Develop Reference

Resizing image with HTML5 canvas

I am trying to size an image with canvas. My goal is to have an image of dimensions A x B sized to M x N without changing proportions - like CSS contain. For example, if the source image is 1000x1000 and the destination is 400x300, it should cut off a piece 100 pixels toll at the bottom, and that should correspond to 250 pixels in the source image.
My code is below:
const canvas = document.createElement('canvas');
const img = new Image();
img.src = promotedImage;
const FINAL_WIDTH = 400;
const FINAL_HEIGHT = 250;
const width = img.width;
const height = img.height;
const ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0, width, height, 0, 0, FINAL_WIDTH, FINAL_HEIGHT);
const finalImage = b64toFile(canvas.toDataURL("image/jpg"));
This is not working, like I want to, though. I am obviously using drawImage incorrectly. For me, if copies source to destination without sizing.
Is this because I need to size (change dimensions) for canvas prior to drawing? Please advise.
I have also tried something like Mozilla image upload. It does even scale the image, but it does not crop. Plus, it sizes a source square to the smaller target side, instead of clipping it.

Setting an image source is asynchronous, can be very fast, but often not fast enough to keep up with still-running code. Generally, to make them work reliably, you set an onload handler first and then set src. The canvas element defaults to 300x150 so would also need to be sized. (Canvas obeys CORS. .crossOrigin = '' sets us as anonymous and imgur has a permissive CORS policy. We wouldn't be able to convert the canvas to an image while using a third-party image in this snippet otherwise.)
const MAX_WIDTH = 400;
const MAX_HEIGHT = 300;
const img = new Image();
img.crossOrigin = '';
img.onload = () => {
const wRatio = MAX_WIDTH / img.width;
const hRatio = MAX_HEIGHT / img.height;
var width, height;
if(wRatio > hRatio) {
width = MAX_WIDTH;
height = wRatio * img.height;
}
else {
width = hRatio * img.width;
height = MAX_HEIGHT;
}
const canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
const ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0, width, height);
//const finalImage = b64toFile(canvas.toDataURL("image/jpg"));
const imgElement = document.createElement('img');
imgElement.src = canvas.toDataURL('image/jpg');
document.body.appendChild(imgElement);
};
img.src = 'https://i.imgur.com/TMeawxt.jpeg';
img { border: 1px solid red; }

Javascript crop image stored in buffer from coordinates

I found code that crops images in javascript using canvas but I need to crop an image that is never actually displayed anywhere since all I need is to get data from the image. There is a screenshot and then the buffered screenshot image needs to be cropped from specific coordinates.
The image is simply stored in a variable to begin with let img = new Image(). How could this code be modified to crop an image stored in the buffer only?
function resizeImage(url, width, height, x, y, callback) {
var canvas = document.createElement("canvas");
var context = canvas.getContext('2d');
var imageObj = new Image();
// set canvas dimensions
canvas.width = width;
canvas.height = height;
imageObj.onload = function () {
context.drawImage(imageObj, x, y, width, height, 0, 0, width, height);
callback(canvas.toDataURL());
};
imageObj.src = url;
}

Since no one answered I will post what I found. In the end I achieved what I needed using the library Sharp's extract feature.
img.onload = resizeImg;
img.src = 'image.png';
function resizeImg() {
this.path = this.path = 'image.png';
sharp(this.path)
.resize(this.width * 2, this.height * 2)
.extract({ width: 100, height: 20, left: 250, top: 100 })
.toBuffer({ resolveWithObject: true })
.then(({ data, info }) => {
//process data
})
}

To cut out the image in Canvas

I have image in Canvas.
const canvas = document.createElement('canvas')
canvas.width = 640
canvas.height = 480
const context = canvas.getContext('2d')
element.appendChild(canvas)
const imageData = context.createImageData(640, 480)
imageData.data.set(new Uint8ClampedArray(data.frame))
context.putImageData(imageData, 0, 0)
I receive data (height, width, x, y).
I want to cut an image from the canvas according to this data and save it as a blob-file.
How can I do this?

You question is unclear. The code example does not match the problem descriptions. As the code example does not work (undefined variable data) I can only go by the description of the problem.
I receive data (height, width, x, y). I want to cut an image from the canvas according to this data and save it as a blob-file. How can I do this?
To cut from Image
To cut a rectangular area from a canvas
function cutArea(fromImage, x, y, width, height) {
const cut = document.createElement("canvas");
cut.width = width;
cut.height = height;
const ctx = cut.getContext("2d");
ctx.drawImage(fromImage, -x, -y);
return cut;
}
You should use an offscreenCanvas but support is limited so add it if you know what your targets are. To create an offscreenCanvas cut
function cutArea(fromImage, x, y, width, height) {
const cut = new OffscreenCanvas(width, height);
const ctx = cut.getContext("2d");
ctx.drawImage(fromImage, -x, -y);
return cut;
}
Note
That the pixel scale is unchanged and that there is not bounds checks. Part or all of the cut image may be empty (0 alpha) if the coordinated do not lay on the image source.
fromImage can be any valid image source, including a canvas.
width and height must be > 0 or the functions will throw an error.
Download
To download the cut to local store as downloaded "image/png" image file. Note filename can be changed by user on download. The image type is the default png. There is no way to know if download is successful.
function downloadCut(cut, name) {
const a = document.createElement("a");
a.download = name + ".png";
const download = blob => {
const url = a.href = URL.createObjectURL(blob);
a.dispatchEvent(new MouseEvent("click", {view: window, bubbles: true, cancelable: true}));
setTimeout(() => URL.revokeObjectURL(url), 1000); // clean up
}
if (cut instanceof OffscreenCanvas) { cut.convertToBlob().then(download) }
else { canvas.toBlob(download) }
}

Draw a square version of image in canvas

I am trying to show previews of uploaded images in a canvas. The preview should be a square version of image.
What I have:
var img = new Image;
img.src = imageSrc;
var c = document.getElementById('file-preview-' + data.response.id);
var ctx = c.getContext("2d");
img.onload = function() {
ctx.drawImage(img, 0, 0);
};
And the preview:
So how can I draw on the canvas a 150x150 square with maintained ratio cropped version of the image?
For example for this image:
I should get:

Here's a method that yields precisely the result you desire, cropped, resized and centered (with commented out lines that yield a left based cropping instead)...
var ctx;
function init(event) {
ctx = document.getElementById('canvas').getContext('2d');
// Your preview is actually 120x120,
// but I've stuck with the textual description of
// your requirements here.
ctx.canvas.width = 150;
ctx.canvas.height = 150;
loadBackground('http://i.stack.imgur.com/PCKEo.png');
}
function loadBackground(path) {
var img = new Image();
img.onload = drawBackground;
img.src = path;
}
function drawBackground(event) {
var img = event.target;
var imgSize = Math.min(img.width, img.height);
// The following two lines yield a central based cropping.
// They can both be amended to be 0, if you wish it to be
// a left based cropped image.
var left = (img.width - imgSize) / 2;
var top = (img.height - imgSize) / 2;
//var left = 0; // If you wish left based cropping instead.
//var top = 0; // If you wish left based cropping instead.
ctx.drawImage(event.target, left, top, imgSize, imgSize, 0, 0, ctx.canvas.width, ctx.canvas.height);
}
window.addEventListener('load', init, false);
<canvas id="canvas"></canvas>

Based on information from this question: SO Question
I was able to modify what was done to create a square cropped image in canvas:
var img = new Image;
img.src = "http://cdn.bmwblog.com/wp-content/uploads/2016/02/M-Performance-Parts-BMW-M2-3.jpg"; //Or whatever image source you have
var c= document.getElementById('file-preview');
var ctx=c.getContext("2d");
img.onload = function(){
ctx.drawImage(img, img.width/2, img.height/2, img.width, img.height, 0, 0, img.width, img.height);
};
You just have to modify the arguments you are passing to the drawImage function as well as add some more.
You can see a fiddle here: https://jsfiddle.net/4d93pkoa/3/
Remember that the first two parameters specify the image's top left coordinates within the canvas element.
Remember to set the width and height of the canvas element in HTML to a square shape.

Drawing another image to canvas

So I am creating a cordova app where I take a photo from the iphone library, draw it to canvas and add another image to it in order to save it as one photo. So far the photo I draw from the iphone photo library gets drawn without problems to the canvas, however the second image doesn't.
When I load the second image, it first gets added to a div with absolute positioning in order to move it to wherever I want. After that I get the actual image it's source and it's positions and try to draw it to canvas. The drawing of the second image happens when I call a method that also performs the canvas2ImagePlugin it's functions. In the end only the first photo without the second image gets saved.
The draw image to canvas function:
function drawImage(image_source, dx, dy)
{
var canvas = document.getElementById('Photo');
var image = new Image();
image.src = image_source;
image.onload = function() {
c=canvas.getContext("2d");
c.canvas.width = window.innerWidth;
c.canvas.height = window.innerHeight;
c.drawImage(image,dx,dy, window.innerWidth, window.innerHeight);
}
}
The method for drawing the second image and saving it:
function saveImage()
{
var img = $('.ObjectImage').attr('src', $('img:first').attr('src'));
var imagePosition = $('.ObjectImage').find('img:first').position();
drawImage(img, imgPosition.left, imgPosition.top);
window.canvas2ImagePlugin.saveImageDataToLibrary(
function(msg){
console.log(msg);
},
function(err){
console.log(err);
},
document.getElementById('Photo')
);
alert("Image saved");
}
The window.innerWidth, window.innerHeight on the canvas is done to get the canvas in full screen of the parent div.
EDIT to the comment:
function drawImage(image_source, dx, dy)
{
var canvas = document.getElementById('Photo');
var image = new Image();
image.onload = function() {
c=canvas.getContext("2d");
c.canvas.width = window.innerWidth;
c.canvas.height = window.innerHeight;
c.drawImage(image,dx,dy, window.innerWidth, window.innerHeight);
}
image.src = image_source;
}
Still not working

The drawImage function works asynchronously, it starts loading an image and exits immediately. Then, when the image loads, the canvas is updated. If you run something like:
drawImage('test.jpg',0,0);
drawImage('test2.jpg',0,0);
you will get both images updating the canvas at approximately the same time and only one will appear.
Also, what wolfhammer said is correct. If you set the size of the canvas, you clear it, so drawing one image after the other, even if they are smaller sizes and should both appear, will only show the last one. Check this link on how to solve it: Preventing Canvas Clear when Resizing Window
Further more, you are drawing all images with the width and height of the window, which doesn't make sense. Probably you want to use the width and height of the image (so this.width instead of window.innerWidth)

When you set the width and height of the canvas the data on "Photo" is cleared. I've provide a resize function if resizing is really needed.
function drawImage(image_source, dx, dy)
{
var canvas = document.getElementById('Photo');
var image = new Image();
image.src = image_source;
image.onload = function() {
c=canvas.getContext("2d");
//c.canvas.width = window.innerWidth;
//c.canvas.height = window.innerHeight;
c.drawImage(image,dx,dy, window.innerWidth, window.innerHeight);
}
}
var can = document.getElementById('can');
var ctx = can.getContext('2d');
ctx.fillStyle = "red";
ctx.beginPath();
ctx.moveTo(20, 90);
ctx.lineTo(50, 10);
ctx.lineTo(80, 90);
ctx.lineTo(10, 40);
ctx.lineTo(90, 40);
ctx.lineTo(20, 90);
ctx.fill();
var btn = document.getElementById('btnResize');
btn.addEventListener('click', function() {
resize(can, can.width * 2, can.height * 2);
});
function resize(can, w, h) {
var ctx = can.getContext('2d');
// copy
var can2 = document.createElement('canvas');
var ctx2 = can2.getContext('2d');
can2.width = can.width;
can2.height = can.height;
ctx2.drawImage(can, 0, 0);
// resize
can.width = w;
can.height = h;
ctx.drawImage(can2, 0, 0, can2.width, can2.height, 0, 0, w, h);
}
#can {
border:1px solid red;
}
<button id='btnResize'>Size x 2</button><br/>
<canvas id="can" width="100" height="100"></canvas>