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Inward Circular Orbit - Canvas

I have a polygon that has circles on its vertices.
What I expect to accomplish is that every circle will be moving to the circle on its right. This is a physics concept which proves that if every circle is moving to the one on its right with a constant speed, soon they will reach the center. I'm trying to accomplish this animation, however I am able to move circles but not in the direction to the one next to it.
Here's my current code that draws the polygon with circles:
function particleGenerator(n){
const ctx = document.getElementById('poly').getContext('2d');
ctx.reset();
drawPolygon(ctx, 154, 71.25 , n, 50, 0, 5, 7.5);
}
const drawPolygon = (ctx, x, y, points, radius, rotation = 0, nodeSize = 0, nodeInset = 0) => {
ctx.beginPath();
ctx.moveTo(
x + radius * Math.cos(rotation),
y + radius * Math.sin(rotation)
);
for (let i = 1; i <= points; i += 1) {
const angle = (i * (2 * Math.PI / points)) + rotation;
ctx.lineTo(
x + radius * Math.cos(angle),
y + radius * Math.sin(angle)
);
}
ctx.fillStyle = "#00818A";
ctx.fill();
if (!nodeSize) return;
const dist = radius - nodeInset;
for (let i = 1; i <= points; i += 1) {
const angle = (i * (2 * Math.PI / points)) + rotation;
let x1 = x + dist * Math.cos(angle);
let y1 = y + dist * Math.sin(angle);
ctx.beginPath();
ctx.arc(x1, y1, nodeSize, 0, 2 * Math.PI);
ctx.fillStyle = "#DBEDF3"
ctx.fill();
}
};
<button onclick="particleGenerator(4)">Click Me!</button>
<canvas id="poly">

You can keep track of a list of corners. You generate them in order, so to get a corner's next neighbor you can do corners[i + 1] || corners[0].
To move the corner in the direction of the next one, you can calculate their differences in x and y coordinates and add a percentage of that difference to a corner's current location.
Here's a running example (I did remove some of the code so I could focus on just the updating problem:
function particleGenerator(n) {
const ctx = document.getElementById('poly').getContext('2d');
ctx.reset();
const originalCorners = createCorners(150, 70, n, 50);
const corners = createCorners(150, 70, n, 50);
const next = () => {
corners.forEach(([x0, y0], i) => {
const [x1, y1] = corners[i + 1] || corners[0];
const dx = x1 - x0;
const dy = y1 - y0;
const SPEED = 0.05;
corners[i][0] = x0 + dx * SPEED;
corners[i][1] = y0 + dy * SPEED;
});
}
const frame = () => {
ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
drawPolygon(ctx, originalCorners, "grey");
drawPolygon(ctx, corners);
drawDots(ctx, corners);
next();
requestAnimationFrame(frame);
};
frame();
}
const createCorners = (x, y, n, radius) => {
const corners = [];
for (let i = 1; i <= n; i += 1) {
const angle = (i * (2 * Math.PI / n));
corners.push([
x + radius * Math.cos(angle),
y + radius * Math.sin(angle)
]);
}
return corners;
}
const drawPolygon = (
ctx,
corners,
color = "#00818A"
) => {
// Draw fill
ctx.beginPath();
corners.forEach((c, i) => {
if (i === 0) ctx.moveTo(...c);
else ctx.lineTo(...c);
});
ctx.fillStyle = color
ctx.fill();
};
const drawDots = (
ctx,
corners,
) => {
// Draw dots
corners.forEach(([x, y], i, all) => {
ctx.beginPath();
ctx.arc(x, y, 5, 0, 2 * Math.PI);
ctx.fillStyle = "red"
ctx.fill();
});
};
<input type="number" value="6" min="3" max="100">
<button onclick="particleGenerator(document.querySelector('input').valueAsNumber)">Click Me!</button>
<canvas id="poly">

Direct the Image positioning used to as strokeStyle in canvas

I am trying to use an image as stroke style, but I have a problem on how to direct how the pattern (arrow image) is placed.
For example I used an arrow as pattern for the strokeStyle. I want the arrow to be pointing forward at the top rectangle, point down at the right side, point backwards at the bottom and then pointing up at the right hand side of the rectangle.
More like the image should follow the shape of the rectangle
Using normal stroke just places the arrow in a straight pattern as shown in the picture.
enter image description here
This is my code
function drawPattern(img, size) {
const canvas = document.getElementById("canvas");
const tempCanvas = document.createElement("canvas");
const tCtx = tempCanvas.getContext("2d");
tempCanvas.width = size;
tempCanvas.height = size;
tCtx.drawImage(img, 0, 0, img.width, img.height, 0, 0, size, size);
const ctx = canvas.getContext("2d");
ctx.clearRect(0, 0, canvas.width, canvas.height);
const pat = ctx.createPattern(tempCanvas, "repeat");
ctx.strokeStyle = pat;
ctx.lineWidth = 100;
ctx.strokeRect(0, 0, canvas.width, canvas.height);
}
const img = new Image();
img.src = "http://freundbild.com/arrow.png";
img.onload = function () {
drawPattern(this, 100);
};

You can use a matrix DOMMatrix or DOMMatrixReadOnly to orientate the pattern using the patterns setTransform function.
UPDATE
See code for how.
Pattern created in code, but can be an image. Just create the pattern and pass to the following function.
Note that the second function is the image the pattern was created from. The example uses a square pattern so the second argument is just the size
Note that the pattern is scaled to fit the lineWidth so will not maintain its aspect.
function orientPattern(ctx, image, pattern, dist, lineWidth, p1, p2) {
const dx = p2.x - p1.x;
const dy = p2.y - p1.y;
const d = (dx * dx + dy * dy) ** 0.5;
const nx = dx / d;
const ny = dy / d;
const w = image.width;
const h = image.height;
const yScale = h / lineWidth;
const mat = new DOMMatrixReadOnly([
nx, ny,
-ny / yScale, nx / yScale,
p1.x - ((ny * h * 0.5) % h) / yScale - (nx * (dist % w)) ,
p1.y + ((nx * h * 0.5) % h) / yScale - (ny * (dist % w))
]);
pattern.setTransform(mat);
return pattern;
}
Note You need to draw each line segment of the shape you want to draw one at a time. You can not fit it to a rect arc or a path that is not a straight line.
Note Line joins will have holes or overlay (depending on the line cap setting) There is no easy way to overcome this problem without writing a full replacement of the 2D stroke function. That would incur a serious performance hit on rendering this type of path.
const ctx = canvas.getContext("2d");
function createArrowPattern(size, bgCol, col) {
const c = document.createElement("canvas");
c.width = c.height = size;
const ctx = c.getContext("2d");
ctx.fillStyle = bgCol;
ctx.fillRect(0,0, size, size);
ctx.fillStyle = col;
const u = size / 5;
ctx.setTransform(size, 0, 0, size, size / 2, size / 2);
ctx.beginPath();
ctx.lineTo(-0.4, -0.2);
ctx.lineTo( 0.1, -0.2);
ctx.lineTo( 0.1, -0.5);
ctx.lineTo( 0.4, 0);
ctx.lineTo( 0.1, 0.5);
ctx.lineTo( 0.1, 0.2);
ctx.lineTo(-0.4, 0.2);
ctx.fill();
return ctx.createPattern(c, "repeat");
}
function orientPattern(ctx, size, pattern, dist, lineWidth, p1, p2) {
const dx = p2.x - p1.x;
const dy = p2.y - p1.y;
const d = (dx * dx + dy * dy) ** 0.5;
const nx = dx / d;
const ny = dy / d;
const yScale = size / lineWidth;
const mat = new DOMMatrixReadOnly([
nx, ny,
-ny / yScale, nx / yScale,
p1.x - ((ny * size * 0.5) % size) / yScale - (nx * (dist % size)) ,
p1.y + ((nx * size * 0.5) % size) / yScale - (ny * (dist % size))
]);
pattern.setTransform(mat);
return pattern;
}
function drawPatternPath(ctx, size, pattern, lineWidth, start, ...points) {
var i = 0;
ctx.lineWidth = lineWidth;
ctx.lineCap = "round";
var dist = 0;
var p1 = points[i++]
while (i < points.length) {
const p2 = points[i++];
ctx.strokeStyle = orientPattern(ctx, size, pattern, -start + dist, lineWidth, p1, p2);
ctx.beginPath();
ctx.lineTo(p1.x, p1.y);
ctx.lineTo(p2.x, p2.y);
ctx.stroke();
// dist += 10
dist += ((p1.x - p2.x) ** 2 + (p1.y - p2.y) ** 2) ** 0.5;
p1 = p2;
}
}
const P = (x, y) => ({x,y});
const ARROW_SIZE = 64;
const LINE_WIDTH = 22;
const arrow = createArrowPattern(ARROW_SIZE, "white", "red");
var pos = 0;
animate()
function animate() {
ctx.clearRect(0,0,400,300);
drawPatternPath(
ctx, ARROW_SIZE, arrow, LINE_WIDTH, pos,
P(30,30),
P(370,30),
P(370, 200),
P(350, 250),
P(300, 270),
P(30,270),
P(30,30)
);
pos += 1;
requestAnimationFrame(animate);
}
<canvas id="canvas" width ="400" height="300"></canvas>
I consider this approach a hack and that there is a need to map images, patterns, gradients, and text, to strokes (as paths) in the 2D API as at the moment there is no acceptable workaround in both quality and performance.
Alternative option is WebGL which is easily able to draw such patterned paths with excellent speed and exceptional quality however to integrate it with the 2D API is non trivial, and once along that path why bother with the 2D API at all.

Rotating Rectangles Around Circle Perimeter on Canvas

I'm trying to create a little circular "equalizer" effect using JavaScript and HTML canvas for a little project I'm working on, and it works great, except one little thing. It's just a series of rectangular bars moving in time to an mp3 - nothing overly fancy, but at the moment all the bars point in one direction (i.e. 0 radians, or 90 degrees).
I want each respective rectangle around the edge of the circle to point directly away from the center point, rather than to the right. I have 360 bars, so naturally, each one should be 1 degree more rotated than the previous.
I thought that doing angle = i*Math.PI/180 would fix that, but it doesn't seem to matter what I do with the rotate function - they always end up pointing in weird and wonderful directions, and being translated a million miles from where they were. And I can't see why. Can anyone see where I'm going wrong?
My frame code, for reference, is as follows:
function frames() {
// Clear the canvas and get the mp3 array
window.webkitRequestAnimationFrame(frames);
musicArray = new Uint8Array(analyser.frequencyBinCount);
analyser.getByteFrequencyData(musicArray);
ctx.clearRect(0, 0, canvas.width, canvas.height);
bars = 360;
for (var i = 0; i < bars; i++) {
// Find the rectangle's position on circle edge
distance = 100;
var angle = i * ((Math.PI * 2) / bars);
var x = Math.cos(angle) * distance + (canvas.width / 2);
var y = Math.sin(angle) * distance + (canvas.height / 2);
barWidth = 5;
barHeight = (musicArray[i] / 4);
// Fill with a blue-green gradient
var grd = ctx.createLinearGradient(x, 0, x + 40, 0);
grd.addColorStop(0, "#00CCFF");
grd.addColorStop(1, "#00FF7F");
ctx.fillStyle = grd;
// Rotate the rectangle according to position
// ctx.rotate(i*Math.PI/180); - DOESN'T WORK
// Draw the rectangle
ctx.fillRect(x, y, barHeight, barWidth);
}

For clarity I've removed part of your code. I'm using rotate as you intended. Also I'm using barHeight = (Math.random()* 50); instead your (musicArray[i]/4); because I wanted to have something to show.
Also I've changed your bars to 180. It's very probable that you won't have 360 bars but 32 or 64 or 128 or 256 . . . Now you can change the numbers of bare to one of these numbers to see the result.
I'm drawing everything around the origin of the canvas and translating the context in the center.
I hope it helps.
const canvas = document.getElementById("c");
const ctx = canvas.getContext("2d");
let cw = canvas.width = 400;
let ch = canvas.height = 400;
let bars = 180;
let r = 100;
ctx.translate(cw / 2, ch / 2)
for (var i = 0; i < 360; i += (360 / bars)) {
// Find the rectangle's position on circle edge
var angle = i * ((Math.PI * 2) / bars);
//var x = Math.cos(angle)*r+(canvas.width/2);
//var y = Math.sin(angle)*r+(canvas.height/2);
barWidth = 2 * Math.PI * r / bars;
barHeight = (Math.random() * 50);
ctx.fillStyle = "green";
// Rotate the rectangle according to position
// ctx.rotate(i*Math.PI/180); - DOESN'T WORK
// Draw the rectangle
ctx.save();
ctx.rotate(i * Math.PI / 180);
ctx.fillRect(r, -barWidth / 2, barHeight, barWidth);
//ctx.fillRect(r ,0, barHeight, barWidth);
ctx.restore();
}
canvas {
border: 1px solid
}
<canvas id="c"></canvas>

Here is another solution, I'm preserving your initial trigonometry approach.
But instead of rectangles I used lines, I don't think it makes a difference for you, if what you need is bars moving in time to an mp3 all you need to do is change the var v = Math.random() + 1; to a reading from the Amplitude, and those bars will be dancing.
const canvas = document.getElementById("c");
canvas.width = canvas.height = 170;
const ctx = canvas.getContext("2d");
ctx.translate(canvas.width / 2, canvas.height / 2)
ctx.lineWidth = 2;
let r = 40;
let bars = 180;
function draw() {
ctx.clearRect(-100, -100, 200, 200)
for (var i = 0; i < 360; i += (360 / bars)) {
var angle = i * ((Math.PI * 2) / bars);
var x = Math.cos(angle) * r;
var y = Math.sin(angle) * r;
ctx.beginPath();
var v = Math.random() + 1;
ctx.moveTo(x, y);
ctx.lineTo(x * v, y * v)
grd = ctx.createLinearGradient(x, y, x*2, y*2);
grd.addColorStop(0, "blue");
grd.addColorStop(1, "red");
ctx.strokeStyle = grd;
ctx.stroke();
}
}
setInterval(draw, 100)
<canvas id="c"></canvas>

Trying to create a pattern out of circles?

I am trying to imitate a pattern I found on the internet, but I get weird lines in the middle and when trying to connect another set of circles on top.
Also, when I try to fill, it becomes fully black.
console.log("grid");
var canvas = document.getElementById("canvas");
var image_b = document.getElementById("brown");
var image_g = document.getElementById("grey");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var ctx = canvas.getContext("2d");
var side = 160;
var side2 = 150;
ctx.strokeStyle = 'black';
ctx.fillStyle = 'white';
function draw() {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
ctx.clearRect(0, 0, window.innerWidth, window.innerHeight);
var widthNbr = Math.ceil(window.innerWidth / side) + 1;
var heightNbr = Math.ceil(window.innerHeight / side) + 1;
var counter = 0;
for (var i = 0; i < widthNbr; i++) {
for (var j = 0; j < heightNbr; j++) {
ctx.beginPath();
var x = side * i + side / 2;
var y = side * j + side / 2;
var a = side * i + side / 2;
var s = side * j + side / 2;
var d = side * i + side / 2;
var f = side * j + side / 2;
var g = side * i + side / 2;
var h = side * j + side / 2;
var q = side * i + side / 2;
var w = side * j + side / 2;
var o = side * i + side / 2;
var p = side * j + side / 2;
var x1 = side2 * i + side2;
var y1 = side2 * j + side2;
var a1 = side2 * i + side2;
var s1 = side2 * j + side2;
var d1 = side2 * i + side2;
var f1 = side2 * j + side2;
var g1 = side2 * i + side2;
var h1 = side2 * j + side2;
var q1 = side2 * i + side2;
var w1 = side2 * j + side2;
var o1 = side2 * i + side2;
var p1 = side2 * j + side2;
ctx.arc(x, y, side / 2, 0, Math.PI * 2);
ctx.arc(a, s, side / 2.5, 0, Math.PI * 2);
ctx.arc(d, f, side / 3.5, 0, Math.PI * 2);
ctx.arc(g, h, side / 5.3, 0, Math.PI * 2);
ctx.arc(q, w, side / 9, 0, Math.PI * 2);
ctx.arc(o, p, side / 18, 0, Math.PI * 2);
ctx.lineWidth = 5;
ctx.arc(x1, y1, side2 / 2, 0, Math.PI * 2);
ctx.arc(a1, s1, side2 / 2.5, 0, Math.PI * 2);
ctx.arc(d1, f1, side2 / 3.5, 0, Math.PI * 2);
ctx.arc(g1, h1, side2 / 5.3, 0, Math.PI * 2);
ctx.arc(q1, w1, side2 / 9, 0, Math.PI * 2);
ctx.arc(o1, p1, side2 / 18, 0, Math.PI * 2);
ctx.stroke();
// ctx.fill();
ctx.closePath();
counter++;
}
}
}
draw();
<canvas id="canvas"></canvas>

You have to think about canvas Path drawings as pencil drawing on a paper :
Just after the path declaration (beginPath), when you say ctx.arc(x, y, rad, 0, Math.PI*2) your pen goes to coordinates (x, y), and because x and y are the center position of your arc it will be putted at a rad distance from this center to draw the circle. Your 0 tells it to start at 3 o'clock, so in this case, we just need to add this rad to the x value.
At this moment, your pen is on the paper.
It draws the arc, and when you tell it arc(x1, y1, rad, ...), it goes directly to coordinates (x1+rad, y1) and draws the new arc.
The problem here is that you never told it to raise the pencil from the paper, so you can see the line that goes from the last point on the first arc to the first point on the next one.
Fortunately, Canvas API comes with a handy set of operations, and the "Raise_the_pen_and_move_to_coordinates_x,y_without_ruining_my_paper" is simply called moveTo.
By telling the context to gently raise the pencil and to move to the next first drawing point, before actually drawing the arc, you'll avoid all these trailing lines.
So basically, for three arcs it would be :
// initialize a new drawing
ctx.beginPath();
// here we can set it directly because the pen is not on the paper yet
ctx.arc(x, y, rad, 0, Math.PI*2);
// tell it to raise the pen off the paper
// and to go to the next starting point (3 o'clock in our case)
ctx.moveTo(x1 + rad, y1);
ctx.arc(x1, y1, rad, 0, Math.PI*2);
// once again
ctx.moveTo(x2 + rad, y2);
ctx.arc(x2, y2, rad, 0, Math.PI*2);
// now we've got clear independents arcs
ctx.stroke();
And with your code (That you could clean a lot by using arrays btw)
var canvas = document.getElementById("canvas");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var ctx = canvas.getContext("2d");
var side = 160;
var side2 = 150;
ctx.strokeStyle = 'black';
ctx.fillStyle = 'white';
function draw() {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
ctx.clearRect(0, 0, window.innerWidth, window.innerHeight);
var widthNbr = Math.ceil(window.innerWidth / side) + 1;
var heightNbr = Math.ceil(window.innerHeight / side) + 1;
var counter = 0;
for (var i = 0; i < widthNbr; i++) {
for (var j = 0; j < heightNbr; j++) {
ctx.beginPath();
var x = side * i + side / 2;
var y = side * j + side / 2;
var a = side * i + side / 2;
var s = side * j + side / 2;
var d = side * i + side / 2;
var f = side * j + side / 2;
var g = side * i + side / 2;
var h = side * j + side / 2;
var q = side * i + side / 2;
var w = side * j + side / 2;
var o = side * i + side / 2;
var p = side * j + side / 2;
var x1 = side2 * i + side2;
var y1 = side2 * j + side2;
var a1 = side2 * i + side2;
var s1 = side2 * j + side2;
var d1 = side2 * i + side2;
var f1 = side2 * j + side2;
var g1 = side2 * i + side2;
var h1 = side2 * j + side2;
var q1 = side2 * i + side2;
var w1 = side2 * j + side2;
var o1 = side2 * i + side2;
var p1 = side2 * j + side2;
ctx.moveTo(x + side / 2, y);
ctx.arc(x, y, side / 2, 0, Math.PI * 2);
ctx.moveTo(a + side / 2.5, s);
ctx.arc(a, s, side / 2.5, 0, Math.PI * 2);
ctx.moveTo(d + side / 3.5, f)
ctx.arc(d, f, side / 3.5, 0, Math.PI * 2);
ctx.moveTo(g + side / 5.3, h)
ctx.arc(g, h, side / 5.3, 0, Math.PI * 2);
ctx.moveTo(q + side / 9, w)
ctx.arc(q, w, side / 9, 0, Math.PI * 2);
ctx.moveTo(o + side / 18, p)
ctx.arc(o, p, side / 18, 0, Math.PI * 2);
ctx.lineWidth = 5;
ctx.moveTo(x1 + side2 / 2, y1)
ctx.arc(x1, y1, side2 / 2, 0, Math.PI * 2);
ctx.moveTo(a1 + side2 / 2.5, s1)
ctx.arc(a1, s1, side2 / 2.5, 0, Math.PI * 2);
ctx.moveTo(d1 + side2 / 3.5, f1)
ctx.arc(d1, f1, side2 / 3.5, 0, Math.PI * 2);
ctx.moveTo(g1 + side2 / 5.3, h1)
ctx.arc(g1, h1, side2 / 5.3, 0, Math.PI * 2);
ctx.moveTo(q1 + side2 / 9, w1)
ctx.arc(q1, w1, side2 / 9, 0, Math.PI * 2);
ctx.moveTo(o1 + side2 / 18, p1)
ctx.arc(o1, p1, side2 / 18, 0, Math.PI * 2);
ctx.stroke();
counter++;
}
}
}
draw();
<canvas id="canvas"></canvas>
As correctly noted by Spencer Wieczorek in comments above, to get the result you wanted, you'll also have to white-fill the largest arcs, but I let you find the way to do it as a training.
Also, a small note on closePath() that you were using in your code, his name might be quite confusing when we see the number of people misusing it, but note that it doesn't ends your Path declaration. All it does is a lineTo(last_time_I_putted_the_pencil). In the case of closed circle, it doesn't have any effect because last_time_I_putted_the_pencil === current_pencil_position_on_the_paper, but it's often the source of a lot of problems.
And an other small note, for users a bit more experienced (probably OP in few days / weeks) :
Other operations allow us to raise our pencil from the paper : the transformation commands.
(mainly setTransform, and its subsets transform, translate, rotate and scale).
These operations will first raise the pen, and then move the paper rather than the pen. This comes handy in a lot of situations.
And to set it back to its normal position, you just have to call setTransform(1,0,0,1,0,0).

As I was far too slow in responding to this question please consider this an addendum to the excellent answer already provided by Kaiido.
When thinking about problems like this it is sometimes useful to separate the calculation of values from their application. Of course, this kind of understanding only comes with experience, unless that is we can plug into the experience of others - which is exactly what sites like StackOverflow are for! :)
The image we're intending to make is made entirely of circles, so we can greatly reduce repetition in our code by creating a function that deals with just that one thing for us. Something like...
/* draw circle */
function drawCircle(x, y, r, LW) {
context.lineWidth = LW;
context.beginPath();
context.arc(x, y, r, 0, Math.PI*2, true);
context.fill();
context.stroke();
}
Although this function only draws a single circle to the canvas we can pass it values that can be used to draw every element we need.
The reference image is built out of sets of circles, where each circle-set has the same (x,y) position. If we know those starting co-ordinates for X & Y, and the radius of the largest circle in the set, then we can create a function to calculate the values we need and pass them into the drawCircles() function above...
function circleSet(X, Y, Radius) {
var count = 5; /* number of circles in each set */
var step = Radius / count;
var ln_width;
var rad;
while (count > 0) {
ln_width = count > 3 ? 3 : (count > 1 ? 2 : 1);
rad = count * step;
drawCircle(X, Y, rad, ln_width);
count--;
}
}
The while-loop reduces the count variable from 5 to 1 and line width and radius values are calculated before being passed into the aforementioned drawCircle() function, along with (x,y) co-ordinates.
The conditional in the first line of the while-loop says:
if count is 4 or 5 then ln_width equals 3;
or else if count is 2 or 3 then ln_width equals 2;
or else ln_width equals 1.
The rad variable holds a value that starts at Radius and is decreased by 1 x steps for each iteration of the loop.
Now that the circle-set parameters are ready to be calculated all that's needed is to calculate the values to pass to the circleSet() function, that is; the (x,y) co-ordinates and starting radius of each set.
As your original code snippet showed, we can use two nested for-loops for this. one to deal with the vertical and one to deal with the horizontal, but first we need to make some decisions.
If we want to have 10 circle-sets across the canvas then the width of each set would be...
var circleSet_Size = canvas.width / 10;
...and the maximum radius of each circle-set would therefore be...
var circleSet_Radius = circleSet_Size / 2;
We also need to draw two lots of circle-sets to make something like the reference image, one lot for the 'background' and one lot for the 'foreground'. So we need to determine the starting (x,y) co-ordinates of each pass as well as width/height of the area we want to draw over. We can create a function for that too. Something like...
function loopXYPosition(startX, startY) {
for (var y = startY; y < (canvas.height + circleSet_Radius); y += circleSet_Size) {
for (var x = startX; x < (canvas.width + circleSet_Radius); x += circleSet_Size) {
circleSet(x, y, setRadius);
}
}
}
/* 'background' pass */
loopXYPosition(0, 0);
/* 'foreground' pass */
loopXYPosition(circleSet_Radius, circleSet_Radius);
With all that in place we can collate our functions into a single script. But before we do that it's worth taking note of those values which need to be calculated each time a function is called and which values remain static once calculated. Bearing all that in mind we end up with something like...
var circlePattern = (function() {
/* define variables available
to all functions */
var canvas
, ctx
, cWidth
, cHeight
, circleCount
, circleSet_Size
, circleSet_Radius
, P360;
/* draw each cicle */
function drawCircle(x, y, r, LW) {
ctx.beginPath();
ctx.lineWidth = LW;
ctx.arc(x, y, r, 0, P360, true);
ctx.fill();
ctx.stroke();
}
/* calculate each set of circles (circle-set) */
function circleSet(X, Y, R) {
var count = circleCount,
radiusSteps = R / count,
ln_width,
rad;
while (count > 0) {
ln_width = count > 3 ? 2.5 : (count > 1 ? 2 : 1.5);
rad = count * radiusSteps;
drawCircle(X, Y, rad, ln_width);
count--;
}
}
function loopXYPosition(startX, startY) {
/* add circleSet_Radius to canvas width
and height to make sure we draw right
up to the edges of the canvas */
var cHcR = cHeight + circleSet_Radius;
var cWcR = cWidth + circleSet_Radius;
/* to get the effect we want we need to create
a little padding around each circle-set,
therefore we reduce the circelSet_Radius value
by 5% before passing to drawCircleSet() */
var setRadius = circleSet_Radius * 0.95;
/* step across and down canvas in
increments of 'circleSet_Size' */
for (var y = startY; y < cHcR; y += circleSet_Size) {
for (var x = startX; x < cWcR; x += circleSet_Size) {
circleSet(x, y, setRadius);
}
}
}
function begin(NoC, cCount) {
var numberOfCircles = NoC;
/* set variables needed later */
circleSet_Size = cWidth / numberOfCircles;
circleSet_Radius = circleSet_Size / 2;
circleCount = cCount;
/* draw rows of circles */
loopXYPosition(0, 0);
loopXYPosition(circleSet_Radius, circleSet_Radius);
}
/* initialise canvas */
function init(e) {
/* Set variables to use later */
canvas = document.getElementById('canvas');
ctx = canvas.getContext('2d');
cWidth = canvas.width;
cHeight = canvas.height;
P360 = Math.PI * 2;
/* fillStyle & strokeStyle are properties
of the Canvas, so we only need to set
them once here */
ctx.fillStyle = '#fff';
ctx.strokeStyle = '#000';
/* fill canvas (background) */
ctx.fillRect(0, 0, cWidth, cHeight);
/* first argument: number of horizontal circle-sets
second argument: number of circles in each set
!Try different values here! */
begin(7, 6);
}
return {
go: init
};
}());
window.onload = circlePattern.go;
body {
color:#000;
background-color:#fff;
font-family:sans-serif;
}
div.box {
width:100%;
height:auto;
text-align:center;
margin:2em auto;
display:block;
}
div.box h3 {
font-size:1.3em;
line-height:2.3em;
}
#refimg, #canvas {
width:600px;
margin:0 auto;
clear:both;
display:block;
}
#refimg {
height:360px;
}
#canvas {
height:400px;
}
<div class="box">
<h3>Refernece Image</h3>
<img id="refimg" src="http://i.stack.imgur.com/Uxm2Z.jpg)" alt="image" title="reference image" />
</div>
<div class="box">
<h3>Canvas Image</h3>
<canvas id="canvas" width="600" height="400" title="Canvas image"></canvas>
</div>
Not exactly the same results, though pretty close. With the exception of the (x,y) co-ordinates which pass through it the circleSet() function produces the same sequence of values each time it is called, so those values could be calculated once and stored in an Object, but I've included it here for simplicity and to highlight the sequence of events.
Feeding different values into the begin() function and playing with setRadius in the loopXYPosition() function yields some interesting results.
I hope the process outlined here gives you some hints in your continuing exploration of the HTML5 Canvas API.
;)

How can I generate a rainbow circle using HTML5 canvas?

I would like to generate a canvas image using gradients in some clever way. I would like the image to looks something like this:
I just can't get my head around it. I need to generate lines in the form and arc - or use gradients with color stops in some clever way. Maybe it would be a lot easier if I converted to HSL and just go through the HUE values?
For example in a rectangle format I could
for (var i = 0; i < h; ++i) {
var ratio = i/h;
var hue = Math.floor(360*ratio);
var sat = 100;
var lum = 50;
line(dc, hslColor(hue,sat,lum), left_margin, top_margin+i, left_margin+w, top_margin+i);
}
Does anybody have any clever tips on how to produce this image using canvas?

This is not perfect (due to drawing steps ...), but it can help you :
http://jsfiddle.net/afkLY/2/
HTML:
<canvas id="colors" width="200" height="200"></canvas>
Javascript:
var canvas = document.getElementById("colors");
var graphics = canvas.getContext("2d");
var CX = canvas.width / 2,
CY = canvas.height/ 2,
sx = CX,
sy = CY;
for(var i = 0; i < 360; i+=0.1){
var rad = i * (2*Math.PI) / 360;
graphics.strokeStyle = "hsla("+i+", 100%, 50%, 1.0)";
graphics.beginPath();
graphics.moveTo(CX, CY);
graphics.lineTo(CX + sx * Math.cos(rad), CY + sy * Math.sin(rad));
graphics.stroke();
}
The idea is to draw the disc line by line with a hue value corresponding to the line direction.
You can change the color base rotation by adding a radius angle to rad variable (adding -pi/2 to rad would make the gradient look like your figure).
EDIT:
I made a new demo that generalizes the concept a bit and renders a rainbow polygon. Here is the CodePen.
To get rid of the small voids beteween the colors, I used quads that overflow to the next color part, except for the last one.

Small adjustment to make it have a white center
var canvas = document.getElementById('colorPicker');
var graphics = canvas.getContext("2d");
var CX = canvas.width / 2,
CY = canvas.height / 2,
sx = CX,
sy = CY;
for (var i = 0; i < 360; i += 0.1) {
var rad = i * (2 * Math.PI) / 360;
var grad = graphics.createLinearGradient(CX, CY, CX + sx * Math.cos(rad), CY + sy * Math.sin(rad));
grad.addColorStop(0, "white");
grad.addColorStop(0.01, "white");
grad.addColorStop(0.99, "hsla(" + i + ", 100%, 50%, 1.0)");
grad.addColorStop(1, "hsla(" + i + ", 100%, 50%, 1.0)");
graphics.strokeStyle = grad;
graphics.beginPath();
graphics.moveTo(CX, CY);
graphics.lineTo(CX + sx * Math.cos(rad), CY + sy * Math.sin(rad));
graphics.stroke();
}

Here is an alternate approach that takes a slightly more functional approach:
var canvas = document.getElementById("radial"),
ctx = canvas.getContext("2d"),
width = canvas.width,
height = canvas.height,
center = { x: width/2, y: height/2 },
diameter = Math.min(width, height);
var distanceBetween = function(x1,y1,x2,y2) {
// Get deltas
var deltaX = x2 - x1,
deltaY = y2 - y1;
// Calculate distance from center
return Math.sqrt(deltaX*deltaX+deltaY*deltaY);
}
var angleBetween = function(x1,y1,x2,y2) {
// Get deltas
var deltaX = x2 - x1,
deltaY = y2 - y1;
// Calculate angle
return Math.atan2(deltaY, deltaX);
}
var radiansToDegrees = _.memoize(function(radians) {
// Put in range of [0,2PI)
if (radians < 0) radians += Math.PI * 2;
// convert to degrees
return radians * 180 / Math.PI;
})
// Partial application of center (x,y)
var distanceFromCenter = _.bind(distanceBetween, undefined, center.x, center.y)
var angleFromCenter = _.bind(angleBetween, undefined, center.x, center.y)
// Color formatters
var hslFormatter = function(h,s,l) { return "hsl("+h+","+s+"%,"+l+"%)"; },
fromHue = function(h) { return hslFormatter(h,100,50); };
// (x,y) => color
var getColor = function(x,y) {
// If distance is greater than radius, return black
return (distanceFromCenter(x,y) > diameter/2)
// Return black
? "#000"
// Determine color
: fromHue(radiansToDegrees(angleFromCenter(x,y)));
};
for(var y=0;y<height;y++) {
for(var x=0;x<width;x++) {
ctx.fillStyle = getColor(x,y);
ctx.fillRect( x, y, 1, 1 );
}
}
It uses a function to calculate the color at each pixel – not the most efficient implementation, but perhaps you'll glean something useful from it.
Note it uses underscore for some helper functions like bind() – for partial applications – and memoize.
Codepen for experimentation.