Wednesday, November 16, 2011

Trials and tribulations with HTML5 Canvas on Android

You may not be aware but I recently released a mobile web app called It is a neat little application that lets you post notes about someones driving abilities based on their license plate number. It leverages several technologies: jquery, jquerymobile, jqScribble (my custom canvas drawing tool), as well as some other things I will get into shortly.

The coolest part of my app, in my opinion, is the jqScribble drawing tool. It is touch enabled, so it will let you draw with either a mouse or your finger. It then provides hooks to let you specify how you want the image saved. In my case I send the image data string to my webservice which creates the image file. To do this the tool leverages the toDataURL function available on the HTML 5 canvas element. Basically toDataURL converts the canvas image as a 64bit string. This canvas feature is amazing and super fast, which I love.

Nearing the final days before my official "launch" one of my co-workers wanted to use the app. It just so happens he has an Android phone. I believe his OS version is 2.2 (not sure which dessert that is). Anyway he wanted to use the drawing tool, which I was expecting to work perfectly, the drawing part worked fine, but the saving part failed. After some research I discovered that up until version 2.3 of Android the toDataURL functionality of canvas was nothing more than an empty function with a comment to the tune of "I don't know what this is supposed to do".

After some obvious ranting and Google cursing, I came across this post by Hans Schmucker. He was having the exact same issue as me and developed his own PNG encoder, which he used to replace toDataURL on Android devices. So I went to work attempting to implement what he did with my code, unfortunately I was never able to get it to work. However he did get me thinking down the right track. This lead me to the realization that toDataURL can be set to return either PNG or JPG formatted data. Now the only trick was finding a JPEG encoder written in javascript. Well that wasn't tough, a good one exists here (Thanks to Arizona for finding a new link after the old one was removed). All that was left was to add some simple detection and execute if the stock toDataURL doesn't work properly.

Here is my code:

var tdu = HTMLCanvasElement.prototype.toDataURL;
HTMLCanvasElement.prototype.toDataURL = function(type)
 var res = tdu.apply(this,arguments);
 //If toDataURL fails then we improvise
 if(res.substr(0,6) == "data:,")
  var encoder = new JPEGEncoder();
  return encoder.encode(this.getContext("2d").getImageData(0,0,this.width,this.height), 90);
 else return res;

The first line puts the toDataURL function into a variable so that if it works correctly we don't blow it away on the next line where we override toDataURL with our own custom function. The custom function will attempt to call the original toDataURL, then checks the returned value. If it doesn't have the correct header information, we know that the original toDataURL failed, so it creates a new JPEGEncoder instance and encodes, then returns the canvas data.

Is this the best way of doing things? Probably not, it is never a good idea to muck around with stock functionality, but I didn't want to go back and change my code that already used toDataURL, so I made a judgement call.

Finally to break it down for you: If you need to support the toDataURL function of canvas in an Android device < 2.3. You should take a similar approach.

1) Download a JPEG Encoder.
2) Add in your own custom toDataURL handler which will use the Encoder if the original toDataURL fails.

The drawing tool works like a charm, and saving images is still reasonably fast. Although I am dealing with fairly small images.