I am trying to set the src of an img tag in order to display an image. The following works on android/mac/windows, but not on iOS:
let b64Bmp = pageModel.image;
this.$currentPageImage.src = `data:image/bmp;base64,${b64Bmp}`;
b64Bmp is a b64 encoded bitmap image. I also tried the same using a blob:
const blobUrl = URL.createObjectURL(resultBlob);
this.$currentPageImage.src = blobUrl;
None of these solutions work on iOS specifically. Also, when trying to create a new Image object like so:
const image = new Image();
image.src = ...
The image.onerror throws an error without any information.
What do I have to change to make this work on iOS?
Found the solution. Apparently, iOS does not display BMP files with a color depth of less than 24 bits (see https://developer.apple.com/forums/thread/715255). It works now, when sending a BMP with color depth of 24 bits
I generate array of pixels in client side JavaScript code and convert it to a blob. Then I pass URL of the blob as image.src and revoke it at image.onload callback. I don't keep any references to the data, generated by the previous steps, so this data may be freed by GC.
There are many images generated this way, and it works fine. But sometimes user may want to save the generated image by clicking on a Save button near the image. I don't want to generate image again, because generation is slow, and the image is already generated and is visible on screen. So I want to get my pixels back from the image. I tried to create canvas again, draw image on it and then call toBlob, but browser treats this image as cross origin and throws an exception: "Failed to execute 'toBlob' on 'HTMLCanvasElement': tainted canvases may not be exported". Similar errors I get with canvas.toDataUrl and canvasContext.getImageData.
Is there a workaround for this problem?
I also tried to create canvases instead of images, but when I create the second canvas, the content of the first one clears.
Added
This error occurs only in the Chrome and other WebKit browsers. Firefox and MS Edge work fine. And when I commented out line of code that revoked blob url, this error disappeared - I can draw the image on the canvas and get its pixel data without CORS issues. But it is pointless to do so, because I already have blob that is not deleted.
But my page may generate many images - it depends on its user and is unlimited. Size of images is also unlimited - it may be useful to generate even 4096x4096 images. So I want to reduce memory consumption of my page as much as possible. And all these images should be downloadable. Generation of most images uses previously generated images, so to regenerate last image in a chain, I must to regenerate all images.
So I need a workaround only for Chrome browser.
Added 2
I tried to reproduce this problem in JS Fiddle, but couldn't. However locally my code doesn't work - I developed my app locally and I haven't tried running it on server. Create test.html file on your computer and open it in browser (locally, without server):
<html>
<body>
<pre id="log"></pre>
</body>
<script>
var log = document.getElementById("log");
var canvas = document.createElement("canvas");
canvas.width = canvas.height = 256;
var ctx = canvas.getContext("2d");
canvas.toBlob(function(blob) {
var img = new Image();
var blobUrl = URL.createObjectURL(blob);
img.src = blobUrl;
img.onload = function()
{
URL.revokeObjectURL(blobUrl);
ctx.drawImage(img, 0, 0);
try { canvas.toBlob(function(blob) { log.textContent += 'success\n'; }); }
catch(e) {log.textContent += e.message + '\n';}
};
});
</script>
</html>
It will print Failed to execute 'toBlob' on 'HTMLCanvasElement': Tainted canvases may not be exported..
So, I think, my workaround is to detect that page is run on combination of WebKit browser and file:/// protocol. And I can to defer revoking blob URL until page unload only for this combination.
That indeed sounds like a bug in chrome's implementation, you may want to report it.
What seems to happen is that chrome only checks if the image has been loaded through a clean origin when it is drawn for the first time on the canvas.
Since at this time, you already did revoke the blobURI, it can't map this URI to a clean origin (file:// protocol is quite strict on chrome).
But there seems to be a simple workaround:
By drawing this image on a [the reviver]* canvas before revoking the blobURI, chrome will mark the image as clean, and will remember it.
*[edit] Actually it seems it needs to be the same canvas...
So you can simply create your export canvas before-hand, and draw each images on it (to save memory you can even set it to 1x1px when you don't use it for the export) before you do revoke the image's src:
// somewhere accessible to the export logic
var reviver = document.createElement('canvas').getContext('2d');
// in your canvas to img logic
canvas.toBlob(function(blob){
var img = new Image();
img.onload = function(){
reviver.canvas.width = reviver.canvas.height = 1; // save memory
reviver.drawImage(this, 0,0); // mark our image as origin clean
URL.revokeObjectURL(this.src);
}
img.src = URL.createObjectURL(blob);
probablySaveInAnArray(img);
};
// and when you want to save your images to disk
function reviveBlob(img){
reviver.canvas.width = img.width;
reviver.canvas.height = img.height;
reviver.drawImage(img,0,0);
reviver.canvas.toBlob(...
}
But note that this method will create a new Blob, not retrieve the previous one, which has probably been Collected by the GarbageCollector at this time. But it is unfortunately your only way since you did revoke the blobURI...
I am attempting to download an entire canvas image using canvas.toDataURL(). The image is a map rendered on the canvas (Open Layers 3).
In Firefox I can use the following to download the map on the click of a link:
var exportPNGElement = document.getElementById('export_image_button');
if ('download' in exportPNGElement) {
map.once('postcompose', function(event) {
var canvas = event.context.canvas;
exportPNGElement.href = canvas.toDataURL('image/png');
});
map.renderSync();
} else {
alert("Sorry, something went wrong during our attempt to create the image");
}
However in Chrome and Opera I'm hitting a size limit on the link. I have to physically make the window smaller for the download to work.
There are size limit differences between browsers, Chrome is particularly limiting. A similar post here (over 2 years old now) suggests an extensive server side workaround:
canvas.toDataURL() for large canvas
Is there a client side work around for this at all?
Check out toBlob https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob
canvas.toBlob(function(blob) {
exportPNGElement.href = URL.createObjectURL(blob);
});
browser support is not as awesome as toDataURL though. But Chrome and Firefox have it, so it solves your biggest issue. The mdn link above also has a polyfill based on toDataURL, so you get the best possible support.
Just in case you didn't know, you can also dramatically reduce the size using jpeg compression
exportPNGElement.href = canvas.toDataURL('image/jpeg', 0.7);
I've created a small test site in which you can upload a picture. And without a round-trip to the backend, the selected picture is shown. So far nothing very interesting
$('input').on('change', function () {
var file = this.files[0];
var reader = new FileReader();
reader.onload = function (event) {
var base64 = event.target.result;
$('<img>').attr('src', base64).appendTo('body');
};
reader.readAsDataURL(file);
});
However, I noticed that on my iPad3 some pictures are shown up-side-down. I found on google about EXIF metadata which is stored in the image (base64) which defines the orientation of the picture. But another thing is, that on my laptop the image are shown normal (with the same pictures of course). Is there any way to prevent/fix this behaviour from happening ? (I want them to show the picture the same way, and if possible I also want them to be shown normal (not up-side-down))
This is not a CSS issue. It's actually an issue with the image. Some browsers interpret the orientation of the image through meta data. Simply open the image in any image editing software and export it. Upload it to your server and let me know if that worked!
EDIT - Reference this URL for a possible solution:
Accessing JPEG EXIF rotation data in JavaScript on the client side
I am looking for a way to resize an image client-side with JavaScript (really resize, not just change width and height).
I know it's possible to do it in Flash but I would like to avoid it if possible.
Is there any open source algorithm somewhere on the web?
Here's a gist which does this:
https://gist.github.com/dcollien/312bce1270a5f511bf4a
(an es6 version, and a .js version which can be included in a script tag)
You can use it as follows:
<input type="file" id="select">
<img id="preview">
<script>
document.getElementById('select').onchange = function(evt) {
ImageTools.resize(this.files[0], {
width: 320, // maximum width
height: 240 // maximum height
}, function(blob, didItResize) {
// didItResize will be true if it managed to resize it, otherwise false (and will return the original file as 'blob')
document.getElementById('preview').src = window.URL.createObjectURL(blob);
// you can also now upload this blob using an XHR.
});
};
</script>
It includes a bunch of support detection and polyfills to make sure it works on as many browsers as I could manage.
(it also ignores gif images - in case they're animated)
The answer to this is yes - in HTML 5 you can resize images client-side using the canvas element. You can also take the new data and send it to a server. See this tutorial:
http://hacks.mozilla.org/2011/01/how-to-develop-a-html5-image-uploader/
If you were resizing before uploading I just found out this http://www.plupload.com/
It does all the magic for you in any imaginable method.
Unfortunately HTML5 resize only is supported with Mozilla browser, but you can redirect other browsers to Flash and Silverlight.
I just tried it and it worked with my android!
I was using http://swfupload.org/ in flash, it does the job very well, but the resize size is very small. (cannot remember the limit) and does not go back to html4 when flash is not available.
http://nodeca.github.io/pica/demo/
In modern browser you can use canvas to load/save image data. But you should keep in mind several things if you resize image on the client:
You will have only 8bits per channel (jpeg can have better dynamic range, about 12 bits). If you don't upload professional photos, that should not be a problem.
Be careful about resize algorithm. The most of client side resizers use trivial math, and result is worse than it could be.
You may need to sharpen downscaled image.
If you wish do reuse metadata (exif and other) from original - don't forget to strip color profile info. Because it's applied when you load image to canvas.
Perhaps with the canvas tag (though it's not portable). There's a blog about how to rotate an image with canvas here, I suppose if you can rotate it, you can resize it. Maybe it can be a starting point.
See this library also.
You can use a javascript image processing framework for client-side image processing before uploading the image to the server.
Below I used MarvinJ to create a runnable code based on the example in the following page:
"Processing images in client-side before uploading it to a server"
Basically I use the method Marvin.scale(...) to resize the image. Then, I upload the image as a blob (using the method image.toBlob()). The server answers back providing a URL of the received image.
/***********************************************
* GLOBAL VARS
**********************************************/
var image = new MarvinImage();
/***********************************************
* FILE CHOOSER AND UPLOAD
**********************************************/
$('#fileUpload').change(function (event) {
form = new FormData();
form.append('name', event.target.files[0].name);
reader = new FileReader();
reader.readAsDataURL(event.target.files[0]);
reader.onload = function(){
image.load(reader.result, imageLoaded);
};
});
function resizeAndSendToServer(){
$("#divServerResponse").html("uploading...");
$.ajax({
method: 'POST',
url: 'https://www.marvinj.org/backoffice/imageUpload.php',
data: form,
enctype: 'multipart/form-data',
contentType: false,
processData: false,
success: function (resp) {
$("#divServerResponse").html("SERVER RESPONSE (NEW IMAGE):<br/><img src='"+resp+"' style='max-width:400px'></img>");
},
error: function (data) {
console.log("error:"+error);
console.log(data);
},
});
};
/***********************************************
* IMAGE MANIPULATION
**********************************************/
function imageLoaded(){
Marvin.scale(image.clone(), image, 120);
form.append("blob", image.toBlob());
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://www.marvinj.org/releases/marvinj-0.8.js"></script>
<form id="form" action='/backoffice/imageUpload.php' style='margin:auto;' method='post' enctype='multipart/form-data'>
<input type='file' id='fileUpload' class='upload' name='userfile'/>
</form><br/>
<button type="button" onclick="resizeAndSendToServer()">Resize and Send to Server</button><br/><br/>
<div id="divServerResponse">
</div>
In my experience, this example has been the best solution for uploading a resized picture: https://zocada.com/compress-resize-images-javascript-browser/
It uses the HTML5 Canvas feature.
The code is as 'simple' as this:
compress(e) {
const fileName = e.target.files[0].name;
const reader = new FileReader();
reader.readAsDataURL(e.target.files[0]);
reader.onload = event => {
const img = new Image();
img.src = event.target.result;
img.onload = () => {
const elem = document.createElement('canvas');
const width = Math.min(800, img.width);
const scaleFactor = width / img.width;
elem.width = width;
elem.height = img.height * scaleFactor;
const ctx = elem.getContext('2d');
// img.width and img.height will contain the original dimensions
ctx.drawImage(img, 0, 0, width, img.height * scaleFactor);
ctx.canvas.toBlob((blob) => {
const file = new File([blob], fileName, {
type: 'image/jpeg',
lastModified: Date.now()
});
}, 'image/jpeg', 1);
},
reader.onerror = error => console.log(error);
};
}
There are two downsides with this solution.
The first one is related with the image rotation, due to ignoring EXIF data. I couldn't tackle this issue, and wasn't so important in my use case, but will be glad to hear any feedback.
The second downside is the lack of support foe IE/Edge (not the Chrome based version though), and I won't put any time on that.
Yes, with modern browsers this is totally doable. Even doable to the point of uploading the file specifically as a binary file having done any number of canvas alterations.
http://jsfiddle.net/bo40drmv/
(this answer is a improvement of the accepted answer here)
Keeping in mind to catch process the result submission in the PHP with something akin to:
//File destination
$destination = "/folder/cropped_image.png";
//Get uploaded image file it's temporary name
$image_tmp_name = $_FILES["cropped_image"]["tmp_name"][0];
//Move temporary file to final destination
move_uploaded_file($image_tmp_name, $destination);
If one worries about Vitaly's point, you can try some of the cropping and resizing on the working jfiddle.