如何在画布上叠加图像，然后在画布上绘制？

Question

I have all the code worked out to draw on a canvas in a separate .js file, so now I just need to put the image on.

I can't get the image to work, though, the normal method isn't working. I've tried this:

var canvasvar;
var contextvar;
canvasvar = document.getElementById("canvas");
contextvar = canvas.getContext("2d");
var imageObj = new Image();
imageObj.src = "http://wannabevc.files.wordpress.com/2010/09/im-cool.jpg";
contextvar.drawImage(imageObj,10,10); 

For one thing, should I put this inside the html file, inside a <script></script> at the end, or should I put it in the js file that has the “draw stuff” code? I can't get either to work, it runs with no errors but I don't see any image over the canvas. So I can still draw, but no image appears. I'm guessing the image is under the canvas or something.

I'm guessing this is because I didn't just make a canvas – I made it with all this programming happening inside it in another js file.

I also tried putting this code inside the initCanvas function in my .js code. This is what actually creates the canvas. Here is the code of initCanvas function:

// Retrieve canvas reference
canvas = document.getElementById("canvas");

// Size canvas
canvas.width  = 600;
canvas.height = 400;

// Retrieve context reference, used to execute canvas drawing commands
context = canvas.getContext('2d');
context.lineCap = "round";

// Set control panel defaults
document.getElementById("thickness").selectedIndex = 0;
document.getElementById("color").selectedIndex = 1;

// Overlay image over the canvas. THIS PART IS WHAT I'M ASKING ABOUT. This is usually
// not in the code

var canvasvar;
var contextvar;
canvasvar = document.getElementById("canvas");
contextvar = canvas.getContext("2d");
var imageObj = new Image();
imageObj.src = "http://wannabevc.files.wordpress.com/2010/09/im-cool.jpg";
contextvar.drawImage(imageObj,10,10); 

So the last part in this code is what I'm talking about. It isn't working, though.

Answer 1

Give your image time to load using imageObj.onload

var canvasvar = document.getElementById("canvas");
var contextvar = canvas.getContext("2d");
var imageObj = new Image();
imageObj.onload=function(){
    contextvar.drawImage(imageObj,10,10);
}
imageObj.src = "http://wannabevc.files.wordpress.com/2010/09/im-cool.jpg";