> 10 нояб. 2019 г., в 1:16 AM, Noam Rosenthal <n...@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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