Develop Reference

Convert ImageData to blob in JS?

I have an ImageData object but Tesseract.js only takes blob objects. How can I convert the ImageData to a blob as performantly as possible?

Referring here, the code should look like -
const ImageDataToBlob = function(imageData){
let w = imageData.width;
let h = imageData.height;
let canvas = document.createElement("canvas");
canvas.width = w;
canvas.height = h;
let ctx = canvas.getContext("2d");
ctx.putImageData(imageData, 0, 0); // synchronous
return new Promise((resolve) => {
canvas.toBlob(resolve); // implied image/png format
});
}

Tesseract.js also takes some other types - https://github.com/naptha/tesseract.js/blob/master/docs/image-format.md - and I have found some code on the internet to convert:
function imgDataToImage(imagedata) {
var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
canvas.width = imagedata.width;
canvas.height = imagedata.height;
ctx.putImageData(imagedata, 0, 0);
var image = new Image();
image.src = canvas.toDataURL();
return image;
}

Converting canvas to PNG returns blank image

I am experiencing an issue with converting a canvas to PNG.
Although the canvas looks exactly as I want it and the conversion from canvas to data URL PNG seems right, the image is blank.
I also tried converting a div to PNG but it did not work for me because I wanted a greyscale filter to be applied. Anyone have any ideas?
JavaScript
var imgis = new Image();
var bubble = new Image();
var canvasWidth;
var canvasHeight;
var ctx = canvas.getContext('2d');
bubble.onload = function() {
var imgis = new Image();
var bubble = new Image();
var ctx = canvas.getContext('2d');
bubble.onload = function() {
// set the canvas' size
canvas.width = this.width;
canvas.height = this.height;
// first fill a rect
ctx.fillStyle = 'rgba(255, 255, 255, 0)';
ctx.fillRect(0, 0, canvas.width, canvas.height);
// set the gCO
ctx.globalCompositeOperation = 'luminosity';
// if the browser doesn't support Blend Modes
console.log(ctx.globalCompositeOperation)
if (ctx.globalCompositeOperation !== 'luminosity')
fallback(this);
else {
// draw the image
ctx.drawImage(this, 0, 0);
ctx.drawImage(imgis, 30, 60);
// reset the gCO
ctx.globalCompositeOperation = 'source-over';
}
}
imgis.crossOrigin = "anonymous";
bubble.crossOrigin = "anonymous";
imgis.src = "image1 src";
bubble.src = "image2 src";
function fallback(img) {
// first remove our black rectangle
ctx.clearRect(0, 0, canvas.width, canvas.height);
//draw the image
ctx.drawImage(img, 0, 0);
ctx.drawImage(imgis, 30, 60);
// get the image data
var imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
var d = imgData.data;
// loop through all pixels
// each pixel is decomposed in its 4 rgba values
for (var i = 0; i < d.length; i += 4) {
// get the medium of the 3 first values
var med = (d[i] + d[i + 1] + d[i + 2]) / 3;
// set it to each value
d[i] = d[i + 1] = d[i + 2] = med;
}
// redraw the new computed image
ctx.putImageData(imgData, 0, 0);
}
canvas = document.getElementById('canvas');
var image = Canvas2Image.convertToPNG(canvas);
console.log(image.src);
// document.getElementById('theDemo').src = image.src;
var image_data = $(image).attr('src');
console.log(image_data);
$("#theDemo").attr('src', image_data);
HTML
<canvas id='canvas' > </canvas>
<img src="" id="theDemo" />

I assume you're using canvas2image. You should replace var image = Canvas2Image.convertToPNG(canvas); with Canvas2Image.convertToPNG(canvas, width, height). Hopefully that helps!
EDIT Since the issue is with the actual canvas to base64 conversion, you can try to use the .toDataURL() method instead of using that library. My comment explains how to test this in your specific code.

HTML 5 Canvas image overlay repeatable

I am trying to take a pattern and apply it over a png image but only cover the non-transparent part of the image similar to this one.
http://jsfiddle.net/eLmmA/1/
$(function() {
var canvas = document.createElement('canvas');
canvas.width = 250;
canvas.height = 250;
var canvas_context = canvas.getContext("2d");
var img = new Image();
img.onload = function(){
var msk = new Image();
msk.onload = function(){
canvas_context.drawImage(img, 0, 0);
canvas_context.globalCompositeOperation = "destination-in";
canvas_context.drawImage(msk, 0, 0);
canvas_context.globalCompositeOperation = "source-over";
};
msk.src = 'http://i.stack.imgur.com/QtQrZ.png';
}
img.src = 'http://i.stack.imgur.com/MDGFY.jpg';
document.body.appendChild(canvas);
});
The example above is really close to what I want but I need to be able to use a smaller texture image and repeat it over the none pattern image. I am not familiar with how to use canvas properly but trying to learn more about it.
Thanks in advance!

Never mind I figured it out.
var canvas = document.createElement('canvas');
canvas.width = 250;
canvas.height = 250;
var canvas_context = canvas.getContext("2d");
var img = new Image();
img.onload = function(){
var msk = new Image();
msk.onload = function(){
var ptrn = canvas_context.createPattern(img, 'repeat');
canvas_context.fillStyle = ptrn;
canvas_context.fillRect(0, 0, canvas.width, canvas.height);
canvas_context.drawImage(img, 0, 0);
canvas_context.globalCompositeOperation = "destination-in";
canvas_context.drawImage(msk, 0, 0);
canvas_context.globalCompositeOperation = "source-over";
};
msk.src = $('#base_url').data('base')+'assets/themes/img/download.png';
}
img.src = $('#base_url').data('base')+'assets/uploads/thumbs/1381413411Purple-Night-Owls-thumb.jpg';
$('#itemPreview').html(canvas);

Feather Boarders of Canvas Image

i have a canvas with the following images loaded:
Background Image:
Front Image:
Both together looks like this image:
Now i want to apply a feather effect to the hand boarders to result in to something like this:
I tried the following solution so far. But the result is not like on the above image.
var temp = document.createElement('canvas'),
tx = temp.getContext('2d');
temp.width = scope.canvas.width;
temp.height = scope.canvas.height;
tx.translate(-temp.width, 0);
tx.shadowOffsetX = temp.width;
tx.shadowOffsetY = 0;
tx.shadowColor = '#000';
tx.shadowBlur = 100;
var img = new Image();
img.onload = function() {
tx.drawImage(img, 0, 0, 512, 512);
};
img.src = imageURL; // the hand image
var temp2 = document.createElement('canvas'),
tx2 = temp2.getContext('2d');
temp2.width = scope.canvas.width;
temp2.height = scope.canvas.height;
var img2 = new Image();
img2.onload = function() {
tx2.drawImage(img2, 0, 0, 512, 512);
tx2.save();
tx2.globalCompositeOperation = 'destination-out';
tx2.drawImage(temp, 0, 0);
tx2.restore();
};
img2.src = temp.toDataURL("image/png");
Any idea how to solve this?
Regards
Steve

To feather an image, create a copy of it via a new canvas, create an invers mask of the image with destination-out comp. Then draw the hand again and then mask with destination-ou but with a shadow to create the feather.
var canvas = document.createElement("canvas");
canvas.width = 1024,canvas.height = 1024;
var ctx = canvas.getContext("2d");
document.body.appendChild(canvas);
var hand = new Image();
hand .src = "http://i.stack.imgur.com/PbAfc.png";
hand .onload = function(){
var can = document.createElement("canvas");
can.width = this.width;
can.height = this.height;
ct = can.getContext("2d");
// create inverted mask
ct.fillStyle = "black";
ct.fillRect(0,0,this.width,this.height);
ct.globalCompositeOperation = "destination-out";
ct.drawImage(this,0,0);
// create second canvas
var can1 = document.createElement("canvas");
can1.width = this.width;
can1.height = this.height;
ct1 = can1.getContext("2d");
// draw image
ct1.drawImage(this,0,0);
ct1.shadowColor = "black";
ct1.shadowBlur = 30; // amount of feather
ct1.globalCompositeOperation = "destination-out";
ct1.drawImage(can,0,0);
ct1.shadowBlur = 20; // amount of feather
ct1.drawImage(can,0,0); // The mor you draw the greater the FX
ct1.shadowBlur = 10; // amount of feather
ct1.drawImage(can,0,0); // The mor you draw the greater the FX
ct1.globalCompositeOperation = "source-over";
ct.globalCompositeOperation = "source-over";
ctx.drawImage(can1,0,0);
// use the new canvas can1 as the hand image in later code.
}
//ctx.fillStyle = "#e19e9e"
ctx.fillRect(0,0,1024,1024);

Why are pixels retrieved from canvas all black?

As an exercise, I would like to fill the background of the browser window with pixels of random color: https://jsfiddle.net/j8fay7bs/3/
My question is how to retrieve the current filling of the background image and randomly change the pixels? Currently the image seems to be reverted to black.
// redraw canvas pixels on resize --------------------------
var render = function resize() {
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
context.canvas.width = window.innerWidth;
context.canvas.height = window.innerHeight;
var imageData = context.getImageData(0, 0, canvas.width, canvas.height);
var t0 = performance.now();
for (var i = 0; i < canvas.width*canvas.height; i++) {
if (Math.random() < 0.5) {
imageData.data[i*4] = Math.round(Math.random()*255);
imageData.data[i*4+1] = Math.round(Math.random()*255);
imageData.data[i*4+2] = Math.round(Math.random()*255);
imageData.data[i*4+3] = 255;
}
}
context.putImageData(imageData, 0, 0);
$('#fps').text(1000/(performance.now() - t0) + " fps");
}
window.onresize = render;
// rendering loop ---------------------------------------
(function loop() {
setInterval(render, 10);
})();

You may retrieve the last image data before u touch the canvas width and height,
once u touched them, the canvas will be reset:
var render = function resize() {
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var oldImageData = context.getImageData(0, 0, canvas.width, canvas.height); // <----
context.canvas.width = window.innerWidth;
context.canvas.height = window.innerHeight;
var imageData = context.getImageData(0, 0, canvas.width, canvas.height);
updated fiddle