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How to adjust the position of the ball in a ruler chart between measurements -100, 0, 100 inside the canvas?

There's a graph that has measurements ranging from -100 to 0 and 0 to 100. What I need to do is adjust the calculation of the ball's position with the number according to the gradientDataChart input, which can be a value between -100 and 100, the problem is proportionality, depending on the screen, the ball is disappearing, I was using distance 16, but it only worked for a width of 586px;
I tested it here if that might make it easier.
Just writing this code, I used 38 for 900 width, the problem is that if you change it to 1000, it goes out of position... it needs to be responsive:
var widthDefault = 900, metric = -100, calcPos = 38 * ((metric / widthDefault) * 100);
ctx.beginPath();
canvas.width = widthDefault;
ctx.translate(calcPos, 0);
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
ctx.arc(centerX, centerY, 24, 0, Math.PI * 2, false);
ctx.fillStyle ="#ffffff";
ctx.fill()
ctx.font = 'bold 14pt sans-serif';
ctx.textAlign = 'center';
ctx.strokeStyle ='#622BCF'
ctx.stroke();
ctx.fillStyle ="#622bcf80";
ctx.fillText(`${medida}`, centerX, centerY+8);
ctx.globalCompositeOperation = 'destination-over';
Image example:

I'm not sure what you are doing in your calcPos, if you need something proportional you need to divide the width by the amount of points, your range is between -100 and 100, so we have 200 points, and we multiply that by the position on this case it is metric + 100...
X = width/200 * (metric + 100)
You can play with the code below:
var canvas = document.getElementById("c");
var ctx = canvas.getContext("2d");
function drawCircle(width, metric) {
var X = width/200 * (metric + 100)
var Y = canvas.height / 2;
ctx.beginPath();
ctx.arc(X, Y, 18, 0, Math.PI * 2, false);
ctx.stroke();
ctx.fillText(metric, X, Y);
}
drawCircle(canvas.width, -80)
drawCircle(canvas.width, -50)
drawCircle(canvas.width, 0)
drawCircle(canvas.width, 25)
drawCircle(canvas.width, 80)
<canvas id="c" width=300 height=140 style="border:solid 1px"></canvas>
Same code different canvas width:
var canvas = document.getElementById("c");
var ctx = canvas.getContext("2d");
function drawCircle(width, metric) {
var X = width/200 * (metric + 100)
var Y = canvas.height / 2;
ctx.beginPath();
ctx.arc(X, Y, 18, 0, Math.PI * 2, false);
ctx.stroke();
ctx.fillText(metric, X, Y);
}
drawCircle(canvas.width, -80)
drawCircle(canvas.width, -50)
drawCircle(canvas.width, 0)
drawCircle(canvas.width, 25)
drawCircle(canvas.width, 80)
<canvas id="c" width=600 height=140 style="border:solid 1px"></canvas>

How to rotate only a line around it's middle point?

I want to display a windmill on js canvas. For now, I display a green line on a cube. I have a problem with rotating the line around it's middle point. Also, I don't want my cube to move. Save() doesn't seem to work? I don't know what I'm doing wrong. I tried looking the answer online but they don't seem to work or I don't understand them. Elements in my canvas somehow disappear.
var x = 600;
function init()
{
window.requestAnimationFrame(draw);
}
function draw()
{
var ctx = document.getElementById('canvas').getContext('2d');
ctx.clearRect(0, 0, 600, 600);
// sciana przednia
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.strokeRect(x/2,x/2,x/4,x/4);
//sciana gorna
ctx.beginPath();
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.moveTo(x/2,x/2);
ctx.lineTo(x-x/3,x/4+x/6);
ctx.lineTo(x-x/8,x/4+x/6);
ctx.lineTo(x/2+x/4,x/2);
ctx.closePath();
ctx.stroke();
//sciana prawa
ctx.beginPath();
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.moveTo(x/2+x/4,x/2+x/4);
ctx.lineTo(x-x/8,x/2+x/7);
ctx.lineTo(x-x/8,x/4+x/6);
ctx.stroke();
ctx.closePath();
//raczka
ctx.beginPath();
ctx.lineWidth = 5;
ctx.strokeStyle = "#808080";
ctx.moveTo(x/2+x/5,x/2-x/5);
ctx.lineTo(x/2+x/5,x/2-x/8+50);
ctx.stroke();
ctx.closePath();
ctx.save();
//smiglo
ctx.beginPath();
ctx.translate(x/2, x/2);
ctx.rotate( (Math.PI / 180) * 25);
ctx.translate(-x/2, -x/2);
ctx.fillStyle = "#00cc00";
ctx.fillRect(x/2+x/5-100,x/2-x/5,200,10);
ctx.closePath();
ctx.restore();
window.requestAnimationFrame(draw);
}
init();
var canvas = document.getElementById('canvas')
var ctx = canvas.getContext('2d');
var x = 600;
function init() {
window.requestAnimationFrame(draw);
}
function draw() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
// sciana przednia
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.strokeRect(x / 2, x / 2, x / 4, x / 4);
//sciana gorna
ctx.beginPath();
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.moveTo(x / 2, x / 2);
ctx.lineTo(x - x / 3, x / 4 + x / 6);
ctx.lineTo(x - x / 8, x / 4 + x / 6);
ctx.lineTo(x / 2 + x / 4, x / 2);
ctx.closePath();
ctx.stroke();
//sciana prawa
ctx.beginPath();
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.moveTo(x / 2 + x / 4, x / 2 + x / 4);
ctx.lineTo(x - x / 8, x / 2 + x / 7);
ctx.lineTo(x - x / 8, x / 4 + x / 6);
ctx.stroke();
ctx.closePath();
//raczka
ctx.beginPath();
ctx.lineWidth = 5;
ctx.strokeStyle = "#808080";
ctx.moveTo(x / 2 + x / 5, x / 2 - x / 5);
ctx.lineTo(x / 2 + x / 5, x / 2 - x / 8 + 50);
ctx.stroke();
ctx.closePath();
ctx.save();
//smiglo
ctx.beginPath();
ctx.translate(x / 2, x / 2);
ctx.rotate((Math.PI / 180) * 25);
ctx.translate(-x / 2, -x / 2);
ctx.fillStyle = "#00cc00";
ctx.fillRect(x / 2 + x / 5 - 100, x / 2 - x / 5, 200, 10);
ctx.closePath();
ctx.restore();
window.requestAnimationFrame(draw);
}
init();
<canvas id="canvas" width=600 height=600></canvas>
Edit: I understand now how to rotate around a certain point. I still don't know how to rotate only the line not the whole thing.

Talking about the rotation, I think that you did well: the green line is rotated by 25 degrees in your example. You just need to rotate its middle point.
But to do so, I think it's better if you make other changes in your code: the part that draws the cube is difficult to handle, whenever you want to edit your code, this part will cause issues. I suggest to isolate it in a drawCube() function and to use proper (x,y) coordinates.
According to me, it should look like this:
function draw() {
angle += 5;
ctx.save();
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.translate(canvas.width / 2, canvas.height / 2);
ctx.rotate(Math.PI / 180 * angle);
ctx.translate(-canvas.width / 2, -canvas.height / 2);
// It rotates
drawLine();
ctx.restore();
// It doesn't rotate
drawCube();
}
Then, your code will work. All you will have to do is to make the line rotate around its middle point, but you said that you know how to do it.
Good luck!
EDIT: I added a snippet with an working example, and with a cube like yours as well, may help.
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var btnExample = document.getElementById('btnExample');
var btnCube = document.getElementById('btnCube');
var angle = 0;
var fps = 1000 / 60;
var propellerWidth = 100;
var propellerHeight = 10;
var towerWidth = 2;
var towerHeight = 100;
var mode = {
CUBE: 'CUBE',
EXAMPLE: 'EXAMPLE'
}
var currentMode = mode.EXAMPLE;
btnExample.onclick = function() {
currentMode = mode.EXAMPLE;
}
btnCube.onclick = function() {
currentMode = mode.CUBE;
}
setInterval(function() {
angle += 3;
draw(canvas, ctx, (canvas.width - propellerWidth) / 2, (canvas.height - propellerWidth) / 2, angle);
}, fps);
function draw(canvas, ctx, cx, cy, angle) {
var towerX = cx + propellerWidth / 2 - towerWidth / 2;
var towerY = cy + propellerWidth / 2;
var propellerX = (canvas.width - propellerWidth) / 2;
var propellerY = (canvas.height - propellerHeight) / 2;
ctx.save();
ctx.clearRect(0, 0, canvas.width, canvas.height);
// Draw other things that don't rotate
if (currentMode === mode.EXAMPLE) {
drawHelp(canvas, ctx);
}
ctx.translate(canvas.width / 2, canvas.height / 2);
ctx.rotate(Math.PI / 180 * angle);
ctx.translate(-canvas.width / 2, -canvas.height / 2);
// Draw things that rotate
drawPropeller(ctx, propellerX, propellerY, propellerWidth, propellerHeight);
ctx.restore();
// Draw other things that don't rotate
if (currentMode === mode.EXAMPLE) {
drawTower(ctx, towerX, towerY, towerWidth, towerHeight);
} else if (currentMode === mode.CUBE) {
drawCube(ctx, towerX, towerY, 30);
}
}
function drawPropeller(ctx, propellerX, propellerY, propellerWidth, propellerHeight) {
ctx.fillStyle = 'black';
ctx.fillRect(propellerX, propellerY, propellerWidth, propellerHeight);
}
function drawTower(ctx, towerX, towerY, towerWidth, towerHeight) {
ctx.fillStyle = 'red';
ctx.fillRect(towerX, towerY, towerWidth, towerHeight);
}
function drawCube(ctx, x, y, size) {
ctx.strokeStyle = 'black';
ctx.beginPath();
ctx.moveTo(x, y);
ctx.lineTo(x, y + size);
ctx.closePath();
ctx.stroke();
var x1 = x - size + (size / 4) + (size / 16);
var y1 = y + (size * 1.25);
var x2 = x1 + (size / 3);
var y2 = y1 - (size * 2 / 3);
var x3 = x2 + size;
var y3 = y2;
var x4 = x3 - (size / 3);
var y4 = y1;
var x5 = x4;
var y5 = y4 + size;
var x6 = x3;
var y6 = y3 + size;
ctx.strokeRect(x1, y1, size, size);
ctx.beginPath();
ctx.moveTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(x2, y2);
ctx.lineTo(x3, y3);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(x3, y3);
ctx.lineTo(x4, y4);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(x3, y3);
ctx.lineTo(x6, y6);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(x5, y5);
ctx.lineTo(x6, y6);
ctx.closePath();
ctx.stroke();
}
function drawHelp(canvas, ctx) {
ctx.globalAlpha = 0.2;
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(canvas.width, canvas.height);
ctx.moveTo(canvas.width, 0);
ctx.lineTo(0, canvas.height);
ctx.stroke();
ctx.closePath();
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.height / 2, propellerWidth / 2, 0, 2 * Math.PI);
ctx.stroke();
ctx.closePath();
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.height / 2, propellerWidth / 2, 0, 2 * Math.PI);
ctx.stroke();
ctx.closePath();
ctx.globalAlpha = 1;
}
canvas {
border: 1px solid black;
}
<canvas id="canvas" width=200 height=200></canvas>
<br>
<button id="btnExample">Example</button>
<button id="btnCube">Cube</button>

hide or remove a specific arc in canvas html5

I want to implement a ripple effect in JavaScript for that I am using a canvas element of HTML 5.
I have created 4 circles in order to implement ripple effect I am thinking of showing only one circle at a time here is my code
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var i = window.innerWidth/4;
var canvas = document.getElementById("myCanvas");
canvas.width = window.innerWidth/2;
canvas.height = window.innerWidth/2;
ctx.lineWidth = 2;
ctx.beginPath();
ctx.arc(i, i, i-i/1.5, 0, 2 * Math.PI);
ctx.stroke();
ctx.beginPath();
ctx.arc(i, i, i-i/2.5, 0, 2 * Math.PI);
ctx.stroke();
ctx.beginPath();
ctx.arc(i, i, i-i/5, 0, 2 * Math.PI);
ctx.stroke();
ctx.beginPath();
ctx.arc(i, i, i, 0, 2 * Math.PI);
ctx.stroke();
output
how can I remove a specific circle here
there is a function ctx.clearRect(x, y, width, height); but it works only on rectangle.
also, let me know whether it's a good approach to create a ripple effect in canvas

If you are looking for something like this, you can do it simply by resetting the canvas after some time.
Also you can trigger the elements to be created at a certain time to make that effect of showing them one by one.
Here is a sample
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var i = window.innerWidth/4;
var canvas = document.getElementById("myCanvas");
canvas.width = window.innerWidth/2;
canvas.height = window.innerWidth/2;
ctx.lineWidth = 2;
ctx.beginPath();
ctx.arc(i, i, i-i/1.5, 0, 2 * Math.PI);
ctx.stroke();
window.setTimeout(() => {
ctx.beginPath();
ctx.arc(i, i, i-i/2.5, 0, 2 * Math.PI);
ctx.stroke();
}, 200*1)
window.setTimeout(() => {
ctx.beginPath();
ctx.arc(i, i, i-i/5, 0, 2 * Math.PI);
ctx.stroke();
}, 200*2)
window.setTimeout(() => {
ctx.beginPath();
ctx.arc(i, i, i, 0, 2 * Math.PI);
ctx.stroke();
}, 200*3)
window.setTimeout(() => {
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var i = window.innerWidth/4;
var canvas = document.getElementById("myCanvas");
canvas.width = window.innerWidth/2;
canvas.height = window.innerWidth/2;
}, 200*6)
<canvas id="myCanvas"></canvas>

Here's a much much better and smoother ripple effect than the one you seek:
NOTE: run it on CodePen because for some reason it's not fully functional on stack overflow, but works fine in an HTML document or on CodePen
CodePen: https://codepen.io/Undefined_Variable/pen/dqZpzE
Code:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>canvas</title>
</head>
<body>
<canvas id="myCanvas"></canvas>
<script>
var canvas = document.body.querySelector("#myCanvas");
var ctx = canvas.getContext("2d");
canvas.width = window.innerWidth * 0.98;
canvas.height = window.innerHeight * 0.97;
var shapeArr = [];
window.addEventListener('resize', function () {
canvas.width = window.innerWidth * 0.98;
canvas.height = window.innerHeight * 0.98;
CW = canvas.width;
CH = canvas.height;
})
var CW = canvas.width;
var CH = canvas.height;
class circle {
constructor(centerX, centerY, radius) {
this.centerX = centerX;
this.centerY = centerY;
this.radius = radius;
this.opacity = 1;
this.strokeStyle = "rgba(0, 0, 0, " + this.opacity + ")";
}
expandCircle() {
this.radius += 3;
this.opacity -= 0.015;
this.strokeStyle = "rgba(0, 0, 0, " + this.opacity + ")";
if (this.radius > window.innerWidth/4) {
this.radius = 0;
this.opacity = 1;
}
}
}
function createCircle(centerX, centerY, radius) {
shapeArr.push(new circle(centerX, centerY, radius));
}
function drawCircle(centerX, centerY, radius, strokeClr) {
ctx.strokeStyle = strokeClr;
ctx.save();
ctx.translate(centerX, centerY);
ctx.beginPath();
ctx.arc(0, 0, radius, 0, Math.PI * 2);
ctx.closePath();
ctx.stroke();
ctx.restore()
}
createCircle(CW / 2, CH / 2, 0);
createCircle(CW / 2, CH / 2, 0);
createCircle(CW / 2, CH / 2, 0);
function firstWave() {
ctx.clearRect(0, 0, CW, CH);
drawCircle(shapeArr[0].centerX, shapeArr[0].centerY, shapeArr[0].radius, shapeArr[0].strokeStyle);
shapeArr[0].expandCircle();
requestAnimationFrame(firstWave);
};
firstWave();
setTimeout(function secondWave() {
drawCircle(shapeArr[1].centerX, shapeArr[1].centerY, shapeArr[1].radius, shapeArr[1].strokeStyle);
shapeArr[1].expandCircle();
requestAnimationFrame(secondWave);
}, 250);
setTimeout(function thirdWave() {
drawCircle(shapeArr[2].centerX, shapeArr[2].centerY, shapeArr[2].radius, shapeArr[2].strokeStyle);
shapeArr[2].expandCircle();
requestAnimationFrame(thirdWave);
}, 500)
</script>
</body>
</html>

How can I change the fill colors of each box for a chessboard in JS, using Canvas?

I just had a quick thought in mind of drawing a chessboard using JS and Canvas, and I have this code that draws the boxes alright with for loops.
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
var x, y,
boxWidth = 30,
boxHeight = 30;
for (x = 0; x < canvas.width; x += boxWidth) {
for (y = 0; y < canvas.height; y += boxHeight) {
ctx.beginPath();
ctx.rect(x, y, boxWidth, boxHeight);
ctx.stroke();
ctx.closePath();
}
}
<canvas id="canvas" width="240" height="240"></canvas>
Now I'm wondering how I can access each odd box on the axes to change their fill colors (e.g. black, white, black, white, and so on).
I know using global variables isn't the best way, but this is a very small project and I just want to get some logic on how I can alternate the colors of the chessboard. Your help is very much appreciated!

You could also try only incrementing your values by 1 (instead of boxWidth), which would make it simpler to check if they are even or odd. Then you would need to either scale or multiply by boxWidth and boxHeight:
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
var x, y,
boxWidth = 30,
boxHeight = 30;
var numbRows = Math.floor(canvas.width / boxWidth),
numbCols = Math.floor(canvas.height / boxHeight);
ctx.save();
ctx.scale(boxWidth, boxHeight);
for (x = 0; x < numbRows; x++) {
for (y = 0; y < numbCols; y++) {
if ((x+y) % 2 == 0) ctx.fillStyle = 'white';
else ctx.fillStyle = 'black';
ctx.beginPath();
ctx.rect(x, y, boxWidth, boxHeight);
ctx.stroke();
ctx.closePath();
}
}
ctx.restore();

You can use fillRect to do so like this:
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
var x, y,
boxWidth = 30,
boxHeight = 30;
for (x = 0; x < canvas.width; x += boxWidth) {
for (y = 0; y < canvas.height; y += boxHeight) {
ctx.fillStyle = (x / boxWidth + y / boxHeight) % 2 === 0? "white": "black"; // determine which color to use depending on the index of x (x / boxWidth) an the index of y (y / boxHeight)
ctx.fillRect(x, y, boxWidth, boxHeight);
}
}
<canvas id="canvas" width="240" height="240"></canvas>

canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
var x, y,
boxWidth = 30,
boxHeight = 30;
for (x = 0; x < canvas.width; x += boxWidth) {
for (y = 0; y < canvas.height; y += boxHeight) {
ctx.beginPath();
ctx.rect(x, y, boxWidth, boxHeight);
// fill odd boxes
(x/boxWidth + y/boxHeight) % 2 && ctx.fill()
ctx.stroke();
ctx.closePath();
}
}
<canvas id="canvas" width="240" height="240"></canvas>

Alternatives to the for loop
Another way without a loop, draw the pattern 2 by 2 square in top corner then repeat that by copying the canvas onto itself.
First create the top 2 by 2 square then fill rest of board with copies.
First 2 by 2 to 4 by 2
then 4 by 2 to 8 by 2
then 8 by 2 to 8 by 4
then 8 by 4 to 8 by 8
Example
const w= 100;
canvas.height = canvas.width = w * 8;
const ctx = canvas.getContext("2d");
ctx.fillStyle = "black";
ctx.fillRect(0, 0, w + w, w + w);
ctx.fillStyle = "white";
ctx.fillRect(0, 0, w, w);
ctx.fillRect(w, w, w, w);
ctx.drawImage(canvas, 0, 0, w * 2, w * 2, w * 2, y , w * 2, w * 2);
ctx.drawImage(canvas, 0, 0, w * 4, w * 2, w * 4, y , w * 4, w * 2);
ctx.drawImage(canvas, 0, 0, w * 8, w * 2, 0 , w * 2, w * 8, w * 2);
ctx.drawImage(canvas, 0, 0, w * 8, w * 4, 0 , w * 4, w * 8, w * 4);
Thus it gets drawn in 7 render calls, if the grid was larger then 2 more calls for 16 by 16, and every doubling in size only needs two more calls.
The pattern can be very complex but not create excessive render stress as in the next example that has shadows and different composite calls.
const squareSize = 72;
const boardSize = 8;
const borderSize = 8;
canvas.height = canvas.width = squareSize * boardSize + borderSize * 2;
const ctx = canvas.getContext("2d");
var x = borderSize;
var y = x;
var w = squareSize;
drawSquare(3, 3, canvas.width - 6, "black", "#F97");
drawSquare(x, y, w, "white", "#964");
drawSquare(w + x, y, w, "black", "#745");
ctx.drawImage(canvas, x, y, w, w, x + w, y + w, w, w);
ctx.drawImage(canvas, x + w, y, w, w, x, y + w, w, w);
ctx.drawImage(canvas, x, y, w * 2, w * 2, x + w * 2, y, w * 2, w * 2);
ctx.drawImage(canvas, x, y, w * 4, w * 2, x + w * 4, y, w * 4, w * 2);
ctx.drawImage(canvas, x, y, w * 8, w * 2, x, y + w * 2, w * 8, w * 2);
ctx.drawImage(canvas, x, y, w * 8, w * 4, x, y + w * 4, w * 8, w * 4);
drawSquare(0,0,canvas.width,"rgba(0,0,0,0.0)","rgba(0,0,0,0.05)");
// done.
// this function is only called twice.
function drawSquare(x,y,size,color,color2){
ctx.save();
ctx.shadowColor = color2;
ctx.shadowBlur = size * 0.2;
ctx.shadowOffsetX = 0;
ctx.shadowOffsetY = 0;
ctx.beginPath();
ctx.rect(x,y,size,size);
ctx.clip();
ctx.lineWidth = size;
ctx.fillStyle = color;
ctx.fillRect(x,y,size,size);
ctx.globalAlpha = 0.5;
ctx.strokeRect(x - size / 2,y - size / 2, size * 2, size * 2);
ctx.shadowBlur = size * 0.5;
ctx.strokeRect(x - size / 2,y - size / 2, size * 2, size * 2);
ctx.shadowColor = "rgba(0,0,0,0)";
ctx.shadowBlur = 0;
ctx.globalAlpha = 1;
ctx.strokeStyle = color2;
ctx.lineWidth = 2;
ctx.strokeRect(x+1,y+1,size -2,size-2);
ctx.globalAlpha = 0.75;
ctx.fillRect(x+1,y+1,size-2,size-2);
ctx.globalCompositeOperation = "screen";
ctx.fillStyle = "white";
ctx.globalAlpha = 0.1;
ctx.fillRect(x,y,4,size);
ctx.fillRect(x,y,2,size);
ctx.fillRect(x+4,y,size-4,4);
ctx.fillRect(x+2,y,size-2,2);
ctx.restore();
}
canvas { border : 2px solid black; }
<canvas id="canvas" ></canvas>
Use a pattern style.
Create an offscreen canvas to hold the pattern of the top 2 by 2, draw the pattern, then assign the fillStyle of the on screen canvas to a new pattern created from the off screen canvas and fill the whole canvas.
const w = 72;
const patCan = document.createElement("canvas");
patCan.height = patCan.width = w * 2;
var ctx = patCan.getContext("2d");
ctx.fillStyle = "black";
ctx.fillRect(0, 0, w + w, w + w);
ctx.fillStyle = "white";
ctx.fillRect(0, 0, w, w);
ctx.fillRect(w, w, w, w);
// setup display canvas
canvas.height = canvas.width = w * 8;
var ctx = canvas.getContext("2d");
ctx.fillStyle = ctx.createPattern(patCan, "repeat");
ctx.fillRect(0, 0, w * 8, w * 8);
canvas { border : 8px solid green; }
<canvas id="canvas" ></canvas>

Rendering with 3 lines of code
Here's a neat little trick you can use to draw a chess board:
var ctx = c.getContext("2d");
for(var x = 0; x < c.width; x += c.width / 4) ctx.fillRect(x, 0, c.width/8, c.height);
ctx.globalCompositeOperation = "xor"; // toggle alpha channel for every 2nd line
for(var y = 0; y < c.height; y += c.height / 4) ctx.fillRect(0, y, c.width, c.height/8);
<canvas id=c width=600 height=600></canvas>
We're using canvas' size to detemine the grid size. You can of course change this and offset to anything you like. You'd still use the divisors 4 (2 cells) and 8 (1 cell) with the actual width and height.
The first step draws vertical black stripes every other column. Then we toggle alpha channel for every other row knowing that the default color is black (rgba(0,0,0,0)) using the "xor" composite mode which toggles the alpha channel.
Just remember to start with a empty canvas (which you probably are anyways due to the need to redraw moves) and to set back composite mode to "source-over" after drawing the board.
If you'd like to change the color itself simply add an extra step at the end:
ctx.globalCompositeOperation = "source-atop"; // will draw on top of what is filled
ctx.fillStyle = "#09a";
ctx.fillRect(0, 0, c.width, c.height);
The fillStyle and fillRect() can be replaced or used with an image, pattern, gradient etc.
To fill the white background simply use composite mode "destination-over" (will draw behind anything filled using the alpha channel), then draw for background.
An alternative is to use a toggle switch when filling each cell one by one:
var ctx = c.getContext("2d");
var toggle = false;
ctx.beginPath();
for(var y=0; y < c.height; y += c.height / 8) {
toggle = !toggle; // toggle for each row so they don't line up
for(var x=0; x < c.width; x += c.width / 8) {
// toggle for each cell and check, only draw if toggle = true
if (toggle = !toggle) ctx.rect(x, y, c.width / 8, c.height / 8);
}
}
ctx.fill(); // remember to use beginPath() for consecutive draw ops
<canvas id=c width=600 height=600></canvas>
Access Logic
To know if you're inside a cell you would simply calculate the mouse position relative to the canvas (see this answer on how to do that) and then quantize (using pseudo variables here, replace with real):
var cellSize = boardWidth / 8; // assumes the board is 1:1 square
var pos = getMousePos(event); // see linked answer above
var cellX = Math.floor(pos.x / cellSize) * cellSize; // start of current cell X
var cellY = Math.floor(pos.y / cellSize) * cellSize; // start of current cell Y
(to get index of cell just drop the * cellSize part).
Example:
var ctx = c.getContext("2d"), x, y, w = c.width, h = c.height, cellSize = w / 8;
render();
ctx.lineWidth = 4; ctx.strokeStyle = "red"; ctx.setLineDash([7, 7]);
// non-optimized - in production only redraw when needed (cellX/Y changes)
c.onmousemove = function(e) {
render();
var cell = getCellPos(getMousePos(e));
if (cell.x >= 0 && cell.x < w && cell.y >=0 && cell.y < h)
ctx.strokeRect(cell.x + 2, cell.y + 2, cellSize - 4, cellSize - 4);
}
function getCellPos(pos) {
return {x: Math.floor(pos.x / cellSize) * cellSize,
y: Math.floor(pos.y / cellSize) * cellSize}
}
function getMousePos(e) {
var rect = c.getBoundingClientRect();
return {x: e.clientX-rect.x, y: e.clientY-rect.y}
}
function render() {
ctx.clearRect(0, 0, w, h);
for(x = 0; x < w; x += w>>2) ctx.fillRect(x, 0, cellSize, c.height);
ctx.globalCompositeOperation = "xor"; // toggle alpha channel for every 2nd line
for(y = 0; y < h; y += h>>2) ctx.fillRect(0, y, w, cellSize);
ctx.globalCompositeOperation = "source-atop"; // fg
ctx.fillStyle = "#3c4168";
ctx.fillRect(0, 0, w, h);
ctx.globalCompositeOperation = "destination-over"; // bg
ctx.fillStyle = "#eee";
ctx.fillRect(0, 0, w, h);
ctx.globalCompositeOperation = "source-over"; // reset
}
body {background:#222;margin:20px 0 0 20px;}
<canvas id=c width=600 height=600></canvas>

html5 canvas loading circle image

I'm trying to create a loading circle from an image in HTML5 canvas.
Here's the result I expect when percentage is at 50%:
here's what I've done after a lot of tests: (the blue stroke is just to see the circle, it'll be removed after)
var img = new Image();
img.onload = draw;
img.src = "http://i.imgur.com/HVJBZ1L.png";
var canvas = document.getElementsByTagName("canvas")[0];
canvas.width = 500;
canvas.height = 500;
var ctx = canvas.getContext("2d");
function draw() {
ctx.globalAlpha = 0.5
ctx.drawImage(img, 0, 0);
ctx.globalAlpha = 1;
var X = 50;
var Y = 50;
var Radius = img.width / 2;
var end = 40;
var start = 0;
var PI2 = Math.PI * 2;
var quart = Math.PI / 2;
var pct = 50 / 100;
var extent = (end - start) * pct;
var current = (end - start) / 100 * PI2 * pct - quart;
var pattern = ctx.createPattern(img, 'no-repeat');
ctx.beginPath();
ctx.arc(X, Y, Radius, -quart, current);
ctx.closePath();
ctx.fillStyle=pattern;
ctx.fill();
ctx.strokeStyle = "blue";
ctx.stroke();
}
<canvas></canvas>
as you can see, the result here isn't as excepted
What is wrong?

First, you need to correctly calculate your center point:
var X = img.width / 2;
var Y = img.height / 2;
Then you need to circle back in counter-clockwise direction tracing the inner radius Radius - 17:
ctx.beginPath();
ctx.arc(X, Y, Radius, -quart, current);
ctx.arc(X, Y, Radius - 17, current, -quart, true);
ctx.closePath();
If you aren't interested in the stroke outline, you could move to the center first, and then arc:
ctx.beginPath();
ctx.moveTo(X, Y);
ctx.arc(X, Y, Radius, -quart, current);
ctx.closePath();
Example:
var img = new Image();
img.onload = draw;
img.src = "http://i.imgur.com/HVJBZ1L.png";
var canvas = document.getElementsByTagName("canvas")[0];
canvas.width = 500;
canvas.height = 500;
var ctx = canvas.getContext("2d");
function draw() {
ctx.globalAlpha = 0.5
ctx.drawImage(img, 0, 0);
ctx.globalAlpha = 1;
var X = img.width / 2;
var Y = img.height / 2;
var Radius = img.width / 2;
var end = 40;
var start = 0;
var PI2 = Math.PI * 2;
var quart = Math.PI / 2;
var pct = 50 / 100;
var extent = (end - start) * pct;
var current = (end - start) / 100 * PI2 * pct - quart;
var pattern = ctx.createPattern(img, 'no-repeat');
ctx.beginPath();
ctx.moveTo(X, Y);
ctx.arc(X, Y, Radius, -quart, current);
ctx.closePath();
ctx.fillStyle = pattern;
ctx.fill();
}
<canvas></canvas>

const img = new Image();
img.src = "http://i.imgur.com/HVJBZ1L.png";
img.onload = imageLoaded;
var W,H; // width and height when image ready
const ctx = canvas.getContext("2d");
// define the distance of the progress 0- 100
const min = 0;
const max = 100;
var pattern;
var radius;
// get pattern, radius canvas size from image ans start animation
function imageLoaded(){
requestAnimationFrame(mainLoop);
W = this.width;
H = this.height;
pattern = ctx.createPattern(this, 'no-repeat');
radius = W / 2;
canvas.width = W;
canvas.height = H;
}
// draw the background and forground images
// amount is the amount of progress. amount >= min amount <= max
function draw(amount) {
ctx.globalAlpha = 0.5
ctx.drawImage(img, 0, 0);
ctx.globalAlpha = 1;
ctx.fillStyle=pattern;
ctx.strokeStyle = "blue";
ctx.beginPath();
ctx.arc( // draw inside circle CCW
W/2,
H/2,
radius - 17,
((amount - min) / (max-min)) * Math.PI * 2 - Math.PI / 2,
-Math.PI / 2,
true
);
ctx.arc( // draw outer circle CW
W/2,
H/2,
radius,
-Math.PI / 2,
((amount - min) / (max-min)) * Math.PI * 2 - Math.PI / 2
);
ctx.closePath();
ctx.fill();
ctx.stroke();
}
// animate the thing.
function mainLoop(time){
ctx.clearRect(0,0,canvas.width,canvas.height);
draw((time / 50) % max);
requestAnimationFrame(mainLoop);
}
canvas { border : 2px solid black;}
<canvas id="canvas"></canvas>