Resize image with javascript canvas (smoothly)
I'm trying to resize some images with canvas but I'm clueless on how to smoothen them. On photoshop, browsers etc.. there are a few algorithms they use (e.g. bicubic, bilinear) but I don't know if these are built into canvas or not.
Here's my fiddle: http://jsfiddle.net/EWupT/
var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
canvas.width=300
canvas.height=234
ctx.drawImage(img, 0, 0, 300, 234);
document.body.appendChild(canvas);
The first one is a normal resized image tag, and the second one is canvas. Notice how the canvas one is not as smooth. How can I achieve 'smoothness'?
You can use down-stepping to achieve better results. Most browsers seem to use linear interpolation rather than bi-cubic when resizing images.
(Update There has been added a quality property to the specs, imageSmoothingQuality which is currently available in Chrome only.)
Unless one chooses no smoothing or nearest neighbor the browser will always interpolate the image after down-scaling it as this function as a low-pass filter to avoid aliasing.
Bi-linear uses 2x2 pixels to do the interpolation while bi-cubic uses 4x4 so by doing it in steps you can get close to bi-cubic result while using bi-linear interpolation as seen in the resulting images.
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var img = new Image();
img.onload = function () {
// set size proportional to image
canvas.height = canvas.width * (img.height / img.width);
// step 1 - resize to 50%
var oc = document.createElement('canvas'),
octx = oc.getContext('2d');
oc.width = img.width * 0.5;
oc.height = img.height * 0.5;
octx.drawImage(img, 0, 0, oc.width, oc.height);
// step 2
octx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5);
// step 3, resize to final size
ctx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5,
0, 0, canvas.width, canvas.height);
}
img.src = "//i.imgur.com/SHo6Fub.jpg";<img src="//i.imgur.com/SHo6Fub.jpg" width="300" height="234">
<canvas id="canvas" width=300></canvas>Depending on how drastic your resize is you can might skip step 2 if the difference is less.
In the demo you can see the new result is now much similar to the image element.
Since Trung Le Nguyen Nhat's fiddle isn't correct at all
(it just uses the original image in the last step)
I wrote my own general fiddle with performance comparison:
FIDDLE
Basically it's:
img.onload = function() {
var canvas = document.createElement('canvas'),
ctx = canvas.getContext("2d"),
oc = document.createElement('canvas'),
octx = oc.getContext('2d');
canvas.width = width; // destination canvas size
canvas.height = canvas.width * img.height / img.width;
var cur = {
width: Math.floor(img.width * 0.5),
height: Math.floor(img.height * 0.5)
}
oc.width = cur.width;
oc.height = cur.height;
octx.drawImage(img, 0, 0, cur.width, cur.height);
while (cur.width * 0.5 > width) {
cur = {
width: Math.floor(cur.width * 0.5),
height: Math.floor(cur.height * 0.5)
};
octx.drawImage(oc, 0, 0, cur.width * 2, cur.height * 2, 0, 0, cur.width, cur.height);
}
ctx.drawImage(oc, 0, 0, cur.width, cur.height, 0, 0, canvas.width, canvas.height);
}
I created a reusable Angular service to handle high quality resizing of images / canvases for anyone who's interested: https://gist.github.com/transitive-bullshit/37bac5e741eaec60e983
The service includes two 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
})
})
While some of those code-snippets are short and working, they aren't trivial to follow and understand.
As i am not a fan of "copy-paste" from stack-overflow, i would like developers to understand the code they are push into they software, hope you'll find the below useful.
DEMO: Resizing images with JS and HTML Canvas Demo fiddler.
You may find 3 different methods to do this resize, that will help you understand how the code is working and why.
https://jsfiddle.net/1b68eLdr/93089/
Full code of both demo, and TypeScript method that you may want to use in your code, can be found in the GitHub project.
https://github.com/eyalc4/ts-image-resizer
This is the final code:
export class ImageTools {
base64ResizedImage: string = null;
constructor() {
}
ResizeImage(base64image: string, width: number = 1080, height: number = 1080) {
let img = new Image();
img.src = base64image;
img.onload = () => {
// Check if the image require resize at all
if(img.height <= height && img.width <= width) {
this.base64ResizedImage = base64image;
// TODO: Call method to do something with the resize image
}
else {
// Make sure the width and height preserve the original aspect ratio and adjust if needed
if(img.height > img.width) {
width = Math.floor(height * (img.width / img.height));
}
else {
height = Math.floor(width * (img.height / img.width));
}
let resizingCanvas: HTMLCanvasElement = document.createElement('canvas');
let resizingCanvasContext = resizingCanvas.getContext("2d");
// Start with original image size
resizingCanvas.width = img.width;
resizingCanvas.height = img.height;
// Draw the original image on the (temp) resizing canvas
resizingCanvasContext.drawImage(img, 0, 0, resizingCanvas.width, resizingCanvas.height);
let curImageDimensions = {
width: Math.floor(img.width),
height: Math.floor(img.height)
};
let halfImageDimensions = {
width: null,
height: null
};
// Quickly reduce the dize by 50% each time in few iterations until the size is less then
// 2x time the target size - the motivation for it, is to reduce the aliasing that would have been
// created with direct reduction of very big image to small image
while (curImageDimensions.width * 0.5 > width) {
// Reduce the resizing canvas by half and refresh the image
halfImageDimensions.width = Math.floor(curImageDimensions.width * 0.5);
halfImageDimensions.height = Math.floor(curImageDimensions.height * 0.5);
resizingCanvasContext.drawImage(resizingCanvas, 0, 0, curImageDimensions.width, curImageDimensions.height,
0, 0, halfImageDimensions.width, halfImageDimensions.height);
curImageDimensions.width = halfImageDimensions.width;
curImageDimensions.height = halfImageDimensions.height;
}
// Now do final resize for the resizingCanvas to meet the dimension requirments
// directly to the output canvas, that will output the final image
let outputCanvas: HTMLCanvasElement = document.createElement('canvas');
let outputCanvasContext = outputCanvas.getContext("2d");
outputCanvas.width = width;
outputCanvas.height = height;
outputCanvasContext.drawImage(resizingCanvas, 0, 0, curImageDimensions.width, curImageDimensions.height,
0, 0, width, height);
// output the canvas pixels as an image. params: format, quality
this.base64ResizedImage = outputCanvas.toDataURL('image/jpeg', 0.85);
// TODO: Call method to do something with the resize image
}
};
}}
I don't understand why nobody is suggesting createImageBitmap.
createImageBitmap(
document.getElementById('image'),
{ resizeWidth: 300, resizeHeight: 234, resizeQuality: 'high' }
)
.then(imageBitmap =>
document.getElementById('canvas').getContext('2d').drawImage(imageBitmap, 0, 0)
);
works beautifully (assuming you set ids for image and canvas).