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variable for input value isn't behaving as expected in JS

I'm working on a project. I'm creating a canvas that can be manipulated by the user. There are currently 3 input fields for the user to select Height, Width, and a 3rd value called "space".
Currently, Height and Width seem to work perfect fine, but the Space input isn't behaving as expected. For "Space" the user should be able to enter a value between 15 and 50. (space represents the distance between the circles I'm drawing).
The loop for drawing the circles is:
const lightsTop = function () {
for (let i = 5; i < cw; i += space) {
let randX = Math.random() * 10;
c.beginPath();
c.arc(i, 5 + randX, 5, 0, Math.PI * 2);
c.fillStyle = "red";
c.fill();
}
};
This code (above) works perfectly fine if I manually put in "15" (or some arbitrary number between 15-50). It generates a row of circles (which eventually will act like a string of lights), but when I use the variable "space", then it only generates 1 circle.
Here is the full JS: (the HTML is just the 3 inputs type="number",ids = #width, #height, #space)
const canvas = document.createElement("canvas");
document.body.appendChild(canvas);
const c = canvas.getContext("2d");
const widthInput = document.querySelector("#width");
const heightInput = document.querySelector("#height");
const spaceInput = document.querySelector("#space");
let cw = 500;
let ch = 300;
let space = 15;
canvas.width = cw;
canvas.height = ch;
widthInput.addEventListener("change", (event) => {
cw = widthInput.value;
if (cw < 200) cw = 200;
if (cw > 1000) cw = 1000;
canvas.width = cw;
lightsTop();
});
heightInput.addEventListener("change", (event) => {
ch = heightInput.value;
if (ch < 200) ch = 200;
if (ch > 600) ch = 600;
canvas.height = ch;
lightsTop();
});
spaceInput.addEventListener("change", (event) => {
space = spaceInput.value;
if (space < 15) space = 15;
if (space > 50) space = 50;
animate();
});
function animate() {
c.clearRect(0,0,cw,ch);
requestAnimationFrame(lightsTop);
}
const lightsTop = function () {
for (let i = 5; i < cw; i += space) {
let randX = Math.random() * 10;
c.beginPath();
c.arc(i, 5 + randX, 5, 0, Math.PI * 2);
c.fillStyle = "red";
c.fill();
}
};

You are assigning a string to the variable:
space = spaceInput.value;
It must be a number
space = parseInt(spaceInput.value);

Canvas Pixelation - Adjust pixel size

I've got a script that cycle's through images. The images start pixelated and then when they are in view, become unpixelated. I achieve that by calling this function x amount of times with requestAnimationFrame
Images.prototype.setPixels = function() {
var sw = this.imageWidth,
sh = this.imageHeight,
imageData = this.context.getImageData( 0, 0, sw, sh ),
data = imageData.data,
y, x, n, m;
for ( y = 0; y < sh; y += this.pixelation ) {
for ( x = 0; x < sw; x += this.pixelation ) {
var red = data[((sw * y) + x) * 4];
var green = data[((sw * y) + x) * 4 + 1];
var blue = data[((sw * y) + x) * 4 + 2];
for ( n = 0; n < this.pixelation; n++ ) {
for ( m = 0; m < this.pixelation; m++ ) {
if ( x + m < sw ) {
data[((sw * (y + n)) + (x + m)) * 4] = red;
data[((sw * (y + n)) + (x + m)) * 4 + 1] = green;
data[((sw * (y + n)) + (x + m)) * 4 + 2] = blue;
}
}
}
}
}
this.context.putImageData( imageData, 0, 0 );
}
Question: How can I make the individual pixels larger blocks than they are right now. Right now they are pretty small, and the effect is a little jarring. I'm hoping to fix this by having less pixel blocks on the screen, by making them bigger.
I hope this makes sense, I'm fairly green with canvas, so anything you could do to point me in the right direction would be great!

The best for this kind of effect is to simply use drawImage and let the browser handle the pixelation thanks to the nearest-neighbor anti-aliasing algorithm that can be set by changing the imageSmoothingEnabled property to false.
It then becomes a two step process to pixelate an image at any pixel_size:
draw the full quality image (or canvas / video ...) at its original size / pixel_size.
At this stage, each "pixel" is one pixel large.
draw this small image again but up-scaled by pixel_size. To do so, you just need to draw the canvas over itself.
Each pixel is now pixel_size large.
Instead of dealing with hard to read many parameters of drawImage, we can deal the scaling quite easily by just using ctx.scale() method.
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
function drawPixelated( source, pixel_size ) {
// scale down
ctx.scale(1 / pixel_size, 1 / pixel_size)
ctx.drawImage(source, 0, 0);
// make next drawing erase what's currently on the canvas
ctx.globalCompositeOperation = 'copy';
// nearest-neighbor
ctx.imageSmoothingEnabled = false;
// scale up
ctx.setTransform(pixel_size, 0, 0, pixel_size, 0, 0);
ctx.drawImage(canvas, 0, 0);
// reset all to defaults
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.globalCompositeOperation = 'source-over';
ctx.imageSmoothingEnabled = true;
}
const img = new Image();
img.onload = animeLoop;
img.src = "https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png";
let size = 1;
let speed = 0.1;
function animeLoop() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
size += speed;
if(size > 30 || size <= 1) {
speed *= -1
}
drawPixelated( img, size );
requestAnimationFrame(animeLoop);
}
<canvas id="canvas" width="800" height="600"></canvas>
Now for the ones that come with a real need to use an ImageData, for instance because they are generating pixel-art, then know that you can simply use the same technique:
put your ImageData with each pixel being 1 pixel large.
scale your context to pixel_size
draw your canvas over itself upscaled
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
function putPixelated( imageData, pixel_size ) {
ctx.putImageData(imageData, 0, 0);
// make next drawing erase what's currently on the canvas
ctx.globalCompositeOperation = 'copy';
// nearest-neighbor
ctx.imageSmoothingEnabled = false;
// scale up
ctx.setTransform(pixel_size, 0, 0, pixel_size, 0, 0);
ctx.drawImage(canvas, 0, 0);
// reset all to defaults
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.globalCompositeOperation = 'source-over';
ctx.imageSmoothingEnabled = true;
}
const img = new ImageData(16, 16);
crypto.getRandomValues(img.data);
let size = 1;
let speed = 0.1;
animeLoop();
function animeLoop() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
size += speed;
if(size > 30 || size <= 1) {
speed *= -1
}
putPixelated( img, size );
requestAnimationFrame(animeLoop);
}
<canvas id="canvas" width="800" height="600"></canvas>

scrambling images on canvas producing whitespace

I'm starting with a canvas element. I'm making the left half red, and the right side blue. Every half second, setInterval calls a function, scramble, which splits both RHS and LHS into pieces, and shuffles them.
Here is a fiddle: https://jsfiddle.net/aeq1g3yb/
The code is below. The reason I'm using window.onload is because this thing is supposed to scramble pictures and I want the pictures to load first. I'm using colors here because of the cross-origin business that I don't know enough about, so this is my accommodation.
var n = 1;
var v = 1;
function scramble() {
//get the canvas and change its width
var c = document.getElementById("myCanvas");
c.width = 600;
var ctx = c.getContext("2d");
//drawing 2 different colors side by side
ctx.fillStyle = "red";
ctx.fillRect(0, 0, c.width/2, c.height);
ctx.fillStyle = "blue";
ctx.fillRect(c.width/2, 0, c.width/2, c.height);
//how big will each shuffled chunk be
var stepsA = (c.width/2) / n;
var stepsB = (c.width/2) / n;
var step = stepsA + stepsB;
var imgDataA = [];
var imgDataB = [];
for (var i = 0; i < n; i++) {
var imgDataElementA = ctx.getImageData(stepsA*i, 0, stepsA, c.height);
var imgDataElementB = ctx.getImageData(c.width/2+stepsB*i, 0, stepsB, c.height);
imgDataA.push(imgDataElementA);
imgDataB.push(imgDataElementB);
}
//clearing out the canvas before laying on the new stuff
ctx.fillStyle = "white";
ctx.fillRect(0, 0, c.width, c.height);
//put the images back
for (var i = 0; i < n; i++) {
ctx.putImageData(imgDataA[i], step*i, 0);
ctx.putImageData(imgDataB[i], step*i+stepsA, 0);
}
//gonna count the steps
var count = document.getElementById("count");
count.innerHTML = n;
n += v;
if (n >= 100 || n <= 1) {
v *= -1;
}
}; //closing function scramble
window.onload = function() { //gotta do this bc code executes before image loads
scramble();
};
window.setInterval(scramble, 500);
More or less, this thing works the way I want it to. But there is one problem: Sometimes there are vertical white lines.
My question is:
Why are there white lines? If you view the fiddle, you will see the degree to which this impairs the effect of the shuffle.

You can`t divide a Pixel
The problem can be solve but will introduce some other artifacts as you can not divide integer pixels into fractions.
Quick solution
The following solution for your existing code rounds down for the start of a section and up for the width.
for (var i = 0; i < n; i++) {
var imgDataElementA = ctx.getImageData(
Math.floor(stepsA * i), 0,
Math.ceil(stepsA + stepsA * i) - Math.floor(stepsA * i), c.height
);
var imgDataElementB = ctx.getImageData(
Math.floor(c.width / 2 + stepsB * i), 0,
Math.ceil(c.width / 2 + stepsB * i + stepsB) - Math.floor(c.width / 2 + stepsB * i), c.height);
imgDataA.push(imgDataElementA);
imgDataB.push(imgDataElementB);
}
Quicker options
But doing this via the pixel image data is about the slowest possible way you could find to do it. You can just use the 2D context.imageDraw function to do the movement for you. Or if you want the best in terms of performance a WebGL solution would be the best with the fragment shader doing the scrambling for you as a parallel solution.
There is no perfect solution
But in the end you can not cut a pixel in half, there are a wide range of ways to attempt to solve this but each method has its own artifacts. Ideally you should only slice an image if the rule image.width % slices === 0 in all other cases you will have one or more slices that will not fit on an integer number of pixels.
Example of 4 rounding methods.
The demo shows 4 different methods and with 2 colors. Mouse over to see a closer view. Each method is separated horizontally with a white line. Hold the mouse button to increase the slice counter.
The top is your original.
The next three are 3 different ways of dealing with the fractional pixel width.
const mouse = {x : 0, y : 0, button : false}
function mouseEvents(e){
const m = mouse;
if(m.element){
m.bounds = m.element.getBoundingClientRect();
m.x = e.pageX - m.bounds.left - scrollX;
m.y = e.pageY - m.bounds.top - scrollY;
m.button = e.type === "mousedown" ? true : e.type === "mouseup" ? false : m.button;
}
}
["down","up","move"].forEach(name => document.addEventListener("mouse"+name,mouseEvents));
const counterElement = document.getElementById("count");
// get constants for the demo
const c = document.getElementById("myCanvas");
mouse.element = c;
// The image with the blue and red
const img = document.createElement("canvas");
// the zoom image overlay
const zoom = document.createElement("canvas");
// the scrambled image
const scram = document.createElement("canvas");
// Set sizes and get context
const w = scram.width = zoom.width = img.width = c.width = 500;
const h = scram.height = zoom.height = img.height = c.height;
const dCtx = c.getContext("2d"); // display context
const iCtx = img.getContext("2d"); // source image context
const zCtx = zoom.getContext("2d"); // zoom context
const sCtx = scram.getContext("2d"); // scrambled context
// some constants
const zoomAmount = 4;
const zoomRadius = 60;
const framesToStep = 10;
function createTestPattern(ctx){
ctx.fillStyle = "red";
ctx.fillRect(0, 0, c.width/2, c.height/2);
ctx.fillStyle = "blue";
ctx.fillRect(c.width/2, 0, c.width/2, c.height/2);
ctx.fillStyle = "black";
ctx.fillRect(0, c.height/2, c.width/2, c.height/2);
ctx.fillStyle = "#CCC";
ctx.fillRect(c.width/2, c.height/2, c.width/2, c.height/2);
}
createTestPattern(iCtx);
sCtx.drawImage(iCtx.canvas, 0, 0);
// Shows a zoom area so that blind men like me can see what is going on.
function showMouseZoom(src,dest,zoom = zoomAmount,radius = zoomRadius){
dest.clearRect(0,0,w,h);
dest.imageSmoothingEnabled = false;
if(mouse.x >= 0 && mouse.y >= 0 && mouse.x < w && mouse.y < h){
dest.setTransform(zoom,0,0,zoom,mouse.x,mouse.y)
dest.drawImage(src.canvas, -mouse.x, -mouse.y);
dest.setTransform(1,0,0,1,0,0);
dest.globalCompositeOperation = "destination-in";
dest.beginPath();
dest.arc(mouse.x,mouse.y,radius,0,Math.PI * 2);
dest.fill();
dest.globalCompositeOperation = "source-over";
dest.lineWidth = 4;
dest.strokeStyle = "black";
dest.stroke();
}
}
function scramble(src,dest,y,height) {
const w = src.canvas.width;
const h = src.canvas.height;
const steps = (w/2) / slices;
dest.fillStyle = "white";
dest.fillRect(0, y, w, height);
for (var i = 0; i < slices * 2; i++) {
dest.drawImage(src.canvas,
((i / 2) | 0) * steps + (i % 2) * (w / 2)- 0.5, y,
steps + 1, height,
i * steps - 0.5, y,
steps+ 1, height
);
}
}
function scrambleFloor(src,dest,y,height) {
const w = src.canvas.width;
const h = src.canvas.height;
const steps = (w/2) / slices;
dest.fillStyle = "white";
dest.fillRect(0, y, w, height);
for (var i = 0; i < slices * 2; i++) {
dest.drawImage(src.canvas,
(((i / 2) | 0) * steps + (i % 2) * (w / 2)- 0.5) | 0, y,
steps + 1, height,
(i * steps - 0.5) | 0, y,
steps + 1, height
);
}
}
function scrambleNoOverlap(src,dest,y,height) {
const w = src.canvas.width;
const h = src.canvas.height;
const steps = (w / 2) / slices;
dest.fillStyle = "white";
dest.fillRect(0, y, w, height);
for (var i = 0; i < slices * 2; i++) {
dest.drawImage(src.canvas,
((i / 2) | 0) * steps + (i % 2) * (w / 2), y,
steps, height,
i * steps - 0.5, y,
steps, height
);
}
}
function scrambleOriginal(src,dest,y,height) {
const w = src.canvas.width;
const h = src.canvas.height;
//how big will each shuffled chunk be
var stepsA = (w/2) / slices;
var stepsB = (w/2) / slices;
var step = stepsA + stepsB;
var imgDataA = [];
var imgDataB = [];
for (var i = 0; i < slices; i++) {
var imgDataElementA = src.getImageData(stepsA*i, y, stepsA, height);
var imgDataElementB = src.getImageData(w/2+stepsB*i, y, stepsB, height);
imgDataA.push(imgDataElementA);
imgDataB.push(imgDataElementB);
}
//clearing out the canvas before laying on the new stuff
dest.fillStyle = "white";
dest.fillRect(0, y, w, height);
//put the images back
for (var i = 0; i < slices; i++) {
dest.putImageData(imgDataA[i], step*i, y);
dest.putImageData(imgDataB[i], step*i+stepsA, y);
}
}; //closing function scramble
const scrambleMethods = [scrambleOriginal,scramble,scrambleFloor,scrambleNoOverlap];
var frameCount = 0;
var sliceStep = 1;
var slices = 1;
function mainLoop(){
if(mouse.button){
if(frameCount++ % framesToStep === framesToStep-1){ // every 30 Frames
slices += sliceStep;
if(slices > 150 || slices < 2){ sliceStep = -sliceStep }
counterElement.textContent = slices; // Prevent reflow by using textContent
sCtx.clearRect(0,0,w,h);
sCtx.imageSmoothingEnabled = true;
const len = scrambleMethods.length;
for(var i = 0; i < len; i ++){
scrambleMethods[i](iCtx,sCtx,(128/len) * i, 128/len-2);
scrambleMethods[i](iCtx,sCtx,(128/len) * i + 128, 128/len-2);
}
}
}
dCtx.fillStyle = "white";
dCtx.fillRect(0,0,w,h);
dCtx.drawImage(sCtx.canvas,0,0);
showMouseZoom(dCtx,zCtx);
dCtx.drawImage(zCtx.canvas,0,0);
requestAnimationFrame(mainLoop);
}
//scramble(iCtx,sCtx);
requestAnimationFrame(mainLoop);
canvas {
border: 1px solid black;
}
#count {
position : absolute;
top : 0px;
left : 10px;
font-family: monospace;
font-size: 20px;
}
<canvas id="myCanvas" height = "256" title="Hold mouse button to chance slice count"></canvas>
<p id="count"></p>

Image resizing before uploading by using HTML5 Canvas and javascript [duplicate]

I'm trying to create a thumbnail image on the client side using javascript and a canvas element, but when I shrink the image down, it looks terrible. It looks as if it was downsized in photoshop with the resampling set to 'Nearest Neighbor' instead of Bicubic. I know its possible to get this to look right, because this site can do it just fine using a canvas as well. I've tried using the same code they do as shown in the "[Source]" link, but it still looks terrible. Is there something I'm missing, some setting that needs to be set or something?
EDIT:
I'm trying to resize a jpg. I have tried resizing the same jpg on the linked site and in photoshop, and it looks fine when downsized.
Here is the relevant code:
reader.onloadend = function(e)
{
var img = new Image();
var ctx = canvas.getContext("2d");
var canvasCopy = document.createElement("canvas");
var copyContext = canvasCopy.getContext("2d");
img.onload = function()
{
var ratio = 1;
if(img.width > maxWidth)
ratio = maxWidth / img.width;
else if(img.height > maxHeight)
ratio = maxHeight / img.height;
canvasCopy.width = img.width;
canvasCopy.height = img.height;
copyContext.drawImage(img, 0, 0);
canvas.width = img.width * ratio;
canvas.height = img.height * ratio;
ctx.drawImage(canvasCopy, 0, 0, canvasCopy.width, canvasCopy.height, 0, 0, canvas.width, canvas.height);
};
img.src = reader.result;
}
EDIT2:
Seems I was mistaken, the linked website wasn't doing any better of a job of downsizing the image. I tried the other methods suggested and none of them look any better. This is what the different methods resulted in:
Photoshop:
Canvas:
Image with image-rendering: optimizeQuality set and scaled with width/height:
Image with image-rendering: optimizeQuality set and scaled with -moz-transform:
Canvas resize on pixastic:
I guess this means firefox isn't using bicubic sampling like its supposed to. I'll just have to wait until they actually add it.
EDIT3:
Original Image

So what do you do if all the browsers (actually, Chrome 5 gave me quite good one) won't give you good enough resampling quality? You implement them yourself then! Oh come on, we're entering the new age of Web 3.0, HTML5 compliant browsers, super optimized JIT javascript compilers, multi-core(†) machines, with tons of memory, what are you afraid of? Hey, there's the word java in javascript, so that should guarantee the performance, right? Behold, the thumbnail generating code:
// returns a function that calculates lanczos weight
function lanczosCreate(lobes) {
return function(x) {
if (x > lobes)
return 0;
x *= Math.PI;
if (Math.abs(x) < 1e-16)
return 1;
var xx = x / lobes;
return Math.sin(x) * Math.sin(xx) / x / xx;
};
}
// elem: canvas element, img: image element, sx: scaled width, lobes: kernel radius
function thumbnailer(elem, img, sx, lobes) {
this.canvas = elem;
elem.width = img.width;
elem.height = img.height;
elem.style.display = "none";
this.ctx = elem.getContext("2d");
this.ctx.drawImage(img, 0, 0);
this.img = img;
this.src = this.ctx.getImageData(0, 0, img.width, img.height);
this.dest = {
width : sx,
height : Math.round(img.height * sx / img.width),
};
this.dest.data = new Array(this.dest.width * this.dest.height * 3);
this.lanczos = lanczosCreate(lobes);
this.ratio = img.width / sx;
this.rcp_ratio = 2 / this.ratio;
this.range2 = Math.ceil(this.ratio * lobes / 2);
this.cacheLanc = {};
this.center = {};
this.icenter = {};
setTimeout(this.process1, 0, this, 0);
}
thumbnailer.prototype.process1 = function(self, u) {
self.center.x = (u + 0.5) * self.ratio;
self.icenter.x = Math.floor(self.center.x);
for (var v = 0; v < self.dest.height; v++) {
self.center.y = (v + 0.5) * self.ratio;
self.icenter.y = Math.floor(self.center.y);
var a, r, g, b;
a = r = g = b = 0;
for (var i = self.icenter.x - self.range2; i <= self.icenter.x + self.range2; i++) {
if (i < 0 || i >= self.src.width)
continue;
var f_x = Math.floor(1000 * Math.abs(i - self.center.x));
if (!self.cacheLanc[f_x])
self.cacheLanc[f_x] = {};
for (var j = self.icenter.y - self.range2; j <= self.icenter.y + self.range2; j++) {
if (j < 0 || j >= self.src.height)
continue;
var f_y = Math.floor(1000 * Math.abs(j - self.center.y));
if (self.cacheLanc[f_x][f_y] == undefined)
self.cacheLanc[f_x][f_y] = self.lanczos(Math.sqrt(Math.pow(f_x * self.rcp_ratio, 2)
+ Math.pow(f_y * self.rcp_ratio, 2)) / 1000);
weight = self.cacheLanc[f_x][f_y];
if (weight > 0) {
var idx = (j * self.src.width + i) * 4;
a += weight;
r += weight * self.src.data[idx];
g += weight * self.src.data[idx + 1];
b += weight * self.src.data[idx + 2];
}
}
}
var idx = (v * self.dest.width + u) * 3;
self.dest.data[idx] = r / a;
self.dest.data[idx + 1] = g / a;
self.dest.data[idx + 2] = b / a;
}
if (++u < self.dest.width)
setTimeout(self.process1, 0, self, u);
else
setTimeout(self.process2, 0, self);
};
thumbnailer.prototype.process2 = function(self) {
self.canvas.width = self.dest.width;
self.canvas.height = self.dest.height;
self.ctx.drawImage(self.img, 0, 0, self.dest.width, self.dest.height);
self.src = self.ctx.getImageData(0, 0, self.dest.width, self.dest.height);
var idx, idx2;
for (var i = 0; i < self.dest.width; i++) {
for (var j = 0; j < self.dest.height; j++) {
idx = (j * self.dest.width + i) * 3;
idx2 = (j * self.dest.width + i) * 4;
self.src.data[idx2] = self.dest.data[idx];
self.src.data[idx2 + 1] = self.dest.data[idx + 1];
self.src.data[idx2 + 2] = self.dest.data[idx + 2];
}
}
self.ctx.putImageData(self.src, 0, 0);
self.canvas.style.display = "block";
};
...with which you can produce results like these!
so anyway, here is a 'fixed' version of your example:
img.onload = function() {
var canvas = document.createElement("canvas");
new thumbnailer(canvas, img, 188, 3); //this produces lanczos3
// but feel free to raise it up to 8. Your client will appreciate
// that the program makes full use of his machine.
document.body.appendChild(canvas);
};
Now it's time to pit your best browsers out there and see which one will least likely increase your client's blood pressure!
Umm, where's my sarcasm tag?
(since many parts of the code is based on Anrieff Gallery Generator is it also covered under GPL2? I don't know)
† actually due to limitation of javascript, multi-core is not supported.

Fast image resize/resample algorithm using Hermite filter with JavaScript. Support transparency, gives good quality. Preview:
Update: version 2.0 added on GitHub (faster, web workers + transferable objects). Finally i got it working!
Git: https://github.com/viliusle/Hermite-resize
Demo: http://viliusle.github.io/miniPaint/
/**
* Hermite resize - fast image resize/resample using Hermite filter. 1 cpu version!
*
* #param {HtmlElement} canvas
* #param {int} width
* #param {int} height
* #param {boolean} resize_canvas if true, canvas will be resized. Optional.
*/
function resample_single(canvas, width, height, resize_canvas) {
var width_source = canvas.width;
var height_source = canvas.height;
width = Math.round(width);
height = Math.round(height);
var ratio_w = width_source / width;
var ratio_h = height_source / height;
var ratio_w_half = Math.ceil(ratio_w / 2);
var ratio_h_half = Math.ceil(ratio_h / 2);
var ctx = canvas.getContext("2d");
var img = ctx.getImageData(0, 0, width_source, height_source);
var img2 = ctx.createImageData(width, height);
var data = img.data;
var data2 = img2.data;
for (var j = 0; j < height; j++) {
for (var i = 0; i < width; i++) {
var x2 = (i + j * width) * 4;
var weight = 0;
var weights = 0;
var weights_alpha = 0;
var gx_r = 0;
var gx_g = 0;
var gx_b = 0;
var gx_a = 0;
var center_y = (j + 0.5) * ratio_h;
var yy_start = Math.floor(j * ratio_h);
var yy_stop = Math.ceil((j + 1) * ratio_h);
for (var yy = yy_start; yy < yy_stop; yy++) {
var dy = Math.abs(center_y - (yy + 0.5)) / ratio_h_half;
var center_x = (i + 0.5) * ratio_w;
var w0 = dy * dy; //pre-calc part of w
var xx_start = Math.floor(i * ratio_w);
var xx_stop = Math.ceil((i + 1) * ratio_w);
for (var xx = xx_start; xx < xx_stop; xx++) {
var dx = Math.abs(center_x - (xx + 0.5)) / ratio_w_half;
var w = Math.sqrt(w0 + dx * dx);
if (w >= 1) {
//pixel too far
continue;
}
//hermite filter
weight = 2 * w * w * w - 3 * w * w + 1;
var pos_x = 4 * (xx + yy * width_source);
//alpha
gx_a += weight * data[pos_x + 3];
weights_alpha += weight;
//colors
if (data[pos_x + 3] < 255)
weight = weight * data[pos_x + 3] / 250;
gx_r += weight * data[pos_x];
gx_g += weight * data[pos_x + 1];
gx_b += weight * data[pos_x + 2];
weights += weight;
}
}
data2[x2] = gx_r / weights;
data2[x2 + 1] = gx_g / weights;
data2[x2 + 2] = gx_b / weights;
data2[x2 + 3] = gx_a / weights_alpha;
}
}
//clear and resize canvas
if (resize_canvas === true) {
canvas.width = width;
canvas.height = height;
} else {
ctx.clearRect(0, 0, width_source, height_source);
}
//draw
ctx.putImageData(img2, 0, 0);
}

Try pica - that's a highly optimized resizer with selectable algorythms. See demo.
For example, original image from first post is resized in 120ms with Lanczos filter and 3px window or 60ms with Box filter and 0.5px window. For huge 17mb image 5000x3000px resize takes ~1s on desktop and 3s on mobile.
All resize principles were described very well in this thread, and pica does not add rocket science. But it's optimized very well for modern JIT-s, and is ready to use out of box (via npm or bower). Also, it use webworkers when available to avoid interface freezes.
I also plan to add unsharp mask support soon, because it's very useful after downscale.

I know this is an old thread but it might be useful for some people such as myself that months after are hitting this issue for the first time.
Here is some code that resizes the image every time you reload the image. I am aware this is not optimal at all, but I provide it as a proof of concept.
Also, sorry for using jQuery for simple selectors but I just feel too comfortable with the syntax.
$(document).on('ready', createImage);
$(window).on('resize', createImage);
var createImage = function(){
var canvas = document.getElementById('myCanvas');
canvas.width = window.innerWidth || $(window).width();
canvas.height = window.innerHeight || $(window).height();
var ctx = canvas.getContext('2d');
img = new Image();
img.addEventListener('load', function () {
ctx.drawImage(this, 0, 0, w, h);
});
img.src = 'http://www.ruinvalor.com/Telanor/images/original.jpg';
};
html, body{
height: 100%;
width: 100%;
margin: 0;
padding: 0;
background: #000;
}
canvas{
position: absolute;
left: 0;
top: 0;
z-index: 0;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<html>
<head>
<meta charset="utf-8" />
<title>Canvas Resize</title>
</head>
<body>
<canvas id="myCanvas"></canvas>
</body>
</html>
My createImage function is called once when the document is loaded and after that it is called every time the window receives a resize event.
I tested it in Chrome 6 and Firefox 3.6, both on the Mac. This "technique" eats processor as it if was ice cream in the summer, but it does the trick.

I've put up some algorithms to do image interpolation on html canvas pixel arrays that might be useful here:
https://web.archive.org/web/20170104190425/http://jsperf.com:80/pixel-interpolation/2
These can be copy/pasted and can be used inside of web workers to resize images (or any other operation that requires interpolation - I'm using them to defish images at the moment).
I haven't added the lanczos stuff above, so feel free to add that as a comparison if you'd like.

This is a javascript function adapted from #Telanor's code. When passing a image base64 as first argument to the function, it returns the base64 of the resized image. maxWidth and maxHeight are optional.
function thumbnail(base64, maxWidth, maxHeight) {
// Max size for thumbnail
if(typeof(maxWidth) === 'undefined') var maxWidth = 500;
if(typeof(maxHeight) === 'undefined') var maxHeight = 500;
// Create and initialize two canvas
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");
var canvasCopy = document.createElement("canvas");
var copyContext = canvasCopy.getContext("2d");
// Create original image
var img = new Image();
img.src = base64;
// Determine new ratio based on max size
var ratio = 1;
if(img.width > maxWidth)
ratio = maxWidth / img.width;
else if(img.height > maxHeight)
ratio = maxHeight / img.height;
// Draw original image in second canvas
canvasCopy.width = img.width;
canvasCopy.height = img.height;
copyContext.drawImage(img, 0, 0);
// Copy and resize second canvas to first canvas
canvas.width = img.width * ratio;
canvas.height = img.height * ratio;
ctx.drawImage(canvasCopy, 0, 0, canvasCopy.width, canvasCopy.height, 0, 0, canvas.width, canvas.height);
return canvas.toDataURL();
}

I'd highly suggest you check out this link and make sure it is set to true.
Controlling image scaling behavior
Introduced in Gecko 1.9.2 (Firefox 3.6
/ Thunderbird 3.1 / Fennec 1.0)
Gecko 1.9.2 introduced the
mozImageSmoothingEnabled property to
the canvas element; if this Boolean
value is false, images won't be
smoothed when scaled. This property is
true by default. view plainprint?
cx.mozImageSmoothingEnabled = false;

If you're simply trying to resize an image, I'd recommend setting width and height of the image with CSS. Here's a quick example:
.small-image {
width: 100px;
height: 100px;
}
Note that the height and width can also be set using JavaScript. Here's quick code sample:
var img = document.getElement("my-image");
img.style.width = 100 + "px"; // Make sure you add the "px" to the end,
img.style.height = 100 + "px"; // otherwise you'll confuse IE
Also, to ensure that the resized image looks good, add the following css rules to image selector:
-ms-interpolation-mode: bicubic: introduce in IE7
image-rendering: optimizeQuality: introduced in FireFox 3.6
As far as I can tell, all browsers except IE using an bicubic algorithm to resize images by default, so your resized images should look good in Firefox and Chrome.
If setting the css width and height doesn't work, you may want to play with a css transform:
-moz-transform: scale(sx[, sy])
-webkit-transform:scale(sx[, sy])
If for whatever reason you need to use a canvas, please note that there are two ways an image can be resize: by resizing the canvas with css or by drawing the image at a smaller size.
See this question for more details.

i got this image by right clicking the canvas element in firefox and saving as.
var img = new Image();
img.onload = function () {
console.debug(this.width,this.height);
var canvas = document.createElement('canvas'), ctx;
canvas.width = 188;
canvas.height = 150;
document.body.appendChild(canvas);
ctx = canvas.getContext('2d');
ctx.drawImage(img,0,0,188,150);
};
img.src = 'original.jpg';
so anyway, here is a 'fixed' version of your example:
var img = new Image();
// added cause it wasnt defined
var canvas = document.createElement("canvas");
document.body.appendChild(canvas);
var ctx = canvas.getContext("2d");
var canvasCopy = document.createElement("canvas");
// adding it to the body
document.body.appendChild(canvasCopy);
var copyContext = canvasCopy.getContext("2d");
img.onload = function()
{
var ratio = 1;
// defining cause it wasnt
var maxWidth = 188,
maxHeight = 150;
if(img.width > maxWidth)
ratio = maxWidth / img.width;
else if(img.height > maxHeight)
ratio = maxHeight / img.height;
canvasCopy.width = img.width;
canvasCopy.height = img.height;
copyContext.drawImage(img, 0, 0);
canvas.width = img.width * ratio;
canvas.height = img.height * ratio;
// the line to change
// ctx.drawImage(canvasCopy, 0, 0, canvasCopy.width, canvasCopy.height, 0, 0, canvas.width, canvas.height);
// the method signature you are using is for slicing
ctx.drawImage(canvasCopy, 0, 0, canvas.width, canvas.height);
};
// changed for example
img.src = 'original.jpg';

For resizing to image with width less that original, i use:
function resize2(i) {
var cc = document.createElement("canvas");
cc.width = i.width / 2;
cc.height = i.height / 2;
var ctx = cc.getContext("2d");
ctx.drawImage(i, 0, 0, cc.width, cc.height);
return cc;
}
var cc = img;
while (cc.width > 64 * 2) {
cc = resize2(cc);
}
// .. than drawImage(cc, .... )
and it works =).

I have a feeling the module I wrote will produce similar results to photoshop, as it preserves color data by averaging them, not applying an algorithm. It's kind of slow, but to me it is the best, because it preserves all the color data.
https://github.com/danschumann/limby-resize/blob/master/lib/canvas_resize.js
It doesn't take the nearest neighbor and drop other pixels, or sample a group and take a random average. It takes the exact proportion each source pixel should output into the destination pixel. The average pixel color in the source will be the average pixel color in the destination, which these other formulas, I think they will not be.
an example of how to use is at the bottom of
https://github.com/danschumann/limby-resize
UPDATE OCT 2018: These days my example is more academic than anything else. Webgl is pretty much 100%, so you'd be better off resizing with that to produce similar results, but faster. PICA.js does this, I believe. –

The problem with some of this solutions is that they access directly the pixel data and loop through it to perform the downsampling. Depending on the size of the image this can be very resource intensive, and it would be better to use the browser's internal algorithms.
The drawImage() function is using a linear-interpolation, nearest-neighbor resampling method. That works well when you are not resizing down more than half the original size.
If you loop to only resize max one half at a time, the results would be quite good, and much faster than accessing pixel data.
This function downsample to half at a time until reaching the desired size:
function resize_image( src, dst, type, quality ) {
var tmp = new Image(),
canvas, context, cW, cH;
type = type || 'image/jpeg';
quality = quality || 0.92;
cW = src.naturalWidth;
cH = src.naturalHeight;
tmp.src = src.src;
tmp.onload = function() {
canvas = document.createElement( 'canvas' );
cW /= 2;
cH /= 2;
if ( cW < src.width ) cW = src.width;
if ( cH < src.height ) cH = src.height;
canvas.width = cW;
canvas.height = cH;
context = canvas.getContext( '2d' );
context.drawImage( tmp, 0, 0, cW, cH );
dst.src = canvas.toDataURL( type, quality );
if ( cW <= src.width || cH <= src.height )
return;
tmp.src = dst.src;
}
}
// The images sent as parameters can be in the DOM or be image objects
resize_image( $( '#original' )[0], $( '#smaller' )[0] );
Credits to this post

So something interesting that I found a while ago while working with canvas that might be helpful:
To resize the canvas control on its own, you need to use the height="" and width="" attributes (or canvas.width/canvas.height elements). If you use CSS to resize the canvas, it will actually stretch (i.e.: resize) the content of the canvas to fit the full canvas (rather than simply increasing or decreasing the area of the canvas.
It'd be worth a shot to try drawing the image into a canvas control with the height and width attributes set to the size of the image and then using CSS to resize the canvas to the size you're looking for. Perhaps this would use a different resizing algorithm.
It should also be noted that canvas has different effects in different browsers (and even different versions of different browsers). The algorithms and techniques used in the browsers is likely to change over time (especially with Firefox 4 and Chrome 6 coming out so soon, which will place heavy emphasis on canvas rendering performance).
In addition, you may want to give SVG a shot, too, as it likely uses a different algorithm as well.
Best of luck!

Fast and simple Javascript image resizer:
https://github.com/calvintwr/blitz-hermite-resize
const blitz = Blitz.create()
/* Promise */
blitz({
source: DOM Image/DOM Canvas/jQuery/DataURL/File,
width: 400,
height: 600
}).then(output => {
// handle output
})catch(error => {
// handle error
})
/* Await */
let resized = await blizt({...})
/* Old school callback */
const blitz = Blitz.create('callback')
blitz({...}, function(output) {
// run your callback.
})
History
This is really after many rounds of research, reading and trying.
The resizer algorithm uses #ViliusL's Hermite script (Hermite resizer is really the fastest and gives reasonably good output). Extended with features you need.
Forks 1 worker to do the resizing so that it doesn't freeze your browser when resizing, unlike all other JS resizers out there.

I converted #syockit's answer as well as the step-down approach into a reusable Angular service for anyone who's interested: https://gist.github.com/fisch0920/37bac5e741eaec60e983
I included both solutions because they both have their own pros / cons. The lanczos convolution approach is higher quality at the cost of being slower, whereas the step-wise downscaling approach produces reasonably antialiased results and is significantly faster.
Example usage:
angular.module('demo').controller('ExampleCtrl', function (imageService) {
// EXAMPLE USAGE
// NOTE: it's bad practice to access the DOM inside a controller,
// but this is just to show the example usage.
// resize by lanczos-sinc filter
imageService.resize($('#myimg')[0], 256, 256)
.then(function (resizedImage) {
// do something with resized image
})
// resize by stepping down image size in increments of 2x
imageService.resizeStep($('#myimg')[0], 256, 256)
.then(function (resizedImage) {
// do something with resized image
})
})

Thanks #syockit for an awesome answer. however, I had to reformat a little as follows to make it work. Perhaps due to DOM scanning issues:
$(document).ready(function () {
$('img').on("load", clickA);
function clickA() {
var img = this;
var canvas = document.createElement("canvas");
new thumbnailer(canvas, img, 50, 3);
document.body.appendChild(canvas);
}
function thumbnailer(elem, img, sx, lobes) {
this.canvas = elem;
elem.width = img.width;
elem.height = img.height;
elem.style.display = "none";
this.ctx = elem.getContext("2d");
this.ctx.drawImage(img, 0, 0);
this.img = img;
this.src = this.ctx.getImageData(0, 0, img.width, img.height);
this.dest = {
width: sx,
height: Math.round(img.height * sx / img.width)
};
this.dest.data = new Array(this.dest.width * this.dest.height * 3);
this.lanczos = lanczosCreate(lobes);
this.ratio = img.width / sx;
this.rcp_ratio = 2 / this.ratio;
this.range2 = Math.ceil(this.ratio * lobes / 2);
this.cacheLanc = {};
this.center = {};
this.icenter = {};
setTimeout(process1, 0, this, 0);
}
//returns a function that calculates lanczos weight
function lanczosCreate(lobes) {
return function (x) {
if (x > lobes)
return 0;
x *= Math.PI;
if (Math.abs(x) < 1e-16)
return 1
var xx = x / lobes;
return Math.sin(x) * Math.sin(xx) / x / xx;
}
}
process1 = function (self, u) {
self.center.x = (u + 0.5) * self.ratio;
self.icenter.x = Math.floor(self.center.x);
for (var v = 0; v < self.dest.height; v++) {
self.center.y = (v + 0.5) * self.ratio;
self.icenter.y = Math.floor(self.center.y);
var a, r, g, b;
a = r = g = b = 0;
for (var i = self.icenter.x - self.range2; i <= self.icenter.x + self.range2; i++) {
if (i < 0 || i >= self.src.width)
continue;
var f_x = Math.floor(1000 * Math.abs(i - self.center.x));
if (!self.cacheLanc[f_x])
self.cacheLanc[f_x] = {};
for (var j = self.icenter.y - self.range2; j <= self.icenter.y + self.range2; j++) {
if (j < 0 || j >= self.src.height)
continue;
var f_y = Math.floor(1000 * Math.abs(j - self.center.y));
if (self.cacheLanc[f_x][f_y] == undefined)
self.cacheLanc[f_x][f_y] = self.lanczos(Math.sqrt(Math.pow(f_x * self.rcp_ratio, 2) + Math.pow(f_y * self.rcp_ratio, 2)) / 1000);
weight = self.cacheLanc[f_x][f_y];
if (weight > 0) {
var idx = (j * self.src.width + i) * 4;
a += weight;
r += weight * self.src.data[idx];
g += weight * self.src.data[idx + 1];
b += weight * self.src.data[idx + 2];
}
}
}
var idx = (v * self.dest.width + u) * 3;
self.dest.data[idx] = r / a;
self.dest.data[idx + 1] = g / a;
self.dest.data[idx + 2] = b / a;
}
if (++u < self.dest.width)
setTimeout(process1, 0, self, u);
else
setTimeout(process2, 0, self);
};
process2 = function (self) {
self.canvas.width = self.dest.width;
self.canvas.height = self.dest.height;
self.ctx.drawImage(self.img, 0, 0);
self.src = self.ctx.getImageData(0, 0, self.dest.width, self.dest.height);
var idx, idx2;
for (var i = 0; i < self.dest.width; i++) {
for (var j = 0; j < self.dest.height; j++) {
idx = (j * self.dest.width + i) * 3;
idx2 = (j * self.dest.width + i) * 4;
self.src.data[idx2] = self.dest.data[idx];
self.src.data[idx2 + 1] = self.dest.data[idx + 1];
self.src.data[idx2 + 2] = self.dest.data[idx + 2];
}
}
self.ctx.putImageData(self.src, 0, 0);
self.canvas.style.display = "block";
}
});

I wanted some well defined functions out of answers here so ended up with these which am hoping would be useful for others also,
function getImageFromLink(link) {
return new Promise(function (resolve) {
var image = new Image();
image.onload = function () { resolve(image); };
image.src = link;
});
}
function resizeImageToBlob(image, width, height, mime) {
return new Promise(function (resolve) {
var canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
canvas.getContext('2d').drawImage(image, 0, 0, width, height);
return canvas.toBlob(resolve, mime);
});
}
getImageFromLink(location.href).then(function (image) {
// calculate these based on the original size
var width = image.width / 4;
var height = image.height / 4;
return resizeImageToBlob(image, width, height, 'image/jpeg');
}).then(function (blob) {
// Do something with the result Blob object
document.querySelector('img').src = URL.createObjectURL(blob);
});
Just for the sake of testing this run it on a image opened in a tab.

I just ran a page of side by sides comparisons and unless something has changed recently, I could see no better downsizing (scaling) using canvas vs. simple css. I tested in FF6 Mac OSX 10.7. Still slightly soft vs. the original.
I did however stumble upon something that did make a huge difference and that was using image filters in browsers that support canvas. You can actually manipulate images much like you can in Photoshop with blur, sharpen, saturation, ripple, grayscale, etc.
I then found an awesome jQuery plug-in which makes application of these filters a snap:
http://codecanyon.net/item/jsmanipulate-jquery-image-manipulation-plugin/428234
I simply apply the sharpen filter right after resizing the image which should give you the desired effect. I didn't even have to use a canvas element.

Looking for another great simple solution?
var img=document.createElement('img');
img.src=canvas.toDataURL();
$(img).css("background", backgroundColor);
$(img).width(settings.width);
$(img).height(settings.height);
This solution will use the resize algorith of browser! :)

Automatically Crop HTML5 canvas to contents

Let's say this is my canvas, with an evil-looking face drawn on it. I want to use toDataURL() to export my evil face as a PNG; however, the whole canvas is rasterised, including the 'whitespace' between the evil face and canvas edges.
+---------------+
| |
| |
| (.Y. ) |
| /_ |
| \____/ |
| |
| |
+---------------+
What is the best way to crop/trim/shrinkwrap my canvas to its contents, so my PNG is no larger than the face's 'bounding-box', like below? The best way seems to be scaling the canvas, but supposing the contents are dynamic...? I'm sure there should be a simple solution to this, but it's escaping me, with much Googling.
+------+
|(.Y. )|
| /_ |
|\____/|
+------+
Thanks!

Edited (see comments)
function cropImageFromCanvas(ctx) {
var canvas = ctx.canvas,
w = canvas.width, h = canvas.height,
pix = {x:[], y:[]},
imageData = ctx.getImageData(0,0,canvas.width,canvas.height),
x, y, index;
for (y = 0; y < h; y++) {
for (x = 0; x < w; x++) {
index = (y * w + x) * 4;
if (imageData.data[index+3] > 0) {
pix.x.push(x);
pix.y.push(y);
}
}
}
pix.x.sort(function(a,b){return a-b});
pix.y.sort(function(a,b){return a-b});
var n = pix.x.length-1;
w = 1 + pix.x[n] - pix.x[0];
h = 1 + pix.y[n] - pix.y[0];
var cut = ctx.getImageData(pix.x[0], pix.y[0], w, h);
canvas.width = w;
canvas.height = h;
ctx.putImageData(cut, 0, 0);
var image = canvas.toDataURL();
}

If I understood well you want to "trim" away all the surronding your image / drawing, and adjust the canvas to that size (like if you do a "trim" command in Photoshop).
Here is how I'll do it.
Run thru all the canvas pixels checking if their alpha component is > 0 (that means that something is drawn in that pixel). Alternativelly you could check for the r,g,b values, if your canvas background is fullfilled with a solid color, for instance.
Get te coordinates of the top most left pixel non-empty, and same for the bottom most right one. So you'll get the coordinates of an imaginay "rectangle" containing the canvas area that is not empty.
Store that region of pixeldata.
Resize your canvas to its new dimensions (the ones of the region we got at step 2.)
Paste the saved region back to the canvas.
Et, voilá :)
Accesing pixeldata is quite slow depending on the size of your canvas (if its huge it can take a while). There are some optimizations around to work with raw canvas pixeldata (I think there is an article about this topic at MDN), I suggest you to google about it.
I prepared a small sketch in jsFiddle that you can use as starting point for your code.
Working sample at jsFiddle
Hope I've helped you.
c:.

Here's my take. I felt like all the other solutions were overly complicated. Though, after creating it, I now see it's the same solution as one other's, expect they just shared a fiddle and not a function.
function trimCanvas(canvas){
const context = canvas.getContext('2d');
const topLeft = {
x: canvas.width,
y: canvas.height,
update(x,y){
this.x = Math.min(this.x,x);
this.y = Math.min(this.y,y);
}
};
const bottomRight = {
x: 0,
y: 0,
update(x,y){
this.x = Math.max(this.x,x);
this.y = Math.max(this.y,y);
}
};
const imageData = context.getImageData(0,0,canvas.width,canvas.height);
for(let x = 0; x < canvas.width; x++){
for(let y = 0; y < canvas.height; y++){
const alpha = imageData.data[((y * (canvas.width * 4)) + (x * 4)) + 3];
if(alpha !== 0){
topLeft.update(x,y);
bottomRight.update(x,y);
}
}
}
const width = bottomRight.x - topLeft.x;
const height = bottomRight.y - topLeft.y;
const croppedCanvas = context.getImageData(topLeft.x,topLeft.y,width,height);
canvas.width = width;
canvas.height = height;
context.putImageData(croppedCanvas,0,0);
return canvas;
}

Here's code in ES syntax, short, fast and concise:
/**
* Trim a canvas.
*
* #author Arjan Haverkamp (arjan at avoid dot org)
* #param {canvas} canvas A canvas element to trim. This element will be trimmed (reference)
* #param {int} threshold Alpha threshold. Allows for trimming semi-opaque pixels too. Range: 0 - 255
* #returns {Object} Width and height of trimmed canvcas and left-top coordinate of trimmed area. Example: {width:400, height:300, x:65, y:104}
*/
const trimCanvas = (canvas, threshold = 0) => {
const ctx = canvas.getContext('2d'),
w = canvas.width, h = canvas.height,
imageData = ctx.getImageData(0, 0, w, h),
tlCorner = { x:w+1, y:h+1 },
brCorner = { x:-1, y:-1 };
for (let y = 0; y < h; y++) {
for (let x = 0; x < w; x++) {
if (imageData.data[((y * w + x) * 4) + 3] > threshold) {
tlCorner.x = Math.min(x, tlCorner.x);
tlCorner.y = Math.min(y, tlCorner.y);
brCorner.x = Math.max(x, brCorner.x);
brCorner.y = Math.max(y, brCorner.y);
}
}
}
const cut = ctx.getImageData(tlCorner.x, tlCorner.y, brCorner.x - tlCorner.x, brCorner.y - tlCorner.y);
canvas.width = brCorner.x - tlCorner.x;
canvas.height = brCorner.y - tlCorner.y;
ctx.putImageData(cut, 0, 0);
return {width:canvas.width, height:canvas.height, x:tlCorner.x, y:tlCorner.y};
}

The top voted answer here, as well as the implementations i found online trim one extra pixel which was very apparent when trying to trim text out of canvas. I wrote my own that worked better for me:
var img = new Image;
img.onload = () => {
var canvas = document.getElementById('canvas');
canvas.width = img.width;
canvas.height = img.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0);
document.getElementById('button').addEventListener('click', ()=>{
autoCropCanvas(canvas, ctx);
document.getElementById('button').remove();
});
};
img.src = 'data:image/png;base64,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';
function autoCropCanvas(canvas, ctx) {
var bounds = {
left: 0,
right: canvas.width,
top: 0,
bottom: canvas.height
};
var rows = [];
var cols = [];
var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
for (var x = 0; x < canvas.width; x++) {
cols[x] = cols[x] || false;
for (var y = 0; y < canvas.height; y++) {
rows[y] = rows[y] || false;
const p = y * (canvas.width * 4) + x * 4;
const [r, g, b, a] = [imageData.data[p], imageData.data[p + 1], imageData.data[p + 2], imageData.data[p + 3]];
var isEmptyPixel = Math.max(r, g, b, a) === 0;
if (!isEmptyPixel) {
cols[x] = true;
rows[y] = true;
}
}
}
for (var i = 0; i < rows.length; i++) {
if (rows[i]) {
bounds.top = i ? i - 1 : i;
break;
}
}
for (var i = rows.length; i--; ) {
if (rows[i]) {
bounds.bottom = i < canvas.height ? i + 1 : i;
break;
}
}
for (var i = 0; i < cols.length; i++) {
if (cols[i]) {
bounds.left = i ? i - 1 : i;
break;
}
}
for (var i = cols.length; i--; ) {
if (cols[i]) {
bounds.right = i < canvas.width ? i + 1 : i;
break;
}
}
var newWidth = bounds.right - bounds.left;
var newHeight = bounds.bottom - bounds.top;
var cut = ctx.getImageData(bounds.left, bounds.top, newWidth, newHeight);
canvas.width = newWidth;
canvas.height = newHeight;
ctx.putImageData(cut, 0, 0);
}
<canvas id=canvas style='border: 1px solid pink'></canvas>
<button id=button>crop canvas</button>