Resizing an image in an HTML5 canvas

Resizing with High-quality results and performance

If you're the type who values high-quality and optimized performance, you'll need more than a vanilla drawImage function. Enter resampling methods like lanczos, these bad boys calculate weights for each pixel during resizing, ensuring your pics remain crispy clear. Browsers like Firefox may need extra charm like image-rendering adjustments and optimizing canvas size to play nice.

Throwing in libraries like pica or thumbnail-generating code takes it to the next level. They employ web workers for non-blocking image processing, thus ensuring you get the best of both worlds: quality and performance. Here's pica in action:

var pica = require('pica')(); // Sweet! Pica is here

var img = new Image(); // That precious image of yours
img.onload = function() {
  var offscreenCanvas = document.createElement('canvas'); // The invisible canvas... spooky!
  offscreenCanvas.width = desiredWidth; // New width swaggering in
  offscreenCanvas.height = desiredHeight; // New height strutting in

  pica.resize(img, offscreenCanvas)
    .then(result => pica.toBlob(result, 'image/jpeg', 0.90)) // What's cooler than being cool? Being .90 cool!
    .then(blob => {
      // Handle the blob file... like a boss!
    });
};
img.src = 'image.jpg'; // Picasso's masterpiece


Tantrum with the number of lobes in your resampling filter, usually 2 to 3, for that sweet spot between visual quality and performance.

Interpolation, resampling and other cool stuff

Loading like a champ

Before going commando on image processing, make sure your image fully loads. Use the img.onload event as your green signal for canvas activities. Attempting to draw prior to this may give you half-baked results.

Possess client hardware

Don't ignore the client's beastly machine. Offload image resampling tasks to avoid UI hiccups and freezes. Techniques like web workers (No, not the spiders) available in libraries like pica, can do this heavy lifting on alternate threads.

The speed hurdle

JavaScript is far from being Usain Bolt when it comes to multi-core performance in browsers, even if you pimp it with web workers. So, make sure to test your image processing across multiple browsers to catch any snail-trails early on.

Raining code and copyrights

Beside impressing your friends with your amazingly reused code or libraries, remember to look into their origin and licensing. You don't want any legal issues for using Hermite filtering from libraries like fast.js or not acknowledging demos assisting your resizing techniques understanding.