Html5 Canvas: Is There A Way To Resize Image With "nearest Neighbour" Resampling?
Solution 1:
I wrote a NN resizing script a while ago using ImageData (around line 1794)
https://github.com/arahaya/ImageFilters.js/blob/master/imagefilters.js
You can see a demo here
http://www.arahaya.com/imagefilters/
unfortunately the builtin resizing should be slightly faster.
Solution 2:
I'll echo what others have said and tell you it's not a built-in function. After running into the same issue, I've made one below.
It uses fillRect() instead of looping through each pixel and painting it. Everything is commented to help you better understand how it works.
//img is the original image, scale is a multiplier. It returns the resized image.
function Resize_Nearest_Neighbour( img, scale ){
//make shortcuts for image width and height
var w = img.width;
var h = img.height;
//---------------------------------------------------------------
//draw the original image to a new canvas
//---------------------------------------------------------------
//set up the canvas
var c = document.createElement("CANVAS");
var ctx = c.getContext("2d");
//disable antialiasing on the canvas
ctx.imageSmoothingEnabled = false;
//size the canvas to match the input image
c.width = w;
c.height = h;
//draw the input image
ctx.drawImage( img, 0, 0 );
//get the input image as image data
var inputImg = ctx.getImageData(0,0,w,h);
//get the data array from the canvas image data
var data = inputImg.data;
//---------------------------------------------------------------
//resize the canvas to our bigger output image
//---------------------------------------------------------------
c.width = w * scale;
c.height = h * scale;
//---------------------------------------------------------------
//loop through all the data, painting each pixel larger
//---------------------------------------------------------------
for ( var i = 0; i < data.length; i+=4 ){
//find the colour of this particular pixel
var colour = "#";
//---------------------------------------------------------------
//convert the RGB numbers into a hex string. i.e. [255, 10, 100]
//into "FF0A64"
//---------------------------------------------------------------
function _Dex_To_Hex( number ){
var out = number.toString(16);
if ( out.length < 2 ){
out = "0" + out;
}
return out;
}
for ( var colourIndex = 0; colourIndex < 3; colourIndex++ ){
colour += _Dex_To_Hex( data[ i+colourIndex ] );
}
//set the fill colour
ctx.fillStyle = colour;
//---------------------------------------------------------------
//convert the index in the data array to x and y coordinates
//---------------------------------------------------------------
var index = i/4;
var x = index % w;
//~~ is a faster way to do 'Math.floor'
var y = ~~( index / w );
//---------------------------------------------------------------
//draw an enlarged rectangle on the enlarged canvas
//---------------------------------------------------------------
ctx.fillRect( x*scale, y*scale, scale, scale );
}
//get the output image from the canvas
var output = c.toDataURL("image/png");
//returns image data that can be plugged into an img tag's src
return output;
}
Below is an example of it in use.
Your image would appear in the HTML like this:
<img id="pixel-image" src="" data-src="pixel-image.png"/>
The data-src tag contains the URL for the image you want to enlarge. This is a custom data tag. The code below will take the image URL from the data tag and put it through the resizing function, returning a larger image (30x the original size) which then gets injected into the src attribute of the img tag.
Remember to put the function Resize_Nearest_Neighbour (above) into the <script> tag before you include the following.
function Load_Image( element ){
var source = element.getAttribute("data-src");
var img = new Image();
img.addEventListener("load", function(){
var bigImage = Resize_Nearest_Neighbour( this, 30 );
element.src = bigImage;
});
img.src = source;
}
Load_Image( document.getElementById("pixel-image") );
Solution 3:
This CSS on the canvas element works:
image-rendering: pixelate;
This works in Chrome 93, as of September 2021.
Solution 4:
There is no built-in way. You have to do it yourself with getImageData.
Solution 5:
Based on Paul Irish's comment:
function resizeBase64(base64, zoom) {
return new Promise(function(resolve, reject) {
var img = document.createElement("img");
// once image loaded, resize it
img.onload = function() {
// get image size
var imageWidth = img.width;
var imageHeight = img.height;
// create and draw image to our first offscreen canvas
var canvas1 = document.createElement("canvas");
canvas1.width = imageWidth;
canvas1.height = imageHeight;
var ctx1 = canvas1.getContext("2d");
ctx1.drawImage(this, 0, 0, imageWidth, imageHeight);
// get pixel data from first canvas
var imgData = ctx1.getImageData(0, 0, imageWidth, imageHeight).data;
// create second offscreen canvas at the zoomed size
var canvas2 = document.createElement("canvas");
canvas2.width = imageWidth * zoom;
canvas2.height = imageHeight * zoom;
var ctx2 = canvas2.getContext("2d");
// draw the zoomed-up pixels to a the second canvas
for (var x = 0; x < imageWidth; ++x) {
for (var y = 0; y < imageHeight; ++y) {
// find the starting index in the one-dimensional image data
var i = (y * imageWidth + x) * 4;
var r = imgData[i];
var g = imgData[i + 1];
var b = imgData[i + 2];
var a = imgData[i + 3];
ctx2.fillStyle = "rgba(" + r + "," + g + "," + b + "," + a / 255 + ")";
ctx2.fillRect(x * zoom, y * zoom, zoom, zoom);
}
}
// resolve promise with the zoomed base64 image data
var dataURI = canvas2.toDataURL();
resolve(dataURI);
};
img.onerror = function(error) {
reject(error);
};
// set the img soruce
img.src = base64;
});
}
resizeBase64(src, 4).then(function(zoomedSrc) {
console.log(zoomedSrc);
});
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