Add support for Content-DPR response header #5112

eeeps · 2019-11-26T04:32:16Z

This WIP PR adds support for the Content-DPR response header on image responses (see explainer here).

At least two implementers are interested (and none opposed):
- Chrome (already shipped)
- WebKit has asked for a clean spec to review, hence the PRs
Tests are written and can be reviewed and commented upon at: https://github.com/web-platform-tests/wpt/tree/master/content-dpr
Implementation bugs are filed:
- Chrome: N/A
- Firefox: #1611532
- Safari: #145380

Bonus: here's Chrome's usage tracking.

💥 Error: Wattsi server error 💥

PR Preview failed to build. (Last tried on Jan 15, 2021, 8:00 AM UTC).

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<head><title>504 Gateway Time-out</title></head>
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<center><h1>504 Gateway Time-out</h1></center>
<hr><center>nginx/1.10.3</center>
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@zcorpan

based on @zcorpan's advice a1ba23a#r36099166

rniwa · 2019-11-26T13:43:38Z

Note that having this PR does not constitute an implementation interest in WebKit. It’s simply a prerequisite.

annevk · 2019-12-02T11:01:53Z

source

+    <p>Wait until the result of fetching is either a <span data-x="concept-response">response</span>
+    or an error. If it is a <span data-x="concept-response">response</span>, and the
+    <span data-x="concept-response">response</span> has an
+    <span data-x="concept-response-image-density">image density</span> that is not null,


Why not parse for the response header here? Are you trying to avoid having service workers set it or something?

No, just wasn't aware that parsing for headers before a Response exists was an option. Any pointers to another section of the spec that does this?

Before a response exists? You're holding a response here, right?

I guess that's my confusion. Is a Response object available as soon as step 21 completes, before we get to the networking task source stuff? How do I wait for the image density property I tried to define here whatwg/fetch#971 to be populated, and is it guaranteed to be populated before the first chunks of image data are ready to paint?

I guess it would be better to make this part of the next step yeah, once the first task is queued. You'll then have a response (and potentially some initial bytes as well, but definitely all the headers).

Moved it, per this and our conversation on IRC

annevk · 2020-01-24T13:41:33Z

I guess another problem here is that image fetching isn't properly abstracted yet and used by all things that fetch images. So technically this would only work for img.

@annevk

per @annevk's advice https://freenode.logbot.info/whatwg/20200124#c3135642

…loads ...and point to an issue about unifying image loading, generally.

annevk · 2020-02-07T10:32:42Z

@eeeps I thought I'd state it again for clarity, I don't think we need any changes to Fetch here. HTML can be in charge of parsing the header and forwarding the information to the image decoder.

- add reference to Fetch's `get a structured header` algorithm - add reference to the Structured Headers spec, and its definitions of Integers and Decimals - get the Content-DPR header and check that it's valid ourselves, right here in HTML, before setting the image density

eeeps · 2020-02-22T00:11:10Z

@annevk Ok, closed the Fetch PR, called out to Fetch's get a strucructured header, and am validating the result against the Structured Headers definitions of Integer/Decimal. That seemed like the cleanest way to parse, but am I off base?

sideshowbarker · 2020-02-22T00:32:51Z

I don't know what's causing the PR Preview error. Maybe it's some intermittent failure. I'll try to figure out if there's a way to trigger it to rebuild.

annevk · 2020-02-25T14:06:03Z

You should add an IANA template similar to what we have for the Refresh and Ping-To headers and such.
I think we should state that it's expected to apply to all image fetches (and have tests for that). That way the only cleanup that remains to be done is abstracting the algorithm and getting it appropriately invoked. That also avoids potential forward compatibility issues with sites not expecting Content-DPR to work for non-img sources.

eeeps · 2020-02-25T15:53:10Z

@annevk great. I agree on №2; we have tests for CSS background-images but not fetches whose destination is image. Other contexts that come to mind:

SVG <image>
<video poster>
Canvas
<button type=image>

Not sure if Content-DPR should have effects within all of these, but I'll think through them and flesh out + toughen up the note (and work on tests). Any other contexts, that I haven't thought of?

Update:

I don't think the intrinsic dimensions of a fetch destination=image Response are inspectable in any way before that image is used somewhere; perhaps that path only makes sense to test in conjunction with some or all of the possible rendering contexts.
In the same vein, we should test <link rel=preload>-fetched images, I think...
<object> is another context...

eeeps · 2020-02-26T00:33:16Z

@annevk added the IANA section. The other headers had their <dfn>s down there; I left mine in the prose? If you have any opinions on where the <dfn> should live, let me know.

@noamr also added a bunch of tests web-platform-tests/wpt#21962

eeeps · 2020-02-26T01:29:39Z

Ok, added the expectation that Content-DPR applies everywhere, too. (Maybe a bit wordy?)

eeeps · 2020-02-26T06:18:17Z

Hm, I think I need to specify that Content-DPR values must be >0 (and what to do when they're not).

noamr · 2020-02-26T08:09:13Z

Hm, I think I need to specify that Content-DPR values must be >0 (and what to do when they're not).

I believe Content-DPR should be ignored for <= 0

annevk · 2020-03-02T16:51:22Z

CSS has a large number of properties that take images in one form or another. And yeah, this can only be tested on a per-endpoint basis unfortunately. Looks pretty good otherwise. I guess the main things remaining are implementer interest and someone verifying the test coverage. (Happy to help with the latter once the former is sorted.)

eeeps · 2021-03-03T05:48:41Z

Abandoned this approach; switched to using EXIF information for these use cases, rather than an HTTP header. #5574

eeeps added 6 commits November 25, 2019 20:13

Copy/paste Yoav's section

e9d3d42

Use image density from fetch in the processing model

502d8c5

Re-word Content-DPR definition

17ba1c2

Wait for a response and queue a task to set state

598f33b

based on @zcorpan's advice a1ba23a#r36099166

Note stating my confusion/questions

d74fffb

tweak

a586e05

eeeps mentioned this pull request Nov 26, 2019

Add support for Content-DPR response header whatwg/fetch#971

Closed

annevk reviewed Dec 2, 2019

View reviewed changes

Move the current-pixel-density-setting logic

e8593c0

per @annevk's advice https://freenode.logbot.info/whatwg/20200124#c3135642

eeeps mentioned this pull request Jan 24, 2020

Abstract out and unify image loading #5234

Closed

Add a note that image densities should apply to other types of image …

a43030b

…loads ...and point to an issue about unifying image loading, generally.

annevk added addition/proposal New features or enhancements needs implementer interest Moving the issue forward requires implementers to express interest topic: img labels Feb 7, 2020

eeeps added 2 commits February 21, 2020 15:29

Add a note about Content-DPR overriding densities from srcset

7f6d0d4

Add Content-DPR to the Permanent Message Header Field Registry

36d76c1

Other image fetching paths MUST honor Content-DPR

7002753

Ignore Content-DPR values ≤ 0

e7b7b6b

eeeps mentioned this pull request Mar 4, 2020

Bumping up decimal precision? httpwg/http-extensions#1102

Closed

Base automatically changed from master to main January 15, 2021 07:57

eeeps closed this Mar 3, 2021

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Add support for Content-DPR response header #5112

Add support for Content-DPR response header #5112

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Add support for Content-DPR response header #5112

Add support for Content-DPR response header #5112

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