I have done my best to find a simple, relevant, and up-to-date example that works for the latest version of Firefox and I'm really struggling.
Titles says it all really. I want the user to able to copy part of an image from an editor such as Windows Paint or use the Print Screen button and then paste that into a canvas element. Bonus points if the canvas resizes to fit exactly the pasted image (literally).
Want to avoid Flash or Java based solutions if reasonable.
I'm half-decent at Javascript but relatively inexperienced with the latest HTML5 features and totally new to the Canvas element. Please help!
Version 2.0: Smaller, cleaner code works on Chrome, Firefox, Edge, Opera. No more hacks. But if you need support IE and Safari, check v1 version.
http://jsfiddle.net/viliusl/xq2aLj4b/5/
Version 1.0 Chrome implementation is simple. Firefox (and IE) has restrictions that user must give command to do paste like keyboard event and editable input must be focused, so we do tricks here - on ctrl down we focusthat input field, on release unfocus.
Browser support (image data):
var CLIPBOARD = new CLIPBOARD_CLASS("my_canvas", true);
/**
* image pasting into canvas
*
* @param {string} canvas_id - canvas id
* @param {boolean} autoresize - if canvas will be resized
*/
function CLIPBOARD_CLASS(canvas_id, autoresize) {
var _self = this;
var canvas = document.getElementById(canvas_id);
var ctx = document.getElementById(canvas_id).getContext("2d");
var ctrl_pressed = false;
var command_pressed = false;
var paste_event_support;
var pasteCatcher;
//handlers
document.addEventListener('keydown', function (e) {
_self.on_keyboard_action(e);
}, false); //firefox fix
document.addEventListener('keyup', function (e) {
_self.on_keyboardup_action(e);
}, false); //firefox fix
document.addEventListener('paste', function (e) {
_self.paste_auto(e);
}, false); //official paste handler
//constructor - we ignore security checks here
this.init = function () {
pasteCatcher = document.createElement("div");
pasteCatcher.setAttribute("id", "paste_ff");
pasteCatcher.setAttribute("contenteditable", "");
pasteCatcher.style.cssText = 'opacity:0;position:fixed;top:0px;left:0px;width:10px;margin-left:-20px;';
document.body.appendChild(pasteCatcher);
// create an observer instance
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(mutation) {
if (paste_event_support === true || ctrl_pressed == false || mutation.type != 'childList'){
//we already got data in paste_auto()
return true;
}
//if paste handle failed - capture pasted object manually
if(mutation.addedNodes.length == 1) {
if (mutation.addedNodes[0].src != undefined) {
//image
_self.paste_createImage(mutation.addedNodes[0].src);
}
//register cleanup after some time.
setTimeout(function () {
pasteCatcher.innerHTML = '';
}, 20);
}
});
});
var target = document.getElementById('paste_ff');
var config = { attributes: true, childList: true, characterData: true };
observer.observe(target, config);
}();
//default paste action
this.paste_auto = function (e) {
paste_event_support = false;
if(pasteCatcher != undefined){
pasteCatcher.innerHTML = '';
}
if (e.clipboardData) {
var items = e.clipboardData.items;
if (items) {
paste_event_support = true;
//access data directly
for (var i = 0; i < items.length; i++) {
if (items[i].type.indexOf("image") !== -1) {
//image
var blob = items[i].getAsFile();
var URLObj = window.URL || window.webkitURL;
var source = URLObj.createObjectURL(blob);
this.paste_createImage(source);
}
}
e.preventDefault();
}
else {
//wait for DOMSubtreeModified event
//https://bugzilla.mozilla.org/show_bug.cgi?id=891247
}
}
};
//on keyboard press
this.on_keyboard_action = function (event) {
k = event.keyCode;
//ctrl
if (k == 17 || event.metaKey || event.ctrlKey) {
if (ctrl_pressed == false)
ctrl_pressed = true;
}
//v
if (k == 86) {
if (document.activeElement != undefined && document.activeElement.type == 'text') {
//let user paste into some input
return false;
}
if (ctrl_pressed == true && pasteCatcher != undefined){
pasteCatcher.focus();
}
}
};
//on kaybord release
this.on_keyboardup_action = function (event) {
//ctrl
if (event.ctrlKey == false && ctrl_pressed == true) {
ctrl_pressed = false;
}
//command
else if(event.metaKey == false && command_pressed == true){
command_pressed = false;
ctrl_pressed = false;
}
};
//draw pasted image to canvas
this.paste_createImage = function (source) {
var pastedImage = new Image();
pastedImage.onload = function () {
if(autoresize == true){
//resize
canvas.width = pastedImage.width;
canvas.height = pastedImage.height;
}
else{
//clear canvas
ctx.clearRect(0, 0, canvas.width, canvas.height);
}
ctx.drawImage(pastedImage, 0, 0);
};
pastedImage.src = source;
};
}1. Copy image data into clipboard or press Print Screen <br>
2. Press Ctrl+V (page/iframe must be focused):
<br /><br />
<canvas style="border:1px solid grey;" id="my_canvas" width="300" height="300"></canvas>