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;
}

getImageData() array is returning all 0s in angular 5

I am trying to get the rgb value of an image after I render the image inside canvas. However I am getting all 0s in return. I am trying to use getImageData() after I render the image not sure why I am getting 0s in return.
My Code :
fileSelected: File;
preview: any;
context: CanvasRenderingContext2D;
hue: any;
myImageData:any
onFileSelect(e: any) {
let canvas = this.mycanvas.nativeElement;
let context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height)
let imgData;
this.fileSelected = e.target.files[0];
let reader = new FileReader()
reader.onload = (e: any) => {
var image = new Image();
image.onload = function () {
image.width = canvas.width;
image.height = canvas.height;
context.drawImage(image, 0, 0, canvas.width, canvas.height)
};
image.src = e.target.result
}
reader.readAsDataURL(this.fileSelected);
this.myImageData = context.getImageData(0, 0, 100, 100);
this.hue = this.calculateHue(this.fileSelected, this.myImageData);
}
calculateHue(file, myImageData) {
console.log("rgb====", myImageData.data)
return null
}
I have gone through these links getImageData() returning all zeros, getImageData() returning all 0's, getImageData always returning 0
But none of the answers solved it. What am I doing wrong?

You're writing to the canvas as soon as the image is loaded, however you're trying to read from the canvas as soon as a file is selected, move:
this.myImageData = context.getImageData(0, 0, 100, 100);
this.hue = this.calculateHue(this.fileSelected, this.myImageData);
into the image.onload and after the context.drawImage(image, 0, 0, canvas.width, canvas.height) call so you're sure there is image data that you can read.
It should look something like this:
onFileSelect(e: any) {
let canvas = this.mycanvas.nativeElement;
let context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height)
let imgData;
this.fileSelected = e.target.files[0];
let reader = new FileReader();
let self = this;
reader.onload = (e: any) => {
var image = new Image();
image.onload = () => {
image.width = canvas.width;
image.height = canvas.height;
context.drawImage(image, 0, 0, canvas.width, canvas.height)
// Move it here
self.myImageData = context.getImageData(0, 0, 100, 100);
self.hue = this.calculateHue(self.fileSelected, self.myImageData);
};
image.src = e.target.result
}
reader.readAsDataURL(this.fileSelected);
}

Convert image on my page to base64

I have an image loaded on my page that I need to convert to base64. I'm currently using the following:
var c = document.createElement("canvas");
var ctx = c.getContext("2d");
var img = document.getElementById("additem_img");
ctx.drawImage(img, 10, 10);
console.log(c.toDataURL());
Which I found in another StackOverflow question. It seems to create a base64 string
data:image/png;base64,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
But when I try to decode this using every online decoder I can, I get nothing. The "additem_img" element is an image who's source is an image from a remote webpage. I want to create a local copy using canvas and base64.

try this:
<img id="additem_img" src="image.png">
<script>
var c = document.createElement("canvas");
var ctx = c.getContext("2d");
var img = document.getElementById("additem_img");
var w = img.width;
var h = img.height;
c.width = w;
c.height = h;
ctx.drawImage(img, 0, 0);
console.log(c.toDataURL());
</script>

fabric.js - create Image object from ImageData object of canvas API

I want to create an image object in fabric.js from ImageData object, we can get ImageData from this:
var imgData=ctx.getImageData(10,10,50,50);
//ctx.putImageData(imgData,10,70);
// something liket that
var image = new fabric.Image.fromImageData (...);
Is there any way to create an image object from ImageData object?

Let me put my idea here, I don't like this way but have no others around -
var ctx = canvas.getContext('2d');
var data = ctx.getImageData(0, 0, 20, 20);
var c = document.createElement('canvas');
c.setAttribute('id', '_temp_canvas');
c.width = 20;
c.height = 20;
c.getContext('2d').putImageData(data, 0, 0);
fabric.Image.fromURL(c.toDataURL(), function(img) {
img.left = 50;
img.top = 50;
canvas.add(img);
img.bringToFront();
c = null;
$('#_temp_canvas').remove();
canvas.renderAll();
});

Rotate canvas 90 degrees clockwise and update width height

Say we have a canvas:
<canvas id="one" width="100" height="200"></canvas>
And on a button click the canvas gets rotated 90 degrees clockwise (around the center) and the dimensions of the canvas get also updated, so in a sense it looks like this afterwards:
<canvas id="one" width="200" height="100"></canvas>
Note that the id of the canvas is the same.
Imagine simply rotating an image clockwise without it being cropped or being padded.
Any suggestions before I do it the long way of creating a new canvas and rotating and copying pixel by pixel?
UPDATE sample code with suggestion from comments still not working:
function imageRotatecw90(){
var canvas = document.getElementById("one");
var context = canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var myImageData = context.getImageData(0,0, cw,ch);
context.save();
context.translate(cw / 2, ch / 2);
context.rotate(Math.PI/2);
context.putImageData(myImageData, 0, 0);
context.restore();
canvas.width=ch;
canvas.height=cw;
}
FiddleJS

Look at this DEMO.
To achieve the results seen in demo, I made use of canvas.toDataURL to cache the canvas into an image, then reset the canvas to their new dimensions, translate and rotate the context properly and finally draw the cached image back to modified canvas.
That way you easily rotate the canvas without need to redraw everything again. But because anti-aliasing methods used by browser, each time this operation is done you'll notice some blurriness in result. If you don't like this behavior the only solution I could figure out is to draw everything again, what is much more difficult to track.
Here follows the code:
var canvas = document.getElementById("one");
var context = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;
// Sample graphic
context.beginPath();
context.rect(10, 10, 20, 50);
context.fillStyle = 'yellow';
context.fill();
context.lineWidth = 7;
context.strokeStyle = 'black';
context.stroke();
// create button
var button = document.getElementById("rotate");
button.onclick = function () {
// rotate the canvas 90 degrees each time the button is pressed
rotate();
}
var myImageData, rotating = false;
var rotate = function () {
if (!rotating) {
rotating = true;
// store current data to an image
myImageData = new Image();
myImageData.src = canvas.toDataURL();
myImageData.onload = function () {
// reset the canvas with new dimensions
canvas.width = ch;
canvas.height = cw;
cw = canvas.width;
ch = canvas.height;
context.save();
// translate and rotate
context.translate(cw, ch / cw);
context.rotate(Math.PI / 2);
// draw the previows image, now rotated
context.drawImage(myImageData, 0, 0);
context.restore();
// clear the temporary image
myImageData = null;
rotating = false;
}
}
}

Rotation
Note it is not possible to rotate a single element.
ctx.save();
ctx.rotate(0.17);
// Clear the current drawings.
ctx.fillRect()
// draw your object
ctx.restore();
Width/height adjustment
The only way I ever found to properly deal with display ratios, screen sizes etc:
canvas.width = 20;// DO NOT USE PIXELS
canvas.height = 40; // AGAIN NO PIXELS
Notice I am intentionally not using canvas.style.width or canvas.style.height. Also for an adjustable canvas don't rely on CSS or media queries to do the transformations, they are a headache because of the pixel ratio differences. JavaScript automatically accounts for those.
Update
You also have to update the width and the height before you draw. Not sure what you are trying to achieve, but I guess this isn't a problem:
Demo here
var canvas = document.getElementById("one");
var context = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;
canvas.width = 200;
canvas.height = 400;
// Sample graphic
context.beginPath();
context.rect(10,10,20,50);
context.fillStyle = 'yellow';
context.fill();
context.lineWidth = 7;
context.strokeStyle = 'black';
context.stroke();
var myImageData = context.getImageData(0, 0, cw, ch);
context.save();
context.translate(cw / 2, ch / 2);
context.putImageData(myImageData, 0, 0);
context.rotate(0.20);

If you want to rotate an image by 90 degrees this might be helpful:
export const rotateBase64Image = async (base64data: string) => {
const image = new Image();
image.src = base64data;
return new Promise<string>((resolve, reject) => {
image.onload = function () {
const canvas = document.createElement("canvas");
const ctx = canvas.getContext("2d");
if (!ctx) throw new Error("cannnot get context '2d'");
canvas.width = image.height;
canvas.height = image.width;
ctx.setTransform(0, 1, -1, 0, canvas.width, 0); // overwrite existing transform
ctx!.drawImage(image, 0, 0);
canvas.toBlob((blob) => {
if (!blob) {
return reject("Canvas is empty");
}
const fileUrl = window.URL.createObjectURL(blob);
resolve(fileUrl);
}, "image/jpeg");
};
});
};
If you don't have image in base64 format you can do it like this:
const handleRotate = async () => {
const res = await fetch(link);
const blob = await res.blob();
const b64: string = await blobToB64(blob);
const rotatedImage = await rotateBase64Image(b64)
setLink(rotatedImage);
}
Here is my blobTob64 function:
export const blobToB64 = async (blob) => {
return new Promise((resolve, _) => {
const reader = new FileReader();
reader.onloadend = () => resolve(reader.result);
reader.readAsDataURL(blob);
});
};