render them as needed is convenient and from a rendering standpoint paths are a little quicker as the sub paths do not need to be created every time you render the path.
It is best to create the sub paths around the origin (0,0) so you can easily move, rotate and scale them as needed.
Example creating some paths with different content
function createRect() {
const path = new Path2D();
path.rect(-70, -45, 140, 90); // add the sub path;
return path;
}
function createCircle() {
const path = new Path2D();
path.arc(0, 0, 50, 0, Math.PI * 2); // add the sub path;
return path;
}
function createRandLines() {
const path = new Path2D();
var i = 10;
while(i--) {
path.moveTo(Math.random() * 20 - 10, Math.random() * 20 - 10);
path.lineTo(Math.random() * 20 - 10, Math.random() * 20 - 10);
}
return path;
}
To create the paths
const myCircle = createCircle();
const myRect = createCircle();
const myLines1 = createRandLines();
const myLines2 = createRandLines();
You can then render any of the paths with a single function.
function strokePath(path, x, y, lineWidth = ctx.lineWidth, color = ctx.strokeStyle) { // defaults to current style
ctx.setTransform(1, 0, 0, 1, x, y); // position the path so its (0,0) origin is at x,y
ctx.lineWidth = lineWidth;
ctx.strokeStyle = color;
ctx.stroke(path);
}
Passing the path position style and line width to draw the path.
const W = ctx.canvas.width;
const H = ctx.canvas.height;
strokePath(myCircle, Math.random() * W, Math.random() * H);
strokePath(myRect, Math.random() * W, Math.random() * H);
strokePath(myLines1, Math.random() * W, Math.random() * H);
strokePath(myLines2, Math.random() * W, Math.random() * H);
Example
A more detailed draw function and some organisation in regards to the create path functions.
The example creates 4 paths once and then draws them many times, randomly positioned, rotated, scaled, alpha faded, colored, and filled.
const W = canvas.width;
const H = canvas.height;
const ctx = canvas.getContext("2d");
ctx.lineCap = ctx.lineJoin = "round";
// Some math utils
Math.TAU = Math.PI * 2;
Math.rand = (m = 0, M = 1) => Math.random() * (M - m) + m;
Math.randItem = array => array[Math.random() * array.length | 0];
const FACE = [[-3,-38,-34,-32,-47,-16,-48,15,-36,34,-5,43,32,38,47,12,47,-21,25,-35],[-31,-19,-42,-6,-32,1,-9,-6,-6,-24],[5,-24,3,-6,29,2,40,-5,33,-19],[-30,15,-14,32,12,31,29,15,15,15,-2,23,-17,16],[-28,-14,-29,-6,-18,-9,-17,-15],[11,-17,12,-8,20,-6,22,-13,18,-16],[2,-39,0,-53,-9,-60],[-14,17,-16,26,-7,28,-5,22],[2,21,1,28,11,27,13,16]];
// Object to hold path types
const paths = {
rect() {
const path = new Path2D();
path.rect(-20, -10, 40, 20); // add the sub path;
return path;
},
ellipse() {
const path = new Path2D();
path.ellipse(0, 0, 20, 10, 0, 0, Math.TAU); // add the sub path;
return path;
},
randLines() {
const path = new Path2D();
var i = 10;
while (i--) {
path.moveTo(Math.rand(-20, 20), Math.rand(-20, 20));
path.lineTo(Math.rand(-20, 20), Math.rand(-20, 20));
}
return path;
},
face() {
const path = new Path2D();
FACE .forEach(sub => { // each sub path
let i = 0;
path.moveTo(sub[i++] / 3, sub[i++] / 3);
while (i < sub.length) { path.lineTo(sub[i++] / 3, sub[i++] / 3) }
path.closePath();
});
return path;
}
};
// Function to draw scaled, rotated, faded, linewidth, colored path
function strokePath(path, x, y, scale, rotate, alpha, lineWidth, color, fillColor) {
ctx.lineWidth = lineWidth * (1 / scale); //Scale line width by inverse scale to ensure the pixel size is constant
ctx.setTransform(scale, 0, 0, scale, x, y); // position the path so its (0,0) origin is at x,y
ctx.rotate(rotate);
if (fillColor) {
ctx.globalAlpha = 1;
ctx.fillStyle = fillColor;
ctx.fill(path, "evenodd");
}
ctx.globalAlpha = alpha;
ctx.strokeStyle = color;
ctx.stroke(path);
}
// create some paths and colors
const pathArray = [paths.ellipse(), paths.rect(), paths.randLines(), paths.face()];
const colors = "#F00,#FA0,#0B0,#0AF,#00F,#F0A,#000,#888".split(",");
drawRandomPath();
function drawRandomPath() {
strokePath(
Math.randItem(pathArray), // random path
Math.rand(0, W), Math.rand(0, H), // random pos
Math.rand(0.25, 1), // random scale
Math.rand(0, Math.TAU), // random rotate
Math.rand(0.5, 1), // random alpha
1, // constant lineWidth
Math.randItem(colors), // random color
Math.rand() < 0.2 ? "#EED" : undefined, // Fill 1 in 5 with white
);
setTimeout(drawRandomPath, 250); // draw another path every quarter second.
}
* {margin:0px}
canvas {border:1px solid}
<canvas id="canvas" width="600" height="190"></canvas>
You can not do that with current standards unfortunately, you will have to redraw the shape, you can do something like:
var shape = x:10,y:20,width:20,height:40
clear the canvas and redraw with created variable:
shape.width = 100;
ctx.rect(shape.x,shape.y,shape.width,shape.height);
how can I fill the "new" canvas circle that appears next to the older one.
There is no problem with rectangle for example:
**
ctx.fillStyle = 'rgba('+quadratto.r+','+quadratto.g+','+quadratto.b+',1)';
quadratto.x += quadratto.speedX;
quadratto.y += quadratto.speedY;
quadratto.speedY += quadratto.speedY*(-0.15);
ctx.fillRect(quadratto.x-quadratto.h/4, quadratto.y-quadratto.h/2, 2, 2);**
What I want to do?
I'm creating animation in canvas where random-sized-color circle will appear and
it will move in a specified direction. The new canvas layaer will appear in the next frame (fps) with a new(old) circle.
var myCanvasPattern = document.createElement('canvas');
myCanvasPattern.width = window.innerWidth;
myCanvasPattern.height = window.innerHeight;
document.body.appendChild(myCanvasPattern);
var ryC = myCanvasPattern.getContext('2d');
function lottery(min, max) {
return Math.floor(Math.random()*(max-min+1))+min;
}
var allQuadro = [];
var fps = 50;
var lastTime = 0;
animationLoop();
function animationLoop(time){
requestAnimationFrame( animationLoop );
if(time-lastTime>=1000/fps){
lastTime = time;
for(var i=0;i<10;i++){
allQuadro.push({
r : lottery(0, 240),
g : lottery(0, 240),
b : lottery(0, 240),
circleR : lottery(10, 30),
x : myCanvasPattern.width/2,
y : myCanvasPattern.height/2,
speedX : lottery(-1000,1000)/100,
speedY : lottery(-1000,1000)/100
})
}
ryC.fillStyle = 'rgba(255,255,255,0.2)';
ryC.fill(0,0,myCanvasPattern.width, myCanvasPattern.height);
for(var i=0; i<allQuadro.length;i++){
var circle = allQuadro[i];
ryC.fillStyle = 'rgba('+circle.r+','+circle.g+','+circle.b+',1)';
circle.x += circle.speedX;
circle.y += circle.speedY;
//HERE's THE PROBLEM BELOW. HOW TO CREATE NEW ONE THAT APPEARS NEXT TO PREVIOUS ONE WITH NEW RANDOM COLOR
ryC.arc(circle.x-circle.circleR/2, circle.y-circle.circleR/2, circleR, 0, 2 * Math.PI);
//ryC.fill();
}
// ryC.fillStyle = 'rgba('+r+','+g+','+b+',1)';
//ryC.arc(x+speedX, y+speedY, circleR, 0, 2 * Math.PI);
//ryC.fill();
}
}
body {
margin: 0;
padding: 0;
overflow: hidden;
}
The fillRect() will fill directly to the canvas without going via a path (versus for example rect()).
The arc() on the other hand will add to a path which needs to be filled later. It also require the path to be cleared in-between the calls using beginPath().
A simple way to think about it is to wrap the necessary code into a function that acts like fillRect():
function fillArc() {
ctx.beginPath(); // clear current path
ctx.arc.apply(ctx, arguments); // apply arguments to arc() call
ctx.fill();
// notice that the arc still exist on the path after this call
// so to "truly" behave like fillRect() you could consider an
// additional beginPath() here.. it will depend on your code
}
In action:
var ctx = c.getContext("2d");
ctx.fillStyle = "#09f";
fillArc(70, 70, 70, 0, 6.28);
ctx.fillStyle = "#0a9";
fillArc(220, 70, 70, 0, 6.28);
function fillArc() {
ctx.beginPath();
ctx.arc.apply(ctx, arguments);
ctx.fill();
}
<canvas id=c></canvas>
If you are bold you can also add the method to the context itself before calling getContext():
CanvasRenderingContext2D.prototype.fillArc = function() {
this.beginPath();
this.arc.apply(this, arguments);
this.fill();
}
The use it like any other method:
ctx.fillArc( ... );
CanvasRenderingContext2D.prototype.fillArc = function() {
this.beginPath();
this.arc.apply(this, arguments);
this.fill();
}
var ctx = c.getContext("2d");
ctx.fillStyle = "#09f";
ctx.fillArc(70, 70, 70, 0, 6.28);
ctx.fillStyle = "#0a9";
ctx.fillArc(220, 70, 70, 0, 6.28);
<canvas id=c></canvas>
I'm struggling a bit to get this to work. I have a 'Canvas' element on my web page, and I need to 'draw' filled circles within each other. I need to use a loop to draw the pattern, alternating between red and blue filled circles. It will use the initial band width value of 25. It will repeat the loop as long as the current radius is greater than 0. It will use a slider to control the band width. The slider has a minimum value of 5,
maximum value of 50 with step 5, and current value as 25. As the value of
the slider changes, it draws the pattern with the current bandwidth. I can make this work with gradients, but that does not do what I need it to do and it does not look right. Here is what I have so far:
var sliderModule = (function(win, doc) {
win.onload = init;
// canvas and context variables
var context;
// center of the pattern
var centerX, centerY;
function init() {
canvas = doc.getElementById("canvas");
context = canvas.getContext("2d");
centerX = canvas.width / 2;
centerY = canvas.height / 2;
// draw the initial pattern
//drawPattern();
}
// called whenever the slider value changes
function drawPattern() {
var canvas;
// clear the drawing area
context.clearRect(0, 0, canvas.width, canvas.height);
// get the current radius
var radius = doc.getElementById("radius").value;
// set fill color to red
const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
const colors = ['#F00', '#0F0', '#00F'];
const outerRadius = 100;
let bandSize = 10; // this would be where you put the value for your slider
for (let r = outerRadius, colorIndex = 0; r > 0; r -= bandSize, colorIndex = (colorIndex + 1) % colors.length) {
ctx.fillStyle = colors[colorIndex];
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.height / 2, r, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
}
}
return {
drawPattern: drawPattern
};
})
(window, document);
<!doctype html>
<html>
<head>
<title></title>
<meta charset="utf-8">
<script src="bullsEye.js"></script>
</head>
<body>
<canvas id="canvas" width="400" height="400"></canvas>
<label for="bandwidth">BandWidth:</label>
<input type="range" id="radius" min="5" max="50" step="5" value="25" oninput="sliderModule.drawPattern()" />
</body>
</html>
var sliderModule = (function(win, doc) {
win.onload = init;
// canvas and context variables
var canvas;
var context;
// center of the pattern
var centerX, centerY;
function init() {
canvas = doc.getElementById("testCanvas");
context = canvas.getContext("2d");
centerX = canvas.width / 2;
centerY = canvas.height / 2;
// draw the initial pattern
drawPattern();
}
// called whenever the slider value changes
function drawPattern() {
// clear the drawing area
context.clearRect(0, 0, canvas.width, canvas.height);
// get the current radius
var radius = doc.getElementById("radius").value;
// set fill color to red
context.fillStyle = '#FF0000';
// draw the pattern
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI);
context.fill();
context.closePath();
}
return {
drawPattern: drawPattern
};
})(window, document);
Your snippet doesn't quite work as provided, but given a value for your slider, you'll just reduce radius and loop while radius is greater than 0.
const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
canvas.width = 200;
canvas.height = 200;
const colors = ['#F00', '#0F0', '#00F'];
const outerRadius = 100;
let bandSize = 10; // this would be where you put the value for your slider
for (let r = outerRadius, colorIndex = 0; r > 0; r -= bandSize, colorIndex = (colorIndex + 1) % colors.length) {
ctx.fillStyle = colors[colorIndex];
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.height / 2, r, 0, Math.PI * 2);
ctx.closePath();
ctx.fill();
}
<canvas />
What you're missing is the loop which would change. To control the colors, I made an array, and in addition to changing my radius each for-loop iterator, I also change the colorIndex.
I use (colorIndex + 1) % colorIndex.length so it'll loop through each of them and not go beyond the index (it'll count 0, 1, 2, and back to 0). You can change or add colors to the array.
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>