Pixel level drawing with the canvas element

Today we’re going to learn how to manipulate the HTML <canvas> element at the pixel level.

To start, we need a canvas element:

<canvas id="canvas" width="640" height="480">

<p>Your browser doesn't support canvas.</p>

</canvas>

Next get a handle to the canvas element and its 2d context.

var canvas = document.getElementById('canvas');var ctx = canvas.getContext('2d');

Next, we create a ImageData element on the canvas that covers the entire canvas. This is we’re actually going to be drawing our pixels on.

var imgd = ctx.createImageData(canvas.width, canvas.height);

The ImageData element is basically just a big array. Each block of 4 array elements corresponds to a single pixel.

The array indices in each pixel block control the color and alpha transparency as follows:

  • i = Red value
  • i+1 = Green value
  • i+2 = Blue value
  • i+3 = Alpha

And here’s a simple function for drawing a single pixel given an x,y value and r,g,b:

var setPixel = function(imageData, point, r, g, b, a) {    a = typeof a == 'undefined' ? 255 : a;    i = (point.x + point.y * imageData.width) * 4;    imageData.data[i  ] = r;    imageData.data[i+1] = g;    imageData.data[i+2] = b;    imageData.data[i+3] = a;};

Let’s say we wanted to draw a red pixel at 100,100. Given the ImageData element we declared above, we’d call setPixel like this:

setPixel(imgd, {x:100,y:100}, 255, 0, 0);

Notice we’re not passing a value for the alpha transparency. The setPixel defaults the alpha to fully opaque if a value is specified.

A more full-featured setPixel function can be found here. This version accepts HTML hex codes for colors and extends the Object prototype so we call it directly on the ImageData element like this:

imgd.setPixel({x:100,y:100}, #f00);

Here’s a sample that draws a Sierpinski triangle using the chaos game