Develop Reference

Save canvas as png locally with javascript with Cross doimain

I want to save an image of the canvas tag, but when i try create an image from the canvas it tells me "SecurityError: The operation is insecure", which i believe is problems with the canvas coming from another domain than what im on. This canvas is a map which is generated with openlayers3.
var canvas = document.getElementsByClassName('ol-unselectable')[0];
var dataURL = canvas.toDataURL('image/png');
var window = window.open();
window.document.write('<img src='+dataURL+'/>');
I've also tried
canvas.toBlob(function(blob) {
saveAs(blob, 'map.png')
}
Is there an easy work around so the canvas is not tainted?

You tried write in new browser window from another window this is impossible without https protocol or localhost. But you can create new img in currently window.
var canvas = document.getElementsByClassName('ol-unselectable')[0];
var dataURL = canvas.toDataURL('image/png');
var img = new Image();
img.width = 1000;
img.height = 1000;
img.src = dataURL;
img.onload = function () {
document.body.append(img);
}

Why is this FileReader onload not firing?

I am starting with this working JavaScript code:
// Begin File Open Code.
function fileOpen()
{
var fileInput = document.getElementById('fileInput');
var fileDisplayArea = document.getElementById('iDisplay');
fileInput.addEventListener('change', function(e)
{
file = fileInput.files[0];
var imageType = /image.*/;
if (file.type.match(imageType))
{
var reader = new FileReader();
reader.onload = function(e)
{
iDisplay.innerHTML = "";
image = new Image();
image.src = reader.result;
image.id = "I"; // Add an id attribute so the image editor code can access the image.
iDisplay.appendChild(image);
image.onload = function()
{
c = document.getElementById("canvas1");
context = c.getContext("2d");
c.width = image.width;
c.height = image.height;
context.drawImage(image,0,0);
}
}
reader.readAsDataURL(file);
moreFiles++;
document.getElementById("fileInput").disabled = true;
document.getElementById("fileInputD").innerHTML = '<p>Please refresh page to open a new image</p>';
}
else
{
iDisplay.innerHTML = "File type not supported."
}
});
}
// End File Open Code
And trying to break it up into two separate functions, fileOpen() and loadImage(). The reason is that loadImage will also be used to load default images. I came up with several non-working versions. After reading about the topic here, most recently this answer Javascript FileReader onload not firing I adjusted the code as follows:
// Begin File Open Code
function fileOpen(radio)
{
fileInput = document.getElementById('fileInput');
fileDisplayArea = document.getElementById('iDisplay');
fileInput.addEventListener('change', function(e)
{
file = fileInput.files[0];
var imageType = /image.*/;
if (file.type.match(imageType))
{
reader = new FileReader();
reader.onload = fileSelected;
function fileSelected(e)
{
iDisplay.innerHTML = "";
image = new Image();
image.src = reader.result;
image.id = "I"; // Add an id attribute so the image editor code can access the image.
iDisplay.appendChild(image);
image.onload = loadImage();
}
}
else
{
iDisplay.innerHTML = "File type not supported."
}
})
}
// End File Open Code
// Begin Load Image Code - This will be used to load a preselected image
function loadImage()
{
c = document.getElementById("canvas1");
context = c.getContext("2d");
c.width = image.width;
c.height = image.height;
context.drawImage(image,0,0);
reader.readAsDataURL(file);
document.getElementById("fileInput").disabled = true;
document.getElementById("fileInputD").innerHTML = '<p>Please refresh page to open a new image</p>';
}
// End Load Image Code
But, after reviewing in the Chrome Inspector, the code never gets past
reader.onload = fileSelected;
After that line, the function exits, and nothing is loaded. I have spent hours on variations of this code and cannot get it to fire past that line. So, my question is "why?" I should note that I want to stay with pure JavaScript.
Thank you in advance.

There appears to be a bug in Chrome 73 where it isn't fired if you have a debugger statement.
I literally have
reader.readAsText(blob);
debugger;
and it never hits the onload
If I change it to
debugger;
reader.readAsText(blob);
then it works.
Fixed by Chrome 75.

You can also use FileReader.readAsDataURL() to parse the file from your . This will create a string in memory containing a base64 representation of the image.
<input type="file" accept="image/*" onchange="loadFile(event)">
<img id="output"/>
<script>
var loadFile = function(event) {
var reader = new FileReader();
reader.onload = function(){
var output = document.getElementById('output');
output.src = reader.result;
};
reader.readAsDataURL(event.target.files[0]);
};
</script>
Ref: Preview an image before it is uploaded

save and display image captured from input type=file

I have a webpage with capture image from the Available camera feature. For the web version it simply places the video capture onto a canvas. However for a phone, I am using <input class="btn btn-info" type="file" accept="image/*" id="cameraCapture" capture="camera"> to capture a picture. It asks the user to either capture the image using the phone's camera or upload from its filesystem/gallery etc. Once the image is clicked it simply places the image's name next to the button.
Is there a way to access this image and display it in the same page.
Thanks in advance

You can do that with JS using FileReader object.
Take a look at this answer: preview-an-image-before-it-is-uploaded
I hope it helps

var input = document.querySelector('input[type=file]');
input.onchange = function () {
var file = input.files[0];
drawOnCanvas(file);
};
function drawOnCanvas(file) {
var reader = new FileReader();
reader.onload = function (e) {
var dataURL = e.target.result,
c = document.querySelector('canvas'),
ctx = c.getContext('2d'),
img = new Image();
img.onload = function() {
c.width = img.width;
c.height = img.height;
ctx.drawImage(img, 0, 0);
};
img.src = dataURL;
};
reader.readAsDataURL(file);
}

FileReader on safari firing to soon

I'm working on a multi part html questionnaire style form that has a lot of text questions along with a few images. On questions that are images the user is selects the image, i create a canvas element and display the resized image in it underneath the file input.
if (window.FileReader)
{
var file = element.files[0];
var $input = $(element);
var $fileName = file.name;
var reader = new FileReader();
reader.onload = function(e) {
var img = document.createElement("img");
img.src = e.target.result;
var dataID = $input.data("questionId");
var canvasID = "canvas_" + dataID;
$("#"+canvasID).remove();
var canvas = document.createElement("canvas");
canvas.setAttribute("id", canvasID);
canvas.setAttribute("height", "200");
canvas.setAttribute("width", "200");
var heightWidth = getHeightWidth(img);
canvas.height = heightWidth[0];
canvas.width = heightWidth[1];
var ctx = canvas.getContext("2d");
ctx.drawImage(img,0,0, canvas.width, canvas.height);
var sectionID = "section_" + dataID;
$("#" + sectionID).append(canvas);
$("#file_title-" + dataID).val($fileName);
}
reader.readAsDataURL(file);
}
else
{
alert("This browser does not support image uploading.");
}
This works fine in chrome but not in safari (safari 8.x) on desktop or iOS. The problem in my code is that on Safari it returns height=0 width=0 from getHeightWidth() which gives me think the img isn't ready to be handled yet. This theory is further validated because if i change to a new picture and change back to the original it displays properly.
I'm really not sure where to start, any help debugging this would be greatly appreciated. Thanks everyone

The code assumes the image loading is synchronous, but it's asynchronous and should be assumed so even with data-URIs. If the image hasn't loaded properly its width and height attributes will be 0.
You can solve this by adding an onload handler for img, then move the code for detecting and setting size inside that handler (remember also to add an onerror handler as well in case the image file is corrupted).

Canvas - DrawImage Method

I got a strange problem with the "DrawImage"-Methode of the Canvas element. With the following code I create the canvas and attache it to the "rightcanvas" div.
var rightcanvas = document.getElementById('rightcanvas');
rcanvas = document.createElement('canvas');
rcanvas.setAttribute('width', canvasLength);
rcanvas.setAttribute('height', canvasHeight);
rcanvas.setAttribute('id', 'rcanvas');
rightcanvas.appendChild(rcanvas);
And now I try to draw a Base64 encoded image.
var rcontext = rcanvas.getContext('2d');
var image = new Image();
image.src = 'data:image/png;base64,iVBOR....';
rcontext.drawImage(image,0,0);
The canvas will be created and attached. But the image will not be displayed.Does somebody know what the problem is? =(

You have to wait for the image to load before drawing it to the canvas.
var image = new Image();
image.onload = function() {
rcontext.drawImage(image, 0, 0);
}
image.src = 'data:image/png;base64,iVBOR....';