[webkit-dev] Content-DPR header

Yoav Weiss yoav at yoav.ws
Sat May 30 15:03:35 PDT 2015

On Sat, May 30, 2015 at 12:32 AM, Simon Fraser <simon.fraser at apple.com>

> As others have said, doing this at the transport layer seems wrong.

HTTP is an application layer protocol, and is already used to convey all
kinds of data about the payload that it delivers, such as content-type,
charset and language. Content-DPR is not very different.
Content-DPR can be thought of as part as the regular content-negotiation
flow, where the browsers may indicate "this is what I need" (e.g. via
Client-Hints request headers) and the server can indicates "this is what I
gave you".

> Why not just invent some new metadata that gets put into the image to
> describe some scaling of the intrinsic size?

Well, several reasons:
* The same image resource can be used over multiple scenarios (e.g. a 600px
wide image can be used as a 300px image on a retina screen, as well as a
600px image on a 1x screen). In each scenario, the image would need to have
a different intrinsic size.
* Introducing meta data to do that would require a significant effort on
multiple fronts:
 - We would need that meta data to work the same way across all currently
used Web image formats (GIF, PNG, JPEG, SVG, JPEG-2000, WebP and JPEG-XR).
 - We would need to add support for that to the decoding code of all these
 - We would need the various utilities that create these images to add
support for editing this meta data. This would require a huge eco-system
wide effort.
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