Canvas Clock
In these chapters we will build an analog clock using HTML canvas.
Part I - Create the Canvas
The clock needs an HTML container. Create an 300 x 300 pixel HTML canvas:
HTML code:
<!DOCTYPE html>
<html>
<body>
<canvas id="canvas" width="300"
height="300" style="background-color:#333"></canvas>
<script>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var radius = canvas.height / 2;
ctx.translate(radius, radius);
radius = radius * 0.90
drawClock();
function drawClock() {
ctx.arc(0, 0, radius, 0 , 2*Math.PI);
ctx.fillStyle = "white";
ctx.fill();
}
</script>
</body>
</html>
Try it Yourself »
Code Explained
Add an HTML <canvas> element to your page:
<canvas id="canvas" width="300"
height="300" style="background-color:#333"></canvas>
Create a canvas object (var canvas) from the HTML canvas element:
var canvas =
document.getElementById("canvas");
Create a 2d drawing object (var ctx) for the canvas object:
var ctx = canvas.getContext("2d");
Calculate the clock radius, using the height of the canvas:
var radius
= canvas.height / 2;
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Using the canvas height to calculate the clock radius, makes the clock work for all canvas sizes. |
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Remap the (0,0) position (of the drawing object) to the center of the canvas:
ctx.translate(radius, radius);
Reduce the clock radius (to 90%) to draw the clock well inside the canvas:
radius = radius * 0.90;
Create a function to draw the clock:
function drawClock() {
ctx.arc(0, 0, radius, 0 , 2*Math.PI);
ctx.fillStyle =
"white";
ctx.fill();
}
