I have a web application that generates images on the fly and then renders (segments of) them on the client using canvas - the general code looks something like this (simplified)...
var width = ...
var height = ...
var img = new Image();
img.onload = function(){ // Set onload before setting image source ;)
var canvas = document.createElement("canvas");
canvas.setAttribute('width',width);
canvas.setAttribute('height',height);
var context = canvas.getContext("2d");
context.drawImage(this,0,0,width,height,0,0,width,height);
}
img.src = ...
In firefox and chrome, this works fine. In IE9 (and sometimes IE10), this sometimes results in a blank image. As a workaround, I put in...
var width = ...
var height = ...
var img = new Image();
img.onload = function(){
var canvas = document.createElement("canvas");
canvas.setAttribute('width',width);
canvas.setAttribute('height',height);
var context = canvas.getContext("2d");
window.setTimeout(function(){
context.drawImage(img,0,0,width,height,0,0,width,height);
}, 10); // 10 millisecond delay here seems to give IE time to make sure the image is actually ready
}
img.src = ...
Obviously, putting in random delays is making me nervous - I would much rather figure out why this happens. Is there another event type / method / property on images that may be used? Could there perhaps be something in the http headers sent down with the images / the encoding / transfer method that is causing this?
Since drawing images on the canvas seems to be something that trips up new users a lot, searches mostly yield instances where the developer set the source before the onload method. The closest other question / reply I was able to find was this : https://stackoverflow.com/a/15186350/470062
Edit:
I have updated the examples to include width and height definitions at the start
Edit 29/Apr/2013:
The 10 millisecond delay is not always sufficient - I have changed the code to check the canvas pixels to determine if anything has been drawn - This feels like such an ugly hack though - I would love to know if there is a better way :
... snip - this is at the end of the image load callback..
if((navigator.appVersion.indexOf("MSIE")) >= 0){
function checkAndRedraw(){
context.clearRect(0,0,width,height);
context.drawImage(img, 0, 0, width, height);
var imageData = context.getImageData(0, 0, width, height),
data = imageData.data,
len = data.length;
for (var i = 0; i < len; i += 4){
if(data[i] || data[i+1] || data[i+2]){
scheduleRedraw = null; // prevent memory leak
return; // A pixel contained some non empty value so something was drawn - done.
}
}
scheduleRedraw();
}
var redrawHackTimeout;
function scheduleRedraw(){
window.clearTimeout(redrawHackTimeout);
redrawHackTimeout = window.setTimeout(checkAndRedraw, 500);
}
scheduleRedraw();
}
//END IE9 Hack
I have the same issue in IE9, what I had to do is this too:
theImage = new Image();
theImage.onload = function() {
that = this;
setTimeout(function() {
that.onload2();
}, 100);
};
theImage.onload2 = function(){
#your old load function code
}
theImage.src = 'http://....';
I don't understand why ie9 triggers load image function when it's not actually loaded or not "usable" yet in a canvas. it makes me pull my hair out.
var img = new Image();
img.onload = function(){ // Set onload before after setting image source ;)
var canvas = document.createElement("canvas");
canvas.setAttribute('width',width);
canvas.setAttribute('height',height);
var context = canvas.getContext("2d");
context.drawImage(this,0,0,width,height,0,0,width,height);
}
You must set the src
, then wait for the image to load.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.