Anwar Montasir

Web Development + UX Design + Instruction

Case Study:
Lazy Load Images


visit project  GitHub

Project Overview

This project was built for the Devsign course I designed and taught in the winter of 2018 at Alchemy Code Lab.

In my experience, one of the most difficult concepts to teach is strategies for responsive image loading. Not merely setting the image width to 100%, but actually ensuring that appropriate media loads for the appropriate device, and only when needed.

This project introduced three concepts:

  • Using the Cloudinary API. One obstacle in the way of providing responsive media is that creating numerous images is a pain, especially for students with no access to Photoshop. Fortunately, Cloudinary offers image storage, dynamic delivery optimized for different devices, and parameters to control image manipulation.
  • Art direction vs resolution switching. Per MDN’s article on responsive images, while choosing an image of appropriate resolution can be handled by the srcset attribute, choosing different aspect ratios for different screens requires the <picture> element.
  • Lazy loading images. Inspired by the behavior of Medium articles, I wanted to load blurred placeholder images initially, with detailed images loading only as each one is scrolled into the viewport.

Prep Work

While this was a one-day class exercise and did not undergo any part of the UX Research process, I did need to plan the layout and image uploads carefully.

For the hero image, I started with a large version of Van Gogh’s 1890 painting Wheatfield With Crows. The original upload was 4921px by 2361px and weighed 1.8 MB, though I had no plans to implement anything so heavy.

Vincent Van Gogh, Wheatfield With Crows, 1890

A key feature of my script for loading the hero image are the preferences, based on Cloudinary’s Image Transformations documentation established in Home.js:

aspectRatios: ['3:1', '5:2', '2:1', '3:2', '1:1'], breakpoints: [2000, 1500, 1200, 900, 600], options: 'c_fill,g_auto,q_auto'

Here I use a few Cloudinary cropping features (crop fill and gravity) while loading smaller images with increasingly square aspect ratios for decreasing screen widths:

Diagram for choosing aspect ratios

I used an array of breakpoints as well for the remaining images, but these did not require art direction. I loaded these images initially with Cloudinary’s e_blur setting at 600, and the width and height divided by 100, to create placeholder images weighing less than 1K:

According to Firefox’s Network Monitor, the page initially loads 943K of data on my retina screen in 1.02 seconds. This is one-sixth of the total page weight, but the rest loads one image at a time.

Challenges Faced

The Cloudinary API was pretty straightforward. The one tricky part of this exercise was working with the Intersection Observer API to detect visibility of my gallery images within the viewport. Dean Hume’s post on Lazy loading images using Intersection Observer helped a ton.

const config = { // If image gets within 50px of Y axis, start download. rootMargin: '-50px 0px', threshold: 0.01 }; const wrapper = dom.querySelector('.lazy'); setTimeout(() => { let observer = new IntersectionObserver(([entry]) => { if(!entry || !entry.isIntersecting) return; observer.unobserve(wrapper); this.getHiResImage(wrapper.querySelector('.hiRes')); }, config); observer.observe(wrapper); });

I’ve also recently noticed a brief FOUS (flash of unstyled content) when the build loads. I’m working on a solution for this issue.

Technologies Used

  • HTML5
  • CSS
  • JavaScript
  • Webpack
  • Node.js
  • PostCSS