How do I mark an MP4 or WebM as 360, VR180 or stereoscopic so YouTube and players treat it correctly, and which stereo layouts are defined?

Partially verified: the curator re-read S9; S11, S4, S5 were not re-read by the curator and rest on the producer's pinned reading. Spherical Video V2 puts projection metadata in sv3d (svhd, proj with cbmp/equi/mshp) and stereo layout in st3d on the video sample entry, with WebM equivalents; V1 and V2 may coexist and V2 wins when they differ. YouTube's help names only left-right stereo for 3D (producer reading), while the RFC defines more stereo modes. Curator qualifications: Partially verified: the curator re-read S9; S11, S4, S5 were not re-read by the curator and rest on the producer's pinned reading. Curator GET 2026-10-23 of the sources not re-read (mechanical source evidence, not semantic verification): S11: HTTP 200, bytes differ from the producer pin; S4: HTTP 200, bytes differ from the producer pin; S5: HTTP 200, bytes differ from the producer pin. Specs/report read 2026-10-22; no file injected or played. YouTube help pages (S4, S5) and V1 RFC (S11) not re-read. Limitations: Specs/report read 2026-10-22; no file injected or played. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://github.com/google/spatial-media/blob/2308497c39bb604a7c3ba56ac17e302f2b1b6a96/docs/spherical-video-rfc.md",
      "note": "spherical-video-v2-rfc.md at 2308497c L18-24 and st3d section; spatial-media LICENSE"
    },
    {
      "url": "https://support.google.com/youtube/answer/6178631?hl=en",
      "note": "spherical-video-v2-rfc.md at 2308497c L18-24 and st3d section; spatial-media LICENSE"
    },
    {
      "url": "https://support.google.com/youtube/answer/7278886?hl=en",
      "note": "spherical-video-v2-rfc.md at 2308497c L18-24 and st3d section; spatial-media LICENSE"
    },
    {
      "url": "https://github.com/google/spatial-media/blob/2308497c39bb604a7c3ba56ac17e302f2b1b6a96/docs/spherical-video-v2-rfc.md",
      "note": "spherical-video-v2-rfc.md at 2308497c L18-24 and st3d section; spatial-media LICENSE"
    }
  ]
}