WebP in HiPS?

[Dubois-Felsmann, Gregory P.]

Dear colleagues,

A part of the Rubin team that's working on generating HiPS data has expressed interest in using the WebP format for HiPS tiles.  Apparently this has been supported by Aladin Lite since v3.1.0 in August 2023.

However, the HiPS standard's language was not written in a way that seems open to the _replacement_ of JPEG/PNG by WebP.  The way the tile-format metadata in the `properties` file is set up is certainly extensible and compatible with it, but the words in the standard itself are restrictive and prescriptive:

§4.2.1.3 "Three image formats *may* be used to package the HiPS tiles for images: FITS, PNG or JPEG. [...] The tile file extension *must* correspond to the format: .fits for FITS, .jpg for JPEG, .png for PNG.  These extensions *must* be in lowercase."

§4.2.3 "As with image HiPS, the format of each cube frame tiles *may* be FITS, JPEG and/or PNG."

§4.4.1 (properties file):

"7. hips_tile_format: Tile formats – Format: list of different HIPS tiles format supported by the survey, space separated (one or many of “fits”, “jpeg”, “png” for image/cube HiPS and “tsv” for catalog HiPS)"

(Note the inconsistency between the required spelling "jpeg" for hips_tile_format and the requirement to use ".jpg" for file extensions.)

The extension to WebP is obvious, but -- especially because the standard mandates particular spellings and capitalization of the file extensions for the original tile types -- it still should be documented, if extending HiPS is the community's intent.

At the moment I feel that that a WebP-only "HiPS" tree would not be reasonably describable as conformant.

It's not clear whether CDS is already ready to promote this as an extension to HiPS.  WebP isn't mentioned in the master HiPS catalog they maintain, even alongside the other file formats:

https://aladin.cds.unistra.fr/java/nph-aladin.pl?frame=aladinHpxList#hipssky

But, if this is a future direction, may I suggest that CDS issue an IVOA Note documenting the use of WebP in HiPS, and provide the required spelling of the extension in parallel with what was done for the others?  This could go on to be a HiPS 1.0.1, but at least getting it written down in a Note would be helpful to the community.

In the mean time we will assume ".webp".

Also: my colleagues are asking: would you also plan to support AVIF?

Many thanks,
Gregory