[webkit-dev] Content-DPR header

[Yoav Weiss]

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

>
> 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
formats.
 - 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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