如何加载新图像并将其绘制到画布？

Question

i have a image and i draw that image on the canvas. now i want to change the source of the image and again draw the image in the canvas. i have tried the below code and here is the jsfiddler . canvas's drawing is not changing.... what to do?

HTML

<button onclick="change_2_new_image();">change_image()</button>  
<img id="test" src="http://static.clickbd.com/global/classified/item_img/374646_0_original.jpg" alt="error" title="This is an image" />
<canvas id="myCanvas" width="320px" height="275px"><canvas>

JS

    var img;
$(function () {
    var c = document.getElementById("myCanvas");
    var ctx = c.getContext("2d");
    img = document.getElementById("test");
    img.ready = function () {
        alert("asasas");
    };
    ctx.drawImage(img, 0, 0);

});

function change_2_new_image() {

    $('#test').attr("src", "http://upload.wikimedia.org/wikipedia/en/9/9a/Grameenphone_Logo.png");
    img = document.getElementById("test");
    ctx.drawImage(img, 0, 0);
}

Answer 1

Your issue is with variable scope. You can't access ctx from the function.

function change_2_new_image() {
    var c = document.getElementById("myCanvas");
    var ctx = c.getContext("2d");
    $('#test').attr("src", "http://upload.wikimedia.org/wikipedia/en/9/9a/Grameenphone_Logo.png");
    img = document.getElementById("test");
    ctx.drawImage(img, 0, 0);
    ctx.render()
}

The above should now work.

Answer 2

ctx is defined inside of your closure and not defined in change_2_new_image() that's why the error message ReferenceError: ctx is not defined is popping up in the console. After fixing that, you may also notice that the image at times, hasn't yet loaded. use jQuery's on('load', ... ) event. for more info, you can have a look at this thread: jQuery event for images loaded

Hope that helps!