ColorPicker implementation using JavaScript and Canvas - javascript

I'm trying to implement ColorPicker using Canvas just for fun. But i seem lost. as my browser is freezing for a while when it loads due to all these for loops.
I'm adding the screenshot of the result of this script:
window.onload = function(){
colorPicker();
}
function colorPicker(){
var canvas = document.getElementById("colDisp"),
frame = canvas.getContext("2d");
var r=0,
g=0,
b= 0;
function drawColor(){
for(r=0;r<255;r++){
for(g=0;g<255;g++){
for(b=0;b<255;b++){
frame.fillStyle="rgb("+r+","+g+","+b+")";
frame.fillRect(r,g,1,1);
}
}
}
}
drawColor();
Currently , i only want a solution about the freezing problem with better algorithm and it's not displaying the BLACK and GREY colors.
Please someone help me.

Instead of calling fillRect for every single pixel, it might be a lot more efficient to work with a raw RGBA buffer. You can obtain one using context.getImageData, fill it with the color values, and then put it back in one go using context.putImageData.
Note that your current code overwrites each single pixel 255 times, once for each possible blue-value. The final pass on each pixel is 255 blue, so you see no grey and black in the output.
Finding a good way to map all possible RGB values to a two-dimensional image isn't trivial, because RGB is a three-dimensional color-space. There are a lot of strategies for doing so, but none is really optimal for any possible use-case. You can find some creative solutions for this problem on AllRGB.com. A few of them might be suitable for a color-picker for some use-cases.

If you want to fetch the rgba of the pixel under the mouse, you must use context.getImageData.
getImageData returns an array of pixels.
var pixeldata=context.getImageData(0,0,canvas.width,canvas.height);
Each pixel is defined by 4 sequential array elements.
So if you have gotten a pixel array with getImageData:
// first pixel defined by the first 4 pixel array elements
pixeldata[0] = red component of pixel#1
pixeldata[1] = green component of pixel#1
pixeldata[2] = blue component of pixel#1
pixeldata[4] = alpha (opacity) component of pixel#1
// second pixel defined by the next 4 pixel array elements
pixeldata[5] = red component of pixel#2
pixeldata[6] = green component of pixel#2
pixeldata[7] = blue component of pixel#2
pixeldata[8] = alpha (opacity) component of pixel#2
So if you have a mouseX and mouseY then you can get the r,g,b,a values under the mouse like this:
// get the offset in the array where mouseX,mouseY begin
var offset=(imageWidth*mouseY+mouseX)*4;
// read the red,blue,green and alpha values of that pixel
var red = pixeldata[offset];
var green = pixeldata[offset+1];
var blue = pixeldata[offset+2];
var alpha = pixeldata[offset+3];
Here's a demo that draws a colorwheel on the canvas and displays the RGBA under the mouse:
http://jsfiddle.net/m1erickson/94BAQ/

A way to go, using .createImageData():
window.onload = function() {
var canvas = document.getElementById("colDisp");
var frame = canvas.getContext("2d");
var width = canvas.width;
var height = canvas.height;
var imagedata = frame.createImageData(width, height);
var index, x, y;
for (x = 0; x < width; x++) {
for (y = 0; y < height; y++) {
index = (x * width + y) * 4;
imagedata.data[index + 0] = x;
imagedata.data[index + 1] = y;
imagedata.data[index + 2] = x + y - 255;
imagedata.data[index + 3] = 255;
}
}
frame.putImageData(imagedata, 0, 0);
};
http://codepen.io/anon/pen/vGcaF

Related

JavaScript canvas, manually cloning a canvas onto another generates a weird pattern

I'm trying to make a text effect similar to the effect found at the bottom of this article
My proposed approach is:
Make two canvasses, one is visible, the other is invisible I use this as a buffer.
Draw some text on the buffer canvas
Loop over getImageData pixels
if pixel alpha is not equal to zero (when there is a pixel drawn on the canvas buffer) with a small chance, ie 2%, draw a randomly generated circle with cool effecs at that pixel on the visible canvas.
I'm having trouble at step 4. With the code below, I'm trying to replicate the text on the second canvas, in full red. Instead I get this weird picture.
code
// create the canvas to replicate the buffer text on.
var draw = new Drawing(true);
var bufferText = function (size, textFont) {
// set the font to Georgia if it isn't defined
textFont = textFont || "Georgia";
// create a new canvas buffer, true means that it's visible on the screen
// Note, Drawing is a small library I wrote, it's just a wrapper over the canvas API
// it creates a new canvas and adds some functions to the context
// it doesn't change any of the original functions
var buffer = new Drawing(true);
// context is just a small wrapper library I wrote to make the canvas API a little more bearable.
with (buffer.context) {
font = util.format("{size}px {font}", {size: size, font: textFont});
fillText("Hi there", 0, size);
}
// get the imagedata and store the actual pixels array in data
var imageData = buffer.context.getImageData(0, 0, buffer.canvas.width, buffer.canvas.height);
var data = imageData.data;
var index, alpha, x, y;
// loop over the pixels
for (x = 0; x < imageData.width; x++) {
for (y = 0; y < imageData.height; y++) {
index = x * y * 4;
alpha = data[index + 3];
// if the alpha is not equal to 0, draw a red pixel at (x, y)
if (alpha !== 0) {
with (draw.context) {
dot(x/4, y/4, {fillColor: "red"})
}
}
}
}
};
bufferText(20);
Note that here, my buffer is actually visible to show where the red pixels are supposed to go compared to where they actually go.
I'm really confused by this problem.
If anybody knows an alternative approach, that's very welcome too.
replace this...
index = x * y * 4;
with...
index = (imageData.width * y) + x;
the rest is good :)

Mirroring right half of webcam image

I saw that you have helped David with his mirroring canvas problem before. Canvas - flip half the image
I have a similar problem and hope that maybe you could help me.
I want to apply the same mirror effect on my webcam-canvas, but instead of the left side, I want to take the RIGHT half of the image, flip it and apply it to the LEFT.
This is the code you've posted for David. It also works for my webcam cancas. Now I tried to change it, so that it works for the other side, but unfortunately I'm not able to get it.
for(var y = 0; y < height; y++) {
for(var x = 0; x < width / 2; x++) { // divide by 2 to only loop through the left half of the image.
var offset = ((width* y) + x) * 4; // Pixel origin
// Get pixel
var r = data[offset];
var g = data[offset + 1];
var b = data[offset + 2];
var a = data[offset + 3];
// Calculate how far to the right the mirrored pixel is
var mirrorOffset = (width - (x * 2)) * 4;
// Get set mirrored pixel's colours
data[offset + mirrorOffset] = r;
data[offset + 1 + mirrorOffset] = g;
data[offset + 2 + mirrorOffset] = b;
data[offset + 3 + mirrorOffset] = a;
}
}
Even if the accepted answer you're relying on uses imageData, there's absolutely no use for that.
Canvas allows, with drawImage and its transform (scale, rotate, translate), to perform many operations, one of them being to safely copy the canvas on itself.
Advantages is that it will be way easier AND way way faster than handling the image by its rgb components.
I'll let you read the code below, hopefully it's commented and clear enough.
The fiddle is here :
http://jsbin.com/betufeha/2/edit?js,output
One output example - i took also a mountain, a Canadian one :-) - :
Original :
Output :
html
<canvas id='cv'></canvas>
javascript
var mountain = new Image() ;
mountain.onload = drawMe;
mountain.src = 'http://www.hdwallpapers.in/walls/brooks_mountain_range_alaska-normal.jpg';
function drawMe() {
var cv=document.getElementById('cv');
// set the width/height same as image.
cv.width=mountain.width;
cv.height = mountain.height;
var ctx=cv.getContext('2d');
// first copy the whole image.
ctx.drawImage(mountain, 0, 0);
// save to avoid messing up context.
ctx.save();
// translate to the middle of the left part of the canvas = 1/4th of the image.
ctx.translate(cv.width/4, 0);
// flip the x coordinates to have a mirror effect
ctx.scale(-1,1);
// copy the right part on the left part.
ctx.drawImage(cv,
/*source */ cv.width/2,0,cv.width/2, cv.height,
/*destination*/ -cv.width/4, 0, cv.width/2, cv.height);
// restore context
ctx.restore();
}

Looping through pixels in an image

My project is to input an image into a canvas tag in an HTML page, and then loop through the pixels and RGBA values of the pixels. While looping through the red values,so every fourth value in the pixel, I want to log the position of the pixels that represent a white pixel. Now, I have the image loading down with some code I got from this blog, http://www.phpied.com/photo-canvas-tag-flip/ .
He has another post in which he gives some code on how to loop through the pixels and log the information I want to log, but I don't understand it, and I don't want to copy his code without knowing what it is I'm doing. So could anybody please either explain the method he's using or perhaps show me another way to do what he's doing? This is the link to the other post http://www.phpied.com/pixel-manipulation-in-canvas/ .
It's straightforward.
All the pixel data for a canvas are stored sequentially in an array.
The first pixel's data occupy array elements #0-3 (red=0/green=1/blue=2/alpha=3).
The second pixel's data occupy array elements #4-7 (red=4/green=5/blue=6/alpha=7).
And so on...
You can load that pixel data by using context.getImageData() and enumerating through the array:
const imgData = context.getImageData(0, 0, canvas.width, canvas.height);
const data = imgData.data;
// enumerate all pixels
// each pixel's r,g,b,a datum are stored in separate sequential array elements
for(let i = 0; i < data.length; i += 4) {
const red = data[i];
const green = data[i + 1];
const blue = data[i + 2];
const alpha = data[i + 3];
}
You can also change those array values and then save the array back to the image using context.putImageData().
// save any altered pixel data back to the context
// the image will reflect any changes you made
context.putImageData(imgData, 0, 0);
The image will then change according to the changes you made to its pixel array.
Each pixel contains 4 components red, green, blue, alpha - each of them is number 0-255. The loop starts from top-left to bottom-right.
I recommend you to use an image processing framework in order to focus on the algorithms instead of manipulating arrays of values. Some frameworks:
fabric.js
processing.js
MarvinJ
In the case of MarvinJ, you can simply loop through pixels iterating column and row coordinates. I use the methods getIntComponentX() to access color components.
for(var y=0; y<image.getHeight(); y++){
for(var x=0; x<image.getWidth(); x++){
var red = image.getIntComponent0(x,y);
var green = image.getIntComponent1(x,y);
var blue = image.getIntComponent2(x,y);
}
}
Therefore you don't need to worry about how the pixel data is represented. In order to check if a pixel is white:
// Is white?
if(red >= 250 && blue >= 250 && green >= 250){
console.log("Pixel at "+x+","+y+" is white");
}
Runnable Example:
var canvas = document.getElementById("canvas");
image = new MarvinImage();
image.load("https://i.imgur.com/eLZVbQG.png", imageLoaded);
function imageLoaded(){
var whitePixels=0;
for(var y=0; y<image.getHeight(); y++){
for(var x=0; x<image.getWidth(); x++){
var red = image.getIntComponent0(x,y);
var green = image.getIntComponent1(x,y);
var blue = image.getIntComponent2(x,y);
var alpha = image.getAlphaComponent(x,y);
// Is white?
if(red >= 250 && green >= 250 && blue >= 250 && alpha > 0){
whitePixels++;
}
}
}
image.draw(canvas);
document.getElementById("result").innerHTML = "white pixels: "+whitePixels;
}
<script src="https://www.marvinj.org/releases/marvinj-0.7.js"></script>
<canvas id="canvas" width="500" height="344"></canvas>
<div id="result"></div>

HTML5 canvas multiply effect - jagged edges

Our company website features a "random shard generator", built in Flash, which creates a number of overlapping coloured shard graphics at random just below the site header.
http://www.clarendonmarketing.com
I am trying to replicate this effect using HTML5, and whilst I can generate the random shards easily enough, the blended overlapping (multiply in Adobe terms) is proving a challenge.
I have a solution which basically creates an array of all the canvas's pixel data before each shard is drawn, then another array with the canvas's pixel data after each shard is drawn. It then compares the two and where it finds a non transparent pixel in the first array whose corresponding pixel in the second array matches the currently selected fill colour, it redraws it with a new colour value determined by a 'multiply' function (topValue * bottomValue / 255).
Generally this works fine and achieves the desired effect, EXCEPT around the edges of the overlapping shards, where a jagged effect is produced.
I believe this has something to do with the browser's anti-aliasing. I have tried replicating the original pixel's alpha channel value for the computed pixel, but that doesn't seem to help.
Javascript:
// Random Shard Generator v2 (HTML5)
var theCanvas;
var ctx;
var maxShards = 6;
var minShards = 3;
var fillArray = new Array(
[180,181,171,255],
[162,202,28,255],
[192,15,44,255],
[222,23,112,255],
[63,185,127,255],
[152,103,158,255],
[251,216,45,255],
[249,147,0,255],
[0,151,204,255]
);
var selectedFill;
window.onload = function() {
theCanvas = document.getElementById('shards');
ctx = theCanvas.getContext('2d');
//ctx.translate(-0.5, -0.5)
var totalShards = getRandom(maxShards, minShards);
for(i=0; i<=totalShards; i++) {
//get snapshot of current canvas
imgData = ctx.getImageData(0,0,theCanvas.width,theCanvas.height);
currentPix = imgData.data
//draw a shard
drawRandomShard();
//get snapshot of new canvas
imgData = ctx.getImageData(0,0,theCanvas.width,theCanvas.height);
pix = imgData.data;
//console.log(selectedFill[0]+','+selectedFill[1]+','+selectedFill[2]);
//alert('break')
//CALCULATE THE MULTIPLIED RGB VALUES FOR OVERLAPPING PIXELS
for (var j = 0, n = currentPix.length; j < n; j += 4) {
if (
//the current pixel is not blank (alpha 0)
(currentPix[j+3]>0)
&& //and the new pixel matches the currently selected fill colour
(pix[j]==selectedFill[0] && pix[j+1]==selectedFill[1] && pix[j+2]==selectedFill[2])
) { //multiply the current pixel by the selected fill colour
//console.log('old: '+currentPix[j]+','+currentPix[j+1]+','+currentPix[j+2]+','+currentPix[j+3]+'\n'+'new: '+pix[j]+','+pix[j+1]+','+pix[j+2]+','+pix[j+3]);
pix[j] = multiply(selectedFill[0], currentPix[j]); // red
pix[j+1] = multiply(selectedFill[1], currentPix[j+1]); // green
pix[j+2] = multiply(selectedFill[2], currentPix[j+2]); // blue
}
}
//update the canvas
ctx.putImageData(imgData, 0, 0);
}
};
function drawRandomShard() {
var maxShardWidth = 200;
var minShardWidth = 30;
var maxShardHeight = 16;
var minShardHeight = 10;
var minIndent = 4;
var maxRight = theCanvas.width-maxShardWidth;
//generate a random start point
var randomLeftAnchor = getRandom(maxRight, 0);
//generate a random right anchor point
var randomRightAnchor = getRandom((randomLeftAnchor+maxShardWidth),(randomLeftAnchor+minShardWidth));
//generate a random number between the min and max limits for the lower point
var randomLowerAnchorX = getRandom((randomRightAnchor - minIndent),(randomLeftAnchor + minIndent));
//generate a random height for the shard
var randomLowerAnchorY = getRandom(maxShardHeight, minShardHeight);
//select a fill colour from an array
var fillSelector = getRandom(fillArray.length-1,0);
//console.log(fillSelector);
selectedFill = fillArray[fillSelector];
drawShard(randomLeftAnchor, randomLowerAnchorX, randomLowerAnchorY, randomRightAnchor, selectedFill);
}
function drawShard(leftAnchor, lowerAnchorX, lowerAnchorY, rightAnchor, selectedFill) {
ctx.beginPath();
ctx.moveTo(leftAnchor,0);
ctx.lineTo(lowerAnchorX,lowerAnchorY);
ctx.lineTo(rightAnchor,0);
ctx.closePath();
fillColour = 'rgb('+selectedFill[0]+','+selectedFill[1]+','+selectedFill[2]+')';
ctx.fillStyle=fillColour;
ctx.fill();
};
function getRandom(high, low) {
return Math.floor(Math.random() * (high-low)+1) + low;
}
function multiply(topValue, bottomValue){
return topValue * bottomValue / 255;
};
Working demo:
http://www.clarendonmarketing.com/html5shards.html
Do you really need multiplication? Why not just use lower opacity blending?
Demo http://jsfiddle.net/wk3eE/
ctx.globalAlpha = 0.6;
for(var i=totalShards;i--;) drawRandomShard();
Edit: If you really need multiplication, then leave it to the professionals, since multiply mode with alpha values is a little tricky:
Demo 2: http://jsfiddle.net/wk3eE/2/
<script type="text/javascript" src="context_blender.js"></script>
<script type="text/javascript">
var ctx = document.querySelector('canvas').getContext('2d');
// Create an off-screen canvas to draw shards to first
var off = ctx.canvas.cloneNode(true).getContext('2d');
var w = ctx.canvas.width, h = ctx.canvas.height;
for(var i=totalShards;i--;){
off.clearRect(0,0,w,h); // clear the offscreen context first
drawRandomShard(off); // modify to draw to the offscreen context
off.blendOnto(ctx,'multiply'); // multiply onto the main context
}
</script>

Image editing with javascript

In the creation of my html5 game engine I've been able to do some nice things and get some cool features. On a contract to make a game I've been asked to see if I can remove the background color from sprite images. And I see the pluses with this since we could use jpgs instead on pngs and decrease the size of the images.
Is there any way I can do this with pure javascript? I'd like to be able to do this without using the a canvas element so it can be faster, but if I have to that's okay.
If I have to do that I have another question, I don't want the canvas object to show that I use, can I use a canvas object with document.createElement without applying it to the document? That would be nice since it wouldn't have to be rendered to the webpage. If not I guess I can just move the canvas object to the left out of view.
Lastly do you think a good way to preprocess the images be to send them to a server cgi script and have it return a json pixel array?
Here is the function for floodfill algorithm, it removed the background from an image which is already drawn on the canvas.
In the following code canvas is the HTML5 canvas element and context it canvas.getContext("2d"). You can change the value of colorRange and try the function with different colors. The last line of the function
imageElement.src=canvas.toDataURL("image/png");
is to show the image inside an img tag. So you need an img and a canvas on your page. If you don't want to show the image in img element just remove the last line.
// Remove backgroud without ajax call, can be used in non IE browsers.
function RemoveBackground(){
var startR,startG,startB;
var canvasData;
var canvasWidth=canvas.width;
var canvasHeight=canvas.height;
canvasData=mainContext.getImageData(0,0,canvasWidth,canvasHeight);
startR = canvasData.data[0];
startG = canvasData.data[1];
startB = canvasData.data[2];
if(startR==0&& startG==0 && startR==0) return;
var pixelStack = [[0, 0]];
while(pixelStack.length)
{
var newPos, x, y, pixelPos, reachLeft, reachRight;
newPos = pixelStack.pop();
x = newPos[0];
y = newPos[1];
pixelPos = (y*canvasWidth + x) * 4;
while(y-- >= 0 && matchStartColor(pixelPos,canvasData,startR,startG,startB)){
pixelPos -= canvasWidth * 4;
}
pixelPos += canvasWidth * 4;
++y;
reachLeft = false;
reachRight = false;
while(y++ < canvasHeight-1 && matchStartColor(pixelPos,canvasData,startR,startG,startB))
{
colorPixel(pixelPos,canvasData);
if(x > 0)
{
if(matchStartColor(pixelPos-4,canvasData,startR,startG,startB))
{
if(!reachLeft){
pixelStack.push([x - 1, y]);
reachLeft = true;
}
}
else if(reachLeft)
{
reachLeft = false;
}
}
if(x < canvasWidth-1)
{
if(matchStartColor(pixelPos+4,canvasData,startR,startG,startB))
{
if(!reachRight)
{
pixelStack.push([x + 1, y]);
reachRight = true;
}
}
else if(reachRight)
{
reachRight = false;
}
}
pixelPos += canvasWidth * 4;
}
}
context.putImageData(canvasData, 0, 0);
imageElement.src=canvas.toDataURL("image/png");
}
// Helper function for remove background color.
function matchStartColor(pixelPos,canvasData,startR,startG,startB)
{
var r = canvasData.data[pixelPos];
var g = canvasData.data[pixelPos+1];
var b = canvasData.data[pixelPos+2];
var colorRange=8;
return ((r >= startR-colorRange && r<=startR+colorRange)
&&( g >= startG-colorRange && g<=startG+colorRange)
&&( b >= startB-colorRange && b<= startB+colorRange));
}
// Helper function for remove background color.
function colorPixel(pixelPos,canvasData)
{
canvasData.data[pixelPos] = 255;
canvasData.data[pixelPos+1] = 255;
canvasData.data[pixelPos+2] = 255;
}
Removing background without choppy borders isn't a trivial task, even by hand in image-editing programs. You'll have to implement some sort of antialiasing, at least.
Moreover, it's not a good idea to manipulate an image compressed into a lossy format.
PNG compression is superior (in terms of size) to JPG on simpler images with continuous fill of the same color and certain types of gradients. JPG is only good for heterogeneous images with lots of different colors mixed in unpredictable manner. Like photos. Which one would not expect in game sprites, I guess. And again – JPG is a lossy format.
As for the Canvas element, it doesn't have to be added to the DOM tree at all.
The most naïve algorithm to make a given color transparent would be such: draw the image, get its pixel data, iterate over the data and compare every pixel color with your given color. If it matches, set the alpha to 0.
Canvas API methods you'll need:
drawImage
getImageData
The somewhat tricky in it's simplicity part is the CanvasPixelArray. To check each pixel in such arrays, you do something like that:
for (var i = 0; i < pixelAr.length; i += 4) {
var r = pixelAr[i];
var g = pixelAr[i + 1];
var b = pixelAr[i + 2];
var alpha = pixelAr[i + 3];
}
Personally I would not go down this path. JPEG images are compressed, which means that whatever you define as a background color may change slightly in the compressed file (ie. you'll get the classic JPEG artifacting). Furthermore, you won't be able to support partial transparency unless you define a range for your background color, which in turn makes the editing more complicated. The tradeoff between file size and performance/quality is nowhere near worth it here, in my opinion.
Having said that, the only way you can access the pixel data from an image is by placing it on a canvas first. You can, as you mentioned, work with the canvas off-screen in memory without having to append it to the document.
If I understand your last question correctly, you cannot work with a canvas element on the server side. To work with pixel data on your server, you'd have to use something like PHP's image library.
If all of that doesn't sway you in favor of just using PNG images, here's some sample code that will remove a specified background color from a loaded image:
$(document).ready(function() {
var matte_color = [0, 255, 0, 255]; // rgba: [0, 255];
// Handles the loaded image element by erasing the matte color
// and appending the transformed image to the document.
function handleLoadedImage() {
eraseMatte(); // Eliminate the matte.
// Append the canvas element to the document where it is needed.
document.getElementById("parent_container").appendChild(canvas);
}
// Given the matte color defined at the top, clear all pixels to 100% transparency
// within the loaded sprite.
function eraseMatte() {
canvas.width = sprite.width;
canvas.height = sprite.height;
context.drawImage(sprite, 0, 0); // Draw the image on the canvas so we can read the pixels.
// Get the pixel data from the canvas.
var image_data = context.getImageData(0, 0, sprite.width, sprite.height);
var data = image_data.data; // Obtaining a direct handle is a huge performance boost.
var end = sprite.width * sprite.height * 4; // W x H x RGBA
// Loop over each pixel from the image and clear matte pixels as needed.
for (var i = 0; i < end; i += 4) {
if (data[i] == matte_color[0] && data[i + 1] == matte_color[1] &&
data[i + 2] == matte_color[2] && data[i + 3] == matte_color[3]) { // Color match.
data[i] = data[i + 1] = data[i + 2] = data[i + 3] = 0; // Set pixel to transparent.
}
}
// Put the transformed image data back on the canvas.
context.putImageData(image_data, 0, 0);
}
var canvas = document.createElement("canvas");
var context = canvas.getContext("2d");
var sprite = new Image();
sprite.onload = handleLoadedImage;
sprite.src = "sprite.jpg";
});
You can do that using a canvas, don't know if it is possible without it.
An easy way to achieve what you are trying to do is using the getImageData on your canvas's context:
imgData = myCanvasContext.getImageData(x1, y1, x2, y2);
x1, y1, x2, y2 are the coordenates of the area you want to get data, for the whole use 0, 0, width, height image. The getImageData will return you an ImageData, wich contains an array with rgba values from each pixel. The values will be ordered like this:
http://i.stack.imgur.com/tdHNJ.png
You can manipulate the array imgData.data[index], editing it values and, consequently, editing the image.
Here is a good article about editing images on html5 with canvas:
http://beej.us/blog/2010/02/html5s-canvas-part-ii-pixel-manipulation/
To doesn't show what you are doing, just create the canvas with the css command display:none;
(...)if I can remove the background color from sprite images. And I see the pluses with this since we could use jpgs instead on pngs(...)
I really recommend you to not do that. The jpg compression of the image can make image editing very hard. Removing the background of a jpg image isn't is easy, and it gets harder with the amount of borders on the image. I'm don't think the size that you will economize will compensate the hard work to remove a background from a jpg image.
Not exactly the same, but you can achieve that. I can give you an headstart on this - checkout this jsFiddle. I built this editor using FabricJS.
var canvas = new fabric.Canvas('c');
var imgInstance = new fabric.Image(imgElement);
canvas.add(imgInstance);//initialize the Canvas with the image

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