Picture Perfect - An Image Optimization Talk

Learn the tips and tricks that help production sites deliver images that have optimal Largest Contentful Paint and minimum Cumulative Layout Shift.

Transcript

1. Addy Osmani Picture Perfect Toronto WebPerf https://unsplash.com/photos/uf-IaeduFfk
3. Optimizing images. How hard could it be?
5. Image optimization is an evolving topic.
6. Major changes in the image landscape • WebP support in all modern browsers • AVIF image decode support landing in Chrome & Firefox • JPEGXL decoding behind a fl ag in Chrome • Lazy-loading images in Chrome, FF, Safari TP. A default in WordPress • Compute img/video aspect ratio from width/height attributes • FF support for preload, Chrome: imagesrcset and imagesizes on link rel=preload • width/height on <source> elements for <picture> • and more! 2020-2021
7. The modern <img> element <img src="donut-800w.jpg" alt="A delicious donut" width="400" height="400" srcset="donut-400w.jpg 400w, donut-800w.jpg 800w" sizes="(max-width: 640px) 400px, 800px" loading="lazy" decoding="async" style="background-size: cover; background-image: url(data:image/svg+xml;base64,[svg text]);"> Lazy-load Async decode SET DIMENSIONS TO AVOID LAYOUT SHIFTS Placeholder
8. 50% of the brain is involved in visual processing Time to get the sense of a visual scene 0.01s How much we recall of what we see or do vs. 20% of what we read. 80% Increase in willingness to read due to color visuals. 80% Neuroscience How much audiences more likely to be persuaded by a talk with visuals vs. a purely verbal one 17% To process a symbol and 100ms to attach meaning to it. 150ms Sources: goo.gle/imgsci
10. There are many ways to load an image. https://goo.gle/3eLzPux
11. Images impact Web Vitals in a number of ways
12. Optimal image serving for Web Vitals • For a fast Largest Contentful Paint • Request your key hero image early • Use <picture>, srcset + e ff i cient modern image formats • Avoid wasting pixels (compress, don't serve overly high DPR images) • Lazy-load o ff screen images (reduce network contention for key resources) • For a low Cumulative Layout Shift • Set dimensions (width, height) on your images • Use CSS aspect-ratio or aspect-ratio boxes to reserve space otherwise • For a low impact First Input Delay • Avoid images causing network contention with other critical resources like CSS and JS.
13. Largest Contentful Paint Element Lighthouse Audit function getLCPDebugTarget(entries) { const lastEntry = entries[entries.length - 1]; return lastEntry.element; } LCP API (RUM) web.dev/debug-web-vitals-in-the- fi eld/ User devices have di ff erent screen resolutions, which results in di ff erent page layouts and thus di ff erent elements being visible within the viewport. Content may be personalized for the current user, so the LCP candidate element could vary wildly from user to user. 🔬 ⛰
14. Images & Developer tooling Identify images that could be served in a more modern format
15. Serving Modern Image Formats <picture> <source srcset="puppy.jxl" type="image/jxl"> <source srcset="puppy.avif" type="image/avif"> <source srcset="puppy.webp" type="image/webp"> <source srcset="puppy.jpg" type="image/jpeg"> <img src="puppy.jpg" alt="Cute puppy"> </picture>
16. Picking an image format Compression • Compression • Speed • Limitations • Animation • Features • Tooling
17. JPEG 80 vs AVIF 70 vs WEBP 85 54% size reduction vs JPEG 26% size reduction vs JPEG https://www.industrialempathy.com/posts/avif-webp-quality-settings/
18. When lossy compression is good enough, both AVIF and JPEG XL deliver significantly better results than the existing web codecs, including WebP.
19. Original 1.7MB How much can we get out of ~70KB?
20. JPEG 68.3KB
21. JPEG 68.3KB
22. WEBP 67.7KB
23. HEIC 69.7KB
24. AVIF 67.6KB
25. JXL 67.0KB
26. Picking an image format Speed & Features
27. Image Optimization Tooling Squoosh • Imagemin • ImageMagick • Sharp / Jimp • Thumbor • ImageOptim • XnConvert • Image CDNs
28. Images & Developer tooling Image format debugging
29. Identify images that can be better sized Properly size images
30. Images without dimensions
31. Layout Shifts from images without dimensions Lighthouse audit function getCLSDebugTarget(entries) { const largestShift = entries.reduce(( return a && a.value > b.value ? a : }); if (largestShift && largestShift.sour const largestSource = largestShift. return a.node && a.previousRect.w b.previousRect.width * b.prev }); if (largestSource) { return largestSource.node; } } PerformanceObserver
32. Lots of factors determine the best image size • Many factors determine the best image size • Screen size and resolution • Viewport • Layout and responsive breakpoints • Device pixel ratio (physical resolution) • Art direction Can one size fi t all?
33. Responsive Images: srcset and sizes <img src="donut-800w.jpg" alt="A delicious pink donut." width="400" height="400" srcset="donut-400w.jpg 400w, donut-800w.jpg 800w" sizes="(max-width: 640px) 400px, 800px">
34. <img srcset> by width and density Responsive images
35. Device Pixel Ratio (DPR) / Pixel Density Capping Responsive images
36. Image Aspect Ratio Reserve space for responsive and lazy-loaded images Padding-top hack Mapped aspect-ratio Explicit aspect-ratio
37. Enhanced image preview in DevTools File size, rendered size, rendered aspect ratio, intrinsic size, intrinsic aspect ratio
38. Image lazy-loading Defer o ff screen images
39. Image lazy-loading Eager image fetching
40. Lazy-loading responsive images <!-- Lazy-load images in <picture>. <img> is the one driving image loading --> <picture> <source media="(min-width: 40em)" srcset="big.jpg 1x, big-hd.jpg 2x"> <source srcset="small.jpg 1x, small-hd.jpg 2x"> <img src="fallback.jpg" loading="lazy"> </picture> <img src="donut.jpg" alt="A delicious pink donut." loading="lazy" width="400" height="400"> Lazy-loading images
41. Image lazy-loading Lazy image fetching
42. Image discovered late - loaded via JS LCP image preloaded
43. Preload Largest Contentful Paint image Lighthouse audit
44. Request your image early <link rel="preload" as="image" href="donut.jpg"> <link rel="preload" as="image" href="donut.webp" type="image/webp"> <link rel="preload" as="image" href="donut.jpg" imagesrcset=" poster_400px.jpg 400w, poster_800px.jpg 800w, poster_1600px.jpg 1600w" imagesizes="50vw">
45. Blurry placeholder images Approximate the fi nal image before it loads in
46. Placeholder using background-image <img src="donut-800w.jpg" alt="A delicious donut" width="400" height="400" srcset="donut-400w.jpg 400w, donut-800w.jpg 800w" sizes="(max-width: 640px) 400px, 800px" loading="lazy" decoding="async" style="background-size: cover; background-image: url(data:image/svg+xml;base64,[svg text]);"> Placeholder Final image
47. https://unsplash.com/photos/qDY9ahp0Mto
48. Next.js Image Component • Automatic srcsets for images • Enforced sizing to prevent layout shift • Automatic lazy-loading of o ff screen assets • Framework-speci fi c asset prioritization optimizations • Flexible layout options • Customizable loader support for any image server or CDN Features and bene fi ts Up to 60% faster LCP and up to 100% reduced CLS
49. Next.js Image Component Features and bene fi ts • Automatic srcsets for images • Enforced sizing to prevent layout shift • Automatic lazy-loading of o ff screen assets • Framework-speci fi c asset prioritization optimizations • Flexible layout options • Customizable loader support for any image server or CDN Up to 60% faster LCP and up to 100% reduced CLS
50. Images in Next.js without Next Image import Head from 'next/head'; export default function Index() { return ( <div> <Head><title>Create Next App</title></Head> <main> <div> <img src="/donut1.jpeg" alt="Donut" height={700} width={700} /> <img src="/donut2.jpeg" alt="Donut" height={700} width={700} /> </div> </main>
51. Next.js Image Component
52. Images in Next.js with Next Image import Head from 'next/head'; import Image from 'next/image'; export default function Index() { return ( <div> <Head><title>Next.js Image Component</title></Head> <main> <div> <Image src="/donut1.jpeg" alt="Donut" height={700} width={700} /> <Image src="/donut2.jpeg" alt="Donut" height={700} width={700} /> </div>
53. Next.js Image Component
54. Next.js Image Component
55. 31% improvement in LCP increased sales by 8% Optimizations to improve LCP included resizing their hero image, optimizing SVGs and using media queries to limit loading o ff screen images. Their optimizations to LCP included a 2.5s saving from switching their fi rst large image from being behind JavaScript (client-side hydration) to being directly in the main HTML document. 70% improvement in LCP correlated with a 76% reduction in load abandonment They determined shifts were caused after their hero images were loaded and snapped in for the fi rst view. They used Aspect Ratio Boxes to reserve space before their image was loaded. Optimizations to CLS helped increase News page views per session by 15% French Fashion house Chloè used Link Preload to preload their 1x and 2x Hero images, which were previously bottlenecked by a render-blocking script. Preloading 1x and 2x Hero images bottlenecked by render- blocking script improved LCP by 500ms (based on CrUX)
56. Thank you! @addyosmani
57. Archive Slides that won't make the cut this time. • We can still summarize or include as notes.
59. Automating Image Optimization in frameworks Image Components • Modern image components that build on <img>, like Next.js <Image> (for React) and Nuxt image (for Vue) try to bake in as many of these concepts as possible by default. • You can of course also do this manually just using the <img> element directly. • If using 11ty for your static sites, try the 11ty high-performance blog template.
60. Avoid the download and shrink pattern Better to use srcset, sizes, <picture> to serve images w/right dimensions
61. Compression
62. Speed
63. Limitations
64. Features
65. Codec Comparison https://www.industrialempathy.com/posts/avif-webp-quality-settings/#visual-veri fi cation
66. JPEG@80 213KB
67. WEBP@90 206KB Size reduction 4% vs JPEG DSSIM 0.00103
68. AVIF@75 88KB Size reduction 59% vs JPEG DSSIM 0.00140
69. JXL@61 80KB Size reduction % vs JPEG DSSIM
70. JPEG 80 vs WebP 85 (26% size reduction vs JPEG)
71. JPEG 80 vs AVIF 70 (54% size reduction vs JPEG)
72. Low Fidelity Comparison https://cloudinary.com/blog/time_for_next_gen_codecs_to_dethrone_jpeg
73. LCP image in the loading filmstrip WebPageTest
74. CLS
75. Images & Developer tooling Largest Contentful Paint element
76. Images & Developer tooling Image Intrinsic Size
77. <img srcset> by width and density Responsive images
78. Device Pixel Ratio (DPR) / Pixel Density Capping Responsive images
79. Device Pixel Ratio (DPR) / Pixel Density Capping Responsive images
80. Responsive Breakpoints from Cloudinary Responsive images
81. Serving modern image formats Compression
82. Images & Developer tooling Cloudinary's image analyst tooling
83. Animation
84. Basic <img> element <img src="donut.jpg" alt="A delicious pink donut." width="400" height="400">
85. Lazy-loading images <img src="donut.jpg" alt="A delicious pink donut." loading="lazy" width="400" height="400">
86. Lazy-loading responsive images <!-- Lazy-load images in <picture>. <img> is the one driving image loading so <picture> and srcset fall off of that --> <picture> <source media="(min-width: 40em)" srcset="big.jpg 1x, big-hd.jpg 2x"> <source srcset="small.jpg 1x, small-hd.jpg 2x"> <img src="fallback.jpg" loading="lazy"> </picture>
87. Request your image early <link rel="preload" as="image" href="donut.jpg"> <link rel="preload" as="image" href="donut.webp" type="image/webp"> <link rel="preload" as="image" href="donut.jpg" imagesrcset=" poster_400px.jpg 400w, poster_800px.jpg 800w, poster_1600px.jpg 1600w" imagesizes="50vw">
88. Request your image early <link rel="preload" as="image" href="donut.jpg" imagesrcset=" poster_400px.jpg 400w, poster_800px.jpg 800w, poster_1600px.jpg 1600w" imagesizes="50vw">
89. Image decoding <img src="donut-800w.jpg" alt="A delicious donut" width="400" height="400" srcset="donut-400w.jpg 400w, donut-800w.jpg 800w" sizes="(max-width: 640px) 400px, 800px" loading="lazy" decoding="async">
90. Responsive Breakpoint Generator
91. Optimal image serving for Web Vitals • For a fast Largest Contentful Paint • Request your key hero image early • Use srcset + e ffi cient modern image formats • Avoid wasting pixels (compress, don't serve overly high DPR images) • Lazy-load o ff screen images (reduce network contention for key resources) • For a low Cumulative Layout Shift • Set dimensions (width, height) on your images • Use CSS aspect-ratio or aspect-ratio boxes to reserve space otherwise • For a low impact First Input Delay • Avoid images causing network contention with other critical resources like CSS and JS.