Having fun with HTML5 – Simple painting app using Canvas

It feels like a little while since I last played around with the <canvas> element, so I spent some time over the weekend and put together a simple painting app using the canvas and here is the end result.

Here’s some screenshots:

image image image

Spray Tool

One of the interesting things with this simple painting app is the spray tool, here’s what I did to achieve the effect:

   1: var _intervalId,    // used to track the current interval ID

   2:     _center;        // the current center to spray


   4: function getRandomOffset() {

   5:     var randomAngle = Math.random() * 360;

   6:     var randomRadius = Math.random() * radius;


   8:     return {

   9:         x: Math.cos(randomAngle) * randomRadius,

  10:         y: Math.sin(randomAngle) * randomRadius

  11:     };

  12: }


  14: this.startDrawing = function (position) {

  15:     _center = position;


  17:     // spray once every 200 milliseconds

  18:     _intervalId = setInterval(this.spray, 10);

  19: };


  21: this.finishDrawing = function (position) {

  22:     clearInterval(_intervalId);

  23: };


  25: this.spray = function () {

  26:     var centerX = _center.X, centerY = _center.Y, i;


  28:     for (i = 0; i < density; i++) {

  29:         var offset = getRandomOffset();

  30:         var x = centerX + offset.x, y = centerY + offset.y;


  32:         drawingCxt.fillRect(x, y, 1, 1);

  33:     }

  34: };

The startDrawing function is called when the mouseDown or a subsequent mouseMove event is raised, it in turn sets up an interval event which is triggered every 10 milliseconds and draws one pixel using the currently configured colour within the radius of the mouse pointer.

When the mouseUp event is raised, the interval event handler is unbound.

Saving the Canvas to an Image

One of the cool things you can do with the <canvas> element is the ability to get a data:URL containing a representation of the image as a PNG file, which you can then use as the source for an <img> element so that the user can then save.

The code required to do this is as simple as calling toDataURL() on the <canvas> element and then use the returned value to set the src property on the <img> element.


You can try out the live demo at http://sketchy.theburningmonk.com or via the shortlink http://lnk.by/fghis.

If you’re interested in the source code, you can get it off github here.



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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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