Posted by Huibao Lin and Eyal Peled, Software Engineers, Google
At Google we believe in designing products with speed as a core principle. The Accelerated Mobile Pages (AMP) format helps ensure that content reliably loads fast, but we can do even better.
Smart caching is one of the key ingredients in the near instant AMP experiences users get in products like Google Search and Google News & Weather. With caching, we can make content be, in general, physically closer to the users who are requesting it so that bytes take a shorter trip over the wire to reach the user. In addition, using a single common infrastructure like a cache provides greater consistency in page serving times despite the content originating from many hosts, which might have very different—and much larger—latency in serving the content as compared to the cache.
Faster and more consistent delivery are the major reasons why pages served in Google Search's AMP experience come from the Google AMP Cache. The Cache's unified content serving infrastructure opens up the exciting possibility to build optimizations that scale to improve the experience across hundreds of millions of documents served. Making it so that any document would be able to take advantage of these benefits is one of the main reasons the Google AMP Cache is available for free to anyone to use.
In this post, we'll highlight two improvements we've recently introduced: (1) optimized image delivery and (2) enabling content to be served more successfully in bandwidth-constrained conditions through a project called "AMP Lite."
Image optimizations by the Google AMP Cache
On average across the web, images make up 64% of the bytesof a page. This means images are a very promising target for impactful optimizations.
Applying image optimizations is an effective way for cutting bytes on the wire. The Google AMP Cache employs the image optimization stack used by the PageSpeed Modules and Chrome Data Compression. (Note that in order to make the above transformations, the Google AMP Cache disregards the "Cache-Control: no-transform" header.) Sites can get the same image optimizations on their origin by installing PageSpeed on their server.
Here's a rundown of some of the optimizations we've made:
1) Removing data which is invisible or difficult to see
We remove image data that is invisible to users, such as thumbnail and geolocation metadata. For JPEG images, we also reduce quality and color samples if they are higher than necessary. To be exact, we reduce JPEG quality to 85 and color samples to 4:2:0 — i.e., one color sample per four pixels. Compressing a JPEG to quality higher than this or with more color samples takes more bytes, but the visual difference is difficult to notice.
The reduced image data is then exhaustively compressed. We've found that these optimizations reduce bytes by 40%+ while not being noticeable to the user's eye.
2) Converting images to WebP format
Some image formats are more mobile-friendly. We convert JPEG to WebP for supported browsers. This transformation leads to an additional 25%+ reduction in bytes with no loss in quality.
3) Adding srcset
We add "srcset" if it has not been included. This applies to "amp-img" tags with "src" but no "srcset" attribute. The operation includes expanding "amp-img" tag as well as resizing the image to multiple dimensions. This reduces the byte count further on devices with small screens.
4) Using lower quality images under some circumstances
We decrease the quality of JPEG images when there is an indication that this is desired by the user or for very slow network conditions (as part of AMP Lite discussed below). For example, we reduce JPEG image quality to 50 for Chrome users who have turned on Data Saver. This transformation leads to another 40%+ byte reduction to JPEG images.
The following example shows the images before (left) and after(right) optimizations. Originally the image has 241,260 bytes, and after applying Optimizations 1, 2, & 4 it becomes 25,760 bytes. After the optimizations the image looks essentially the same, but 89% of the bytes have been saved.
AMP Lite for Slow Network Conditions
Many people around the world access the internet with slow connection speeds or on devices with low RAM and we've found that some AMP pages are not optimized for these severely bandwidth constrained users. For this reason, Google has also launched AMP Lite to remove even more bytes from AMP pages for these users.
With AMP Lite, we apply all of the above optimizations to images. In particular, we always use lower quality levels (see Bullet 4 above).
In addition, we optimize external fonts by using the amp-fonttag, setting the font loading timeout to 0 seconds so pages can be displayed immediately regardless of whether the external font was previously cached or not.
AMP Lite is rolling out for bandwidth-constrained users in several countries such as Vietnam and Malaysia and for holders of low ram devices globally. Note that these optimizations may modify the fine details of some images, but do not affect other parts of the page including ads.
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All told, we see a combined 45% reduction in bytes across all optimizations listed above.
We hope to go even further in making more efficient use of users' data to provide even faster AMP experiences.