360/panorama photo viewer

[dichro]

Equirectangular 360 photos seem to be rendered as standard (large) images. There are several examples of content-sensitive handling for this type of image on the web. Facebook, for example, creates a continuously 2d-scrollable view-port onto the wrapped image, and allows an initial viewport to be specified on upload. Can we replicate such an interface?

• https://pannellum.org/ is one example of a WebGL-based viewer seemingly designed for webapp integration.

• Given local caching of images, maybe we can do an entirely local decision for when to swap in the image viewer, based on exif data?

• Is there an obvious way to add initial-viewport metadata to a toot with 360 media? Appending polar coordinates to the URL?

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• [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.

• [ ] This bug happens on a tagged release and not on master (If you're a user, don't worry about this).

[zougloub]

Note:

• The photosphere image metadata ("accepted" standard based on XMP) would need to be preserved for this to work (#2710). From what I've seen, the code is using imagemagick process calls to process images.
• Of course the next request will be about videos, which use the spatial media spherical video "accepted" standard.