# Data Sources and Licenses

Huira's code is released under the [MIT License](https://github.com/huira-render/huira/blob/main/LICENSE).
The data that Huira's [examples](https://github.com/huira-render/huira/tree/main/examples) use
is not part of Huira. Some of it is hosted at
[media.huira.space](https://media.huira.space): 3D models, textures and star catalogs. The rest
comes straight from its provider, such as SPICE kernels from NASA's NAIF. Each dataset has its
own license, listed on this page, and each example's README lists the files it uses.

Please credit the data's providers when you publish work made with it, including rendered
images.

> **The Tycho-2 star catalog is licensed for non-commercial use only.** Huira's MIT license does
> not change that. If you render stars for commercial work, check that your use is allowed: see
> [Tycho-2 star catalog](#tycho-2-star-catalog).

| Data | Where to get it | License |
|---|---|---|
| [Tycho-2 star catalog](#tycho-2-star-catalog) | [media.huira.space/data/stars/tycho2/](https://media.huira.space/data/stars/tycho2/) | CC BY-NC 3.0 IGO (non-commercial) |
| [SPICE kernels](#spice-kernels) | [NASA's NAIF](https://naif.jpl.nasa.gov/naif/data.html) | Redistributable unmodified |
| [NASA spacecraft models](#nasa-spacecraft-models) | [media.huira.space/data/models/nasa/](https://media.huira.space/data/models/nasa/) | No copyright; NASA's media guidelines |
| [Solar System textures](#solar-system-textures) | [media.huira.space/data/textures/solar_system/](https://media.huira.space/data/textures/solar_system/) | CC BY 4.0 |
| [Kernels that ship with Huira](#kernels-that-ship-with-huira) | Included with Huira | Redistributable unmodified |

Each folder on media.huira.space holds an `ATTRIBUTIONS.txt`, with its data's source, license and
how to credit it, and a `SHA256SUMS`, to check downloads with `sha256sum -c SHA256SUMS`.


## Tycho-2 star catalog
The Tycho-2 Catalogue of the 2.5 million brightest stars, from ESA's Hipparcos mission, as
distributed by CDS, Strasbourg (VizieR catalog I/259).

- **Get it:** download the pre-built catalog, in Huira's star catalog format, from
  [media.huira.space/data/stars/tycho2/tycho2.hrsc](https://media.huira.space/data/stars/tycho2/tycho2.hrsc):

  ```sh
  curl -fLO https://media.huira.space/data/stars/tycho2/tycho2.hrsc
  ```

  `tycho2.hrsc` is always the latest build. Every build stays at
  [media.huira.space/data/stars/tycho2/versions/](https://media.huira.space/data/stars/tycho2/versions/)
  as `tycho2-d<N>.hrsc`, where `N` is the build's data version (below). The catalog's
  original files from CDS are at
  [media.huira.space/data/stars/tycho2/archive/](https://media.huira.space/data/stars/tycho2/archive/).
  Each folder has its own `ATTRIBUTIONS.txt` and `SHA256SUMS`.
- **Source:** [cdsarc.cds.unistra.fr/viz-bin/cat/I/259](https://cdsarc.cds.unistra.fr/viz-bin/cat/I/259)
- **License:** [CC BY-NC 3.0 IGO](https://creativecommons.org/licenses/by-nc/3.0/igo/). You may
  share and adapt the catalog, with credit, for non-commercial purposes only.
- **Huira's star catalog:** `tycho2.hrsc` is an adaptation of Tycho-2, under the same license.
  The conversion keeps each star's identifier, position (brought to epoch J2000.0) and proper
  motion; computes an approximate Johnson V magnitude, an effective temperature and a solid angle
  from the catalog's BT and VT magnitudes; drops the stars without magnitudes and the catalog's
  other columns; and sorts the stars by V magnitude.
- **Versions:** an `.hrsc` file carries two version numbers in its header, which Huira logs when
  it loads one. The *format version* is the file's layout; Huira refuses a format it cannot
  read and says whether Huira or the catalog is the older side. The *data version* is how the
  values were computed, and goes up whenever a new build changes them (a better temperature
  from B-V, say); Huira loads any data version. If your stars differ from someone else's,
  compare data versions. The format is documented in
  [`star_catalog.hpp`](https://github.com/huira-render/huira/blob/main/include/huira/stars/io/star_catalog.hpp).
- **Building it yourself:** the catalog is built from the CDS files by `build_star_catalog`,
  in Huira's [`tools/`](https://github.com/huira-render/huira/tree/main/tools) directory,
  built with `-DHUIRA_BUILD_TOOLS=ON`. It reads the files either as CDS's FTP archive
  serves them or as VizieR's text export writes them (the form in `archive/`, with
  `#Table:` header lines); the two differ by one column from the HIP number on. You only
  need this to change how the values are computed.
- **Cite:**
  - Høg, E., et al. 2000, "The Tycho-2 Catalogue of the 2.5 Million Brightest Stars",
    *Astronomy & Astrophysics*, 355, L27
  - Høg, E., et al. 2000, "Construction and verification of the Tycho-2 Catalogue",
    *Astronomy & Astrophysics*, 357, 367
  - and acknowledge CDS: "This research has made use of the VizieR catalogue access tool, CDS,
    Strasbourg, France (DOI: 10.26093/cds/vizier)."


## SPICE kernels
SPICE kernels hold ephemerides, orientations, frames and clocks. Huira does not host them: they
come straight from NASA's Navigation and Ancillary Information Facility (NAIF), which keeps them
at permanent addresses. The examples that use SPICE list the kernels they load, with links.

- **Get them:** NAIF's [generic kernels](https://naif.jpl.nasa.gov/pub/naif/generic_kernels/)
  (planetary and satellite ephemerides, planetary constants, the Earth's orientation), and
  missions' kernels in NAIF's [Planetary Data System archives](https://naif.jpl.nasa.gov/pub/naif/pds/).
- **License:** NAIF permits redistributing the kernels it distributes, as long as they are not
  modified ([NAIF's rules](https://naif.jpl.nasa.gov/naif/rules.html)).
- **Credit:** NAIF asks that SPICE, NAIF, and the teams that produced the data be acknowledged
  ([how to cite](https://naif.jpl.nasa.gov/naif/credit.html)). A kernel's comments name its
  producer, and often the papers to cite; JPL's planetary ephemeris DE440, for example, is
  Park, R. S., et al. 2021, "The JPL Planetary and Lunar Ephemerides DE440 and DE441",
  *The Astronomical Journal*, 161, 105. For SPICE and NAIF:
  - Acton, C. H. 1996, "Ancillary Data Services of NASA's Navigation and Ancillary Information
    Facility", *Planetary and Space Science*, 44(1), 65-70,
    [doi:10.1016/0032-0633(95)00107-7](https://doi.org/10.1016/0032-0633(95)00107-7)
  - Acton, C., Bachman, N., Semenov, B., and Wright, E., "A look toward the future in the
    handling of space science mission geometry", *Planetary and Space Science*,
    [doi:10.1016/j.pss.2017.02.013](https://doi.org/10.1016/j.pss.2017.02.013)
  - A mission's archive is cited by its DOI, which the PDS Engineering Node's DOI search finds
    from the archive's identifier (for example, `urn:nasa:pds:orex.spice` for OSIRIS-REx).


## NASA spacecraft models
NASA's 3D models of spacecraft: the Gateway core module and OSIRIS-REx. The folder's
`ATTRIBUTIONS.txt` gives each model's source page and original file name.

- **Get them:** [media.huira.space/data/models/nasa/](https://media.huira.space/data/models/nasa/)
  - `gateway.glb`: the Gateway core module, from [NASA's 3D Resources](https://science.nasa.gov/3d-resources/), published there as `Gateway Core.glb`.
  - `orex.glb`: the OSIRIS-REx spacecraft, from [NASA Science](https://science.nasa.gov/resource/osiris-rex-3d-model/), published there as `Osiris_Rex.glb`, credited to NASA's Visualization Technology Applications and Development group (VTAD).

  Huira's copies are renamed, and otherwise unchanged.
- **License:** NASA's media are generally not copyrighted and may be used without permission.
  NASA's [media usage guidelines](https://www.nasa.gov/nasa-brand-center/images-and-media/)
  apply: credit NASA, do not use the models to imply that NASA endorses anything, and do not
  use NASA's insignia.
- **Credit:** "Gateway model: NASA." and "OSIRIS-REx model: NASA/VTAD." Or, for several models
  at once, "Spacecraft models: NASA."


## Solar System textures
Textures of the Earth and the planets, from Solar System Scope: Mercury, Venus (surface and
atmosphere), Mars, Jupiter, Saturn and its rings, Uranus and Neptune, and for the Earth its
surface by day and by night, its clouds, a normal map, a roughness map and a city emission map.

- **Get them:** [media.huira.space/data/textures/solar_system/](https://media.huira.space/data/textures/solar_system/)
- **Source:** [Solar System Scope](https://www.solarsystemscope.com/textures/), based on NASA elevation and imagery data, with colors tuned to true-color photographs from Messenger, Viking, Cassini and the Hubble Space Telescope. Solar System Scope notes that unmapped regions are filled with fictional terrain, and that colors are slightly more saturated than true color. Uranus and Neptune are only published at 2k; the rest are 8k (Venus's atmosphere, 4k).
- **Modifications:** Two Earth maps were made for Huira by Christopher Gnam: 
  - The roughness map (`8k_earth_roughness_map.tif`), by modifying the specular map.
  - The city emission map (`8k_earth_city_emission.jpg`), by modifying the night map.
- **License:** [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). The roughness and city
  emission maps, adaptations, are distributed under the same license.
- **Credit:** "Textures: Solar System Scope (solarsystemscope.com), CC BY 4.0." For the Earth,
  add "Roughness map adapted from its specular map; city emission map adapted from its night
  map." Images rendered with the textures should carry this credit too.


## Kernels that ship with Huira
Huira includes two of NAIF's generic kernels, in its `data/kernels/` folder:

- `lsk/naif0012.tls`: leap seconds, which Huira loads when it first converts a time.
- `pck/pck00011.tpc`: planetary constants (sizes and orientations), which
  `huira.spice.load_default_pck()` loads.

Both are unmodified, and redistributed under [NAIF's rules](https://naif.jpl.nasa.gov/naif/rules.html).

 > Leap seconds are to be discontinued by 2035 (CGPM resolution, 2022), so `naif0012.tls` is unlikely to change often, if ever. It stays included either way: SPICE's time system needs a leap-seconds kernel to run.