Tag Archives: Draw single pixel

Playing with the HTML Canvas Again

I’ve previously written about how you can write games using tools like React. I’ve also written about creating a basic game in Html and Javascript, and things like rotating shapes.

In this post, I’m going to demonstrate a sort of Etch A Sketch type program.

The HTML is very simple here:

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" type="text/css" href="test.css">
    <script src="test.js"></script>
</head>
<body onload="doDrawing()">
    <canvas id="canvas">
    </canvas>
</body>
</html>

Basically just bringing in a Javascript file, CSS and putting a canvas on the screen. I’m also calling a function onload – it’s worth bearing in mind that, as it goes, this won’t resize should you change the size of the browser. If you want that behaviour, then have a look at one of my previous posts mentioned above.

The CSS is even simpler:

* { 
    margin:0; 
    padding:0; 
}
canvas {  
    display: block;    
}

All we’re doing here is removing the default margin, and stopping scroll bars from appearing.

Javascript

The Javascript is where everything is happening; let’s start with some variables:

let x = 10;
let y = 10;
let directionHorizontal = 1;
let directionVertical = 0;

The four variables determine the position that we want to draw in, and which way we’re heading. We can now render this to the screen like this:

const doDrawing = () => {
    var c = document.getElementById("canvas");
    
    c.width = window.innerWidth;
    c.height = window.innerHeight;

    var ctx = c.getContext("2d");
    setInterval(() => {
        ctx.fillRect(x, y, 1, 1);
        x += directionHorizontal;
        y += directionVertical;
    }, 10);
}

The canvas width and height are the first things to note: when I started playing with this, I made the mistake of trying to set this in CSS; if you do, it actually doesn’t change the size of the canvas, but stretches it across the screen; this was the only way that I could get the canvas to display full screen without that stretching effect (if you know another / better way, please say so in the comments!)

Next we get the context from the canvas – this allows us to render to it, and then we simply set-up an interval, and draw a rectangle 1px x 1px each iteration.

Summary

That it – as with previous posts, there’s not a whole lot to using the HTML canvas, but I do like to re-experiment every so often.