Writing a pixel editor in javascript - Part three

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Pixel Editor

In part two we finished the main image canvas and got to the point where you could draw onto the grid of pixels with one mouse button and erase with the other. In this, the third part, we shall be adding the preview to the pixel editor. You can grab all the files from this tutorial series here at github.


As you can see from the below code the Preview object is quite simple, containing a handful of private members and just two public methods. In its constructor we create another canvas object which we will use to draw to before caching its image data to the private.cCache variable for rendering to the main DOM <canvas> element in the same way as our ImageCanvas object does.

var Preview = function( options ){

    var private      = {};
    private.xPixels  = (options !== undefined && options.xPixels !== undefined) ? options.xPixels : 16;
    private.yPixels  = (options !== undefined && options.yPixels !== undefined) ? options.yPixels : 16;
    private.offset   = (options !== undefined && options.offset !== undefined) ? options.offset : { x: 341, y: 295 };
    private.loaded   = false;
    private.cCanvas  = $('<canvas/>').attr({ width: 43, height: 36 });
    private.cContext = private.cCanvas.get(0).getContext("2d");
    private.cCache   = null;

    return {

        update: function( step, canvas, context ){


        render: function( step, canvas, context ){




To begin place var iPreview = new Preview(); on a new line after our declaration of iCanvas followed by iPreview.render( step, canvas, context ); in the render method within App and iPreview.update( step, canvas, context); inside the App update method, both on new lines after the calls to their iCanvas counterparts.

Next paste the above code before your ImageCanvas object declaration, refreshing the index.html file in your browser will do nothing different than before, and if all went well you will have no errors in your console.

Once you are sure you have copied everything correctly, paste the below code into your Preview objects render method. You should be quite familiar with the following lines as it is very similar to the method we used in our ImageCnavas object in part two.

private.cContext.clearRect(0, 0, 43, 36);

private.cContext.font      = '10px Arial';
private.cContext.fillStyle = '#000000';
private.cContext.fillText( 'Preview', 3.5, 10);

private.cContext.fillRect( 13, 15, 18, 18);

private.cContext.fillStyle = '#FFFFFF';
private.cContext.fillRect( 14, 16, 16, 16);

private.cCache = private.cContext.getImageData( 0, 0, 43, 36);
private.loaded = true;

The Preview.update method begins by clearing the local private canvas context created by the constructor and writing the text "Preview" to it followed by drawing a black square with a slightly smaller white square over the top to provide a border around a 16x16 pixel preview area.

The render method of our Preview object is the below two lines. Because the render method can sometimes be called before the update method has finished executing we first check that the cache has been loaded before writing the stored image data to the context passed to it from our main render method in the App object.

if ( ! private.loaded ){ return; }
context.putImageData( private.cCache, private.offset.x, private.offset.y );

Pixel Editor

Now upon refreshing index.html in your browser, you should see the above. As you can see from my doodle, the preview doesn't yet provide an actual preview of our drawn pixels; the object that stores our pixel data is privatly held within the ImageCanvas object and normally that would mean that it is hidden from access. Fortunatly however our ImageCanvas object has a getter that we can use to grab the pixel object from within it as shown in the following code snippet:

var mPixels = iCanvas.get('pixels');

for (var y = 1; y <= private.yPixels; y+= 1)
    for (var x = 1; x <= private.xPixels; x+= 1)
        var currentPixel = mPixels.getPixel( x, y);
        if ( currentPixel.on === true )
            private.cContext.fillStyle = '#000000';
            private.cContext.fillRect( (14 + x - 1) ,( 16 + y - 1), 1, 1);

The above code belongs inside your Preview objects update method, between the line private.cContext.fillRect( 14, 16, 16, 16); and private.cCache = private.cContext.getImageData( 0, 0, 43, 36);. It is very similar to the double for loop used within the ImageCanvas object to output the big representation of the pixel data.

Pixel Editor

Upon refreshing index.html within your browser and drawing on the image canvas, you should now be able to see a preview of how your drawing looks, just as the above image shows. Now "1-bit" colour depth is all fun and games but what we really want to have is a choice of what colour we draw with.

Thank you so much for reading this tutorial, next in part four we will add a pallet selector to the application. If you have any comments, questions or suggestions please leave them below in the comment form, or drop me a tweet @carbontwelve.