Here we are going to study about the creation of Canvas dynamically and the basic mouse interactivity with the Canvas elements.
The source files for this example is here for you to download and play.
We are going to use a separate sprite sheet and try to make the scene a little more interesting than the last one. I would say to download the source files and see the example for yourself to see what is it doing. Basically there is a scene on the background with 6 tables and a girl doing some experiment on third table. Towards the left top corner of the scene, there is another girl trying to do some magic with its magic wand. If you look at the code, the scene with the girl doing experiment is a straight forward drawing from the sprite sheet as we did that in last tutorial. The girl with the magic wand is drawn in a dynamically created Canvas. The code looks as below.
//creating canvas dynamically
canvasGirl=document.createElement('canvas');
canvasGirl.width=100;
canvasGirl.height=100;
//get the context
contextGirl=canvasGirl.getContext('2d');
//customise canvas
canvasGirl.style.background='transparent';
canvasGirl.style.zIndex=5;
//add the canvas to the document
document.body.appendChild(canvasGirl);
The first thing is to create a new canvas with “createElement(‘canvas’)”. That will make a new canvas. Now we have to get the context to draw something on it. This is done with “getContext(’2d’)” as usual, nothing new here. And lastly, we have to add our newly created Canvas to the document and this is done with “document.body.appendChild(canvasGirl);” code. Thats beautiful. Everything else in the code is just configuring the canvas.
Once the canvas and context are available to us, we can do anything we are able to do to a Canvas. So the girl with the magic wand is drawn to the context of this Canvas element. The code looks as below.
resetContext(canvasGirl,contextGirl);
contextGirl.drawImage(photo, 108*counter+10, 900, 96, 64, 0, 0, 96, 64);
First we are resetting the Canvas, so as to draw new content on each timer. Since the girl with magic wand is actually an animation, we call the code above repeatedly on a timer event. And first we clear the context with our own function and then redraw an image from sprite sheet at the same place on the canvas, so as to make an animation. If you are following along with the previous tutorials then the reseting of context code should not be new to you, else the code looks as as below.
resetContext=function(canvasRef,contextRef){
contextRef.clearRect(0,0,canvasRef.width,canvasRef.height);
};
All we have, are two layers of drawing and animation drawn from the same sprite sheet.
Lets do some mouse interactivity with these elements. The code looks as below.
//------------ adding mouse interaction --------------
$('#board_one').mouseup(function() {
alert('Canvas with Tables');
});
$(canvasGirl).mouseup(function() {
alert('Canvas with Girl, doing magic.');
});
The trick here is JQuery’s neat job. Just remember anything wrapped with $() will give a JQuery element of the same HTML element. So our dynamically created Canvas is made an JQuery element by
$(canvasGirl)
Notice that there is no ‘#’ before the name as it is before ‘board_one’, thats for the ‘board_one’ we are referring the HTML element with id ‘board_one’, while the other one we are creating dynamically in javascript and converting it to a JQuery equivalent.
Thats nice and if you have not done yet, please go ahead and click on the board to get an alert showing different message and if you click on the girl with magic wand, that will alert a different message.
This is quite simple and straight forward interactivity, but I think you got the point.
Happy coding 
And yes, I made it to one month of continuous posting, highly influenced by Keith Peters. Though it was not all javascript and I missed some dates, but all in all, the number of posts in a month is achieved. I am quite happy about my effort and a big thanks to all of you, who read it.
