[webkit-dev] Content-DPR: Image resolution optimization at CDN/transport layer

Alexey Proskuryakov ap at webkit.org
Mon Nov 11 17:28:03 PST 2019

> 10 нояб. 2019 г., в 1:16 AM, Noam Rosenthal <noam at webkit.org> написал(а):
> Hola
> I would like to open a discussion that has started a few years back and has never reached a consensus: https://bugs.webkit.org/show_bug.cgi?id=145380 <https://bugs.webkit.org/show_bug.cgi?id=145380>
> In a nutshell: content-dpr is a response-only header for images, that allows servers at transient layers (CDNs/proxies) to optimize images by changing their resolution, allowing the user agents to compensate for the change in intrinsic size caused by the resolution change - thus making the resolution change transparent to users. It's the header that makes the resolution-encoding transparent.
> The feature is already up and running in chrome since version 67, and is used by some CDNs.
> Since there was lack of consensus in the bug discussion, I wanted to make the case for it here, and see if opinions about the subject can be voiced before we reach a decision/consensus. I tried to find a good balance between being detailed and being concise. 
> Players in CDN, proxy and other transient parts of the internet world have innovated a lot in the past few years. There are lots of interesting companies and competition there. One of the areas of this innovation is in optimizing images in a transparent way at transient layers (proxy/CDN). This makes a lot of sense considering how much of internet traffic is taken by image download.
> What the research at the CDN companies found, was that modifying resolution at the transient layer can have a tremendous effect on performance/user-experience balance, for example by serving lower-resolution versions of the images when network traffic is costly/slow (the exact formula for that is part of where the CDN can innovate). More details on that innovation in the links below.
> The thing is, that modifying image resolution at the CDN/proxy is not inherently transparent, due to one reason - layout is affected by the image’s intrinsic size, and changing the image’s resolution inadvertently changes the image’s intrinsic size. To make this transparent, the user agent has to be involved, to compensate for this optimization when reading the image’s intrinsic size.
> The main case against this feature was that image resolution is a feature that should be handled at the markup layer and not at the transport layer (https://bugs.webkit.org/show_bug.cgi?id=145380#c7 <https://bugs.webkit.org/show_bug.cgi?id=145380#c7>).
> I would argue that http-headers are not a transport layer (TCP/UDP etc.), but rather part of an application-layer protocol that is designed, in part, to pass information to the transient layers such as CDNs, and that resolution compression is a (new, image-specific) equivalent of transfer-encoding. 
> Also, as a response to the view that this should be part of markup, I would argue that those transparent decisions by the servers should not concern web authors at all. It’s an optimization decision to be handled by the servers, with the user agent doing nothing about it but allow that decision to be transparent in terms of layout (the same way gzip and cache-control are transparent).

I don't see this as a precedent, because cache control and compression are invisible to users. Whereas image quality most certainly is. Changing it behind both web developer and user back would cause lots of undesirable behaviors - say, I e-mail an image from a webpage to someone else, who doesn't have a small display, or just zoom in to see the details.

This basically results in the website being untestable by the author (or by UA implementers who will be getting bug reports about site behavior differences).

Also, maybe I'm just pessimistic here, but it seems almost inevitable that disagreements between markup, Content-DPR and dpi information embedded in images will be handled by UAs differently, even if the spec ends up being very detailed on this point.

- Alexey

> What is offered here is a win-win in terms of delivering images, and would allow webkit-browser users to enjoy some of the innovations in the image delivery world without modifying their HTML content and without concern of breaking their layouts. Less battery and data used for downloading big images at certain situations, for example.
> I hope this provides enough background without being too tedious. I would be happy to shed more light on whatever is missing in this longish essay :)
> Additional info:
> https://github.com/eeeps/content-dpr-explainer <https://github.com/eeeps/content-dpr-explainer>
> https://cloudinary.com/blog/client_hints_and_responsive_images_what_changed_in_chrome_67 <https://cloudinary.com/blog/client_hints_and_responsive_images_what_changed_in_chrome_67>
> https://www.chromestatus.com/metrics/feature/timeline/popularity/906 <https://www.chromestatus.com/metrics/feature/timeline/popularity/906>_______________________________________________
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