Develop Reference

Sending image manipulated via JS in AJAX POST request

I'm a server-side dev learning the ropes of vanilla JS. I need to clear my concepts regarding sending an Ajax POST request for an image object I'm creating in JS - this question is about that.
Imagine a web app where users upload photos for others to see. At the point of each image's upload, I use vanilla JS to confirm the image's mime-type (via interpreting magic numbers), and then resize the image for optimization purposes.
After resizing, I do:
var canvas = document.createElement('canvas');
canvas.width = resized_width;
canvas.height = resized_height;
var ctx = canvas.getContext("2d");
ctx.drawImage(source_img, 0, 0, resized_width, resized_height);
var resized_img = new Image();
resized_img.src = canvas.toDataURL("image/jpeg",0.7);
return resized_img;
The image object returned has to be sent to the backend via an Ajax request. Something like:
function overwrite_default_submit(e) {
e.preventDefault();
var form = new FormData();
form.append("myfile", resized_img, img_name);
var xhr = new XMLHttpRequest();
xhr.open('POST', e.target.action);
// xhr.send(form); // uncomment to really send the request
}
However, the image object returned after resizing is essentially an HTML element like so <img src="data:image/jpeg;base64>. Whereas the object expected in the FormData object ought to be a File object, e.g. something like: File { name: "example.jpg", lastModified: 1500117303000, lastModifiedDate: Date 2017-07-15T11:15:03.000Z, webkitRelativePath: "", size: 115711, type: "image/jpeg" }.
So what do I do to fix this issue? Would prefer to learn the most efficient way of doing things here.
Btw, I've seen an example on SO of using the JS FILE object, but I'd prefer a more cross-browser method, given File garnered support from Safari, Opera Mobile and built-in Android browsers relatively recently.
Moreover, only want pure JS solutions since I'm using this as an exercise to learn the ropes. JQuery is on my radar, but for later.
The rest of my code is as follows (only included JPEG processing for brevity):
var max_img_width = 400;
var wranges = [max_img_width, Math.round(0.8*max_img_width), Math.round(0.6*max_img_width),Math.round(0.4*max_img_width),Math.round(0.2*max_img_width)];
// grab the file from the input and process it
function process_user_file(e) {
var file = e.target.files[0];
var reader = new FileReader();
reader.onload = process_image;
reader.readAsArrayBuffer(file.slice(0,25));
}
// checking file type programmatically (via magic numbers), getting dimensions and returning a compressed image
function process_image(e) {
var img_width;
var img_height;
var view = new Uint8Array(e.target.result);
var arr = view.subarray(0, 4);
var header = "";
for(var i = 0; i < arr.length; i++) {
header += arr[i].toString(16);
}
switch (header) {
case "ffd8ffe0":
case "ffd8ffe1":
case "ffd8ffe2":
case "ffd8ffe3":
case "ffd8ffe8":
// magic numbers represent type = "image/jpeg";
// use the 'slow' method to get the dimensions of the media
img_file = browse_image_btn.files[0];
var fr = new FileReader();
fr.onload = function(){
var dataURL = fr.result;
var img = new Image();
img.onload = function() {
img_width = this.width;
img_height = this.height;
resized_img = resize_and_compress(this, img_width, img_height, 80);
}
img.src = dataURL;
};
fr.readAsDataURL(img_file);
to_send = browse_image_btn.files[0];
load_rest = true;
subform.disabled = false;
break;
default:
// type = "unknown"; // Or one can use the blob.type as fallback
load_rest = false;
subform.disabled = true;
browse_image_btn.value = "";
to_send = null;
break;
}
}
// resizing (& compressing) image
function resize_and_compress(source_img, img_width, img_height, quality){
var new_width;
switch (true) {
case img_width < wranges[4]:
new_width = wranges[4];
break;
case img_width < wranges[3]:
new_width = wranges[4];
break;
case img_width < wranges[2]:
new_width = wranges[3];
break;
case img_width < wranges[1]:
new_width = wranges[2];
break;
case img_width < wranges[0]:
new_width = wranges[1];
break;
default:
new_width = wranges[0];
break;
}
var wpercent = (new_width/img_width);
var new_height = Math.round(img_height*wpercent);
var canvas = document.createElement('canvas');
canvas.width = new_width;
canvas.height = new_height;
var ctx = canvas.getContext("2d");
ctx.drawImage(source_img, 0, 0, new_width, new_height);
console.log(ctx);
var resized_img = new Image();
resized_img.src = canvas.toDataURL("image/jpeg",quality/100);
return resized_img;
}
Update: I'm employing the following:
// converting image data uri to a blob object
function dataURItoBlob(dataURI,mime_type) {
var byteString = atob(dataURI.split(',')[1]);
var ab = new ArrayBuffer(byteString.length);
var ia = new Uint8Array(ab);
for (var i = 0; i < byteString.length; i++) { ia[i] = byteString.charCodeAt(i); }
return new Blob([ab], { type: mime_type });
}
Where the dataURI parameter is canvas.toDataURL(mime_type,quality/100)

You should call the canvas.toBlob() to get the binary instead of using a base64 string.
it's async so you would have to add a callback to it.
canvas.toBlob(function(blob) {
resized_img.onload = function() {
// no longer need to read the blob so it's revoked
URL.revokeObjectURL(this.url);
};
// Preview the image using createObjectURL
resized_img.src = URL.createObjectURL(blob);
// Attach the blob to the FormData
var form = new FormData();
form.append("myfile", blob, img_name);
}, "image/jpeg", 0.7);

See to this SO post: How to get base64 encoded data from html image
I think you need to call 'canvas.toDataURL()' to get the actual base64 stream of the image.
var image = canvas.toDataURL();
Then upload it with a Form: Upload a base64 encoded image using FormData?
var data = new FormData();
data.append("image_data", image);
Untested, but this should be about it.

Canvas image parsing orientation not in right sequence

i've image uploader which using canvas and trying to get orientation using load-image.all.min.js is fine. but when i choose multiple image orientation parsing function saving data not one by one.
which means if i choose 1 image. it transferring data to 'upload_canvas.php?ori='+ori with correct ori variable.
but when i choose multiple image to upload example 3 images (with orientation 1, 1, 8)
it passing data to server upload_canvas.php?ori=8, upload_canvas.php?ori=8, upload_canvas.php?ori=8. only last ori variable.
maybe orientation parsing function already looped before uploading image data to server one by one.
how to transfer image with correct orientation to server?
below my using code.
document.querySelector('form input[type=file]').addEventListener('change', function(event){
// Read files
var files = event.target.files;
var ori = 1;
// Iterate through files
for (var i = 0; i < files.length; i++) {
// Ensure it's an image
if (files[i].type.match(/image.*/)) {
//Get image orienatation
loadImage.parseMetaData(files[i], function (data) {
if (data.exif) {
ori = data.exif.get('Orientation');
console.log("ori: "+ori);
} else {ori = 1;}
});
// Load image
var reader = new FileReader();
reader.onload = function (readerEvent) {
var image = new Image();
image.onload = function (imageEvent) {
canvas.width = image.width;
canvas.height = image.height;
drawImageIOSFix(canvas.getContext('2d'),image, 0, 0, image.width, image.height, 0, 0, width, height);
// Upload image
var xhr = new XMLHttpRequest();
if (xhr.upload) {
// Update progress
xhr.upload.addEventListener('progress', function(event) {
var percent = parseInt(event.loaded / event.total * 100);
progressElement.style.width = percent+'%';
}, false);
// File uploaded / failed
xhr.onreadystatechange = function(event) {
if (xhr.readyState == 4) {
if (xhr.status == 200) {
//some code
} else {
imageElement.parentNode.removeChild(imageElement);
}
}
}
xhr.open('post', 'upload_canvas.php?t=' + Math.random()+'&ori='+ori, true);
xhr.send(canvas.toDataURL('image/jpeg'));
}
}
image.src = readerEvent.target.result;
}
reader.readAsDataURL(files[i]);
}
}
// Clear files
event.target.value = '';});

Your variable ori is a global variable, that is shared between all images. The code in the .onload functions aren't run immediately, but only after your for() loop has gone through all the images. At this point ori will contain the orientation of the last image.
To fix, move the variable and parseMetaData into the reader.onload function.
...
var reader = new FileReader();
reader.onload = function (readerEvent) {
var ori;
loadImage.parseMetaData(files[i], ...)
var image = new Image();
image.onload = function (imageEvent) {
...
Warning: Not tested!

Upload the same image file twice - Javascript

I have an image uploader in my drawing application that I've written in Javascript. I want to allow the user to place multiple of the same image on the canvas. However, when I try to upload an image that's already on the canvas, nothing happens and a breakpoint in my event handler for the uploader never gets hit. What's going on and how can I fix it? Thanks!
Here's the code for my image handler:
function handleImage(e) {
var reader = new FileReader();
reader.onload = function(event) {
var img = new Image();
img.onload = function() {
img.className = 'drag';
img.style.left = 0;
img.style.top = 0;
context.drawImage(img, parseInt(img.style.left, 10) , parseInt(img.style.top, 10));
images.push(img);
}
img.src = event.target.result;
}
reader.readAsDataURL(e.target.files[0]);
};

I do tend to agree with Rene Pot to use the same image again (duplicate button), but you still can't prevent the user from inserting/loading the same image again. I've encountered the problem a while ago and used this bit of code to check if the image is already cached (if cached, there is no load, hence the onload won't fire either).
var img = new Image();
img.src = event.target.result;
var insertImage = function() {
img.className = 'drag';
img.style.left = 0;
img.style.top = 0;
context.drawImage(img, parseInt(img.style.left, 10) , parseInt(img.style.top, 10));
images.push(img);
}
if(img.complete){
img.onload = insertImage;
} else {
insertImage();
}
Hope that helps.

HTML5 - drag&drop a JPG and receive its width

I want to receive the width of a JPG after dragging it into the browser window. The strange thing is, sometimes it works perfectly and sometimes a width of zero is returned.
fiddle
document.body.addEventListener('drop', function ( e )
{
e.stopPropagation();
e.preventDefault();
file = e.dataTransfer.files[0];
reader.readAsDataURL( file );
reader.onloadend = function()
{
var source = this.result;
var currentImg = new Image();
currentImg.src = source;
var someDiv = document.createElement("div");
someDiv.innerHTML = '<li>'+currentImg.width+'</li>';
document.getElementById("logs").appendChild(someDiv);
}
});

This happens because you probably get width before the image is loaded.
Put some code like this and you works correctly.
...
var currentImg = new Image();
currentImg.onload = function() {
var someDiv = document.createElement("div");
someDiv.innerHTML = '<li>'+currentImg.width+'</li>';
document.getElementById("logs").appendChild(someDiv);
}
currentImg.src = source;
...
put the onload before setting the sorce because onload is a callback function.

image,onload event not working in chrome

I'm using html5 to create drag and drop image upload functionality. This works great for me in firefox but in chrome the image onload event only fires the first time. If I drag multiple images in only the first works and if I drag a second in it fails. I believe the problem is with the image onload.
here is the way my code works I have removed the irrelevant sections:
var img = document.createElement("img");
var reader = new FileReader();
var canvas = document.createElement("canvas");
var canvasData;
var ctx = canvas.getContext("2d");
var myFiles;
var i = 0;
reader.onload = (function (aImg)
{
return function (e)
{
aImg.src = e.target.result;
};
})(img);
img.onload = function (){
//resizes image
//draws it to the canvas
//posts to server
i++;
if(i < myFiles.length){
processNext(i);
}
}
function processNext(filei) {
var file = myFiles[filei];
img.file = file;
reader.readAsDataURL(file);
}
i = 0;
myFiles = files;
processNext(0);
Does anyone know why this works in firefox but not chrome?

Explanation from chromium tracker:
This is not a bug. WebKit is just more strict. You must instantiate a new Image() object before the replacement, like this:
var photo = document.getElementById('image_id');
var img = new Image();
img.addEventListener('load', myFunction, false);
img.src = 'http://newimgsource.jpg';
photo.src = img.src;
source: http://code.google.com/p/chromium/issues/detail?id=7731#c12

This is strange, none of the above worked for me. I was defining the image variable as local and change it to global and it started working. Does this make sense? Can somebody explain it?
This didnt worked for me:
function loadImage() {
var ImageToLoad = new Image();
ImageToLoad.onload = function() {
console.log("finish loading");
};
ImageToLoad.src = "myimage.png";
}
This did work:
var ImageToLoad = new Image();
function loadImage() {
ImageToLoad.onload = function() {
console.log("finish loading");
};
ImageToLoad.src = "myimage.png";
}