ReadStar — NeoForge Astronomy Mod

A Minecraft sky mod driven by real celestial mechanics and star catalog data, featuring planetary orbital systems, star catalogs, FOV-aware rendering, and server-synchronized celestial configuration.

Author: FrozenStream

Features

• Real Star Catalog — 12,000+ real stars from Gaia DR3 (color) + BSC5 (bright star fallback), with per-star brightness and glow
• Real Celestial Mechanics — Keplerian orbital system with unlimited nesting depth
• Atmospheric Scattering — HSV-based atmosphere color & glow computation with full-sky overlay and stellar halo
• Comet Rendering — Bézier dust + ion dual-tail structure with dynamic wobble
• FOV-Aware Rendering — Custom shader keeps star screen size constant across FOV changes
• Zero-Code Customization — Add 8 moon phases to any body by simply placing PNGs
• Server Sync — Celestial config managed server-side, auto-synced to all clients

Quick Start

./gradlew build          # Build
./gradlew runClient      # Run client
./gradlew runData        # Data generation (atlas config)
./gradlew runServer      # Run server

The celestial system is defined via data pack at data/readstar/celestial/system.json, and the star catalog via resource pack at assets/readstar/custom/stars/stars.json.

Config changes take effect with F3+T. Data pack changes require /reload or restart.

Configuration

Config file: run/config/readstar-common.toml

FOV-Aware Rendering

Stars use a custom shader that separates each point into position and offset:

Formula: FovCompensation = tan(fov/2) / tan(35°) × strength + (1 - strength)

Shader: worldPos = Position + Offset × FovCompensation

Per-Star Brightness

Based on Vmag with independent decay thresholds:

Glow quads only for Vmag < 2.0: < 0.5 → high / < 1.5 → medium / < 2.0 → low.

Data Pack — Celestial System

Path: data/readstar/celestial/system.json

Place at saves/<world>/datapacks/<pack>/data/readstar/celestial/system.json or server world/datapacks/.

Structure

{
  "System": {
    "<name>": {
      "mass": <double>,
      "radius": <double>,
      "luminance": <int>,
      "unstableDirtySnowball": <bool>,
      "hasAtmosphere": <bool>,
      "atmosphereHSV": <int>,
      "starHSV": <int>,
      "axis": [<x>, <y>, <z>],
      "orbit": {
        "semiMajorAxis": <double>,
        "eccentricity": <double>,
        "inclination": <double>,
        "argumentOfPeriapsis": <double>,
        "longitudeOfAscendingNode": <double>,
        "initialMeanAnomaly": <double>
      },
      "children": { "<name>": { ... } }
    }
  }
}

Field Reference

Names are case-insensitive. children: {} = no children. Nesting depth is unlimited.

Complete Example

Sun → Earth + Mars → Moon. Real solar system data.

{
  "System": {
    "Sun": {
      "mass": 1.989e30, "radius": 6.957e8, "luminance": 15,
      "unstableDirtySnowball": false, "hasAtmosphere": false, "atmosphereHSV": 0, "starHSV": 2172671,
      "axis": [0, 0, 0],
      "orbit": { "semiMajorAxis": 0, "eccentricity": 0, "inclination": 0, "argumentOfPeriapsis": 0, "longitudeOfAscendingNode": 0, "initialMeanAnomaly": 0 },
      "children": {
        "Earth": {
          "mass": 5.972e24, "radius": 6.371e6, "luminance": 0,
          "unstableDirtySnowball": false, "hasAtmosphere": true, "atmosphereHSV": 9738751, "starHSV": 9732275,
          "axis": [0, 0, 0],
          "orbit": { "semiMajorAxis": 1.496e11, "eccentricity": 0.0167, "inclination": 0, "argumentOfPeriapsis": 1.796, "longitudeOfAscendingNode": 0, "initialMeanAnomaly": 6.240 },
          "children": {
            "Moon": {
              "mass": 7.342e22, "radius": 1.737e6, "luminance": 0,
              "unstableDirtySnowball": false, "hasAtmosphere": false, "atmosphereHSV": 0, "starHSV": 1707468,
              "axis": [0, 0, 0.5],
              "orbit": { "semiMajorAxis": 3.844e8, "eccentricity": 0.0549, "inclination": 0.0899, "argumentOfPeriapsis": 0, "longitudeOfAscendingNode": 0, "initialMeanAnomaly": 0 },
              "children": {}
            }
          }
        },
        "Mars": {
          "mass": 6.417e23, "radius": 3.390e6, "luminance": 0,
          "unstableDirtySnowball": false, "hasAtmosphere": true, "atmosphereHSV": 1336835, "starHSV": 1356697,
          "axis": [0, 0, 0],
          "orbit": { "semiMajorAxis": 2.279e11, "eccentricity": 0.0934, "inclination": 0.0323, "argumentOfPeriapsis": 0, "longitudeOfAscendingNode": 0.865, "initialMeanAnomaly": 0 },
          "children": {}
        }
      }
    }
  }
}

Inheritance & Time

• hostStar: resolved recursively upward to nearest luminance > 0 ancestor
• Position = parent.position + orbit(parent.mass, gameTime)
• Root fixed at (0, 0, 0)
• gameTime drives orbital motion; daylightTime (0~24000) drives rotation/zenith
• Moon phases auto-computed from observer-satellite-star geometry

Atmosphere System

Each body can define atmosphere color (HSV) and its own color (HSV), both as packed ints:

atmosphereHSV / starHSV = (H << 16) | (S << 8) | V

Example values:

Pack/unpack via CelestialBody.packHSV(h, s, v) and getHueFloat/getSaturationFloat/getValueFloat.

Glow computation (computeGlowColor): halo color = starlight × atmospheric scattering. Hue shifts toward atmosphere; saturation and brightness slightly boosted.

Rendering layers:

1. renderSkyDisc       → vanilla biome sky background
2. Bodies + stars      → luminous / non-luminous + star catalog
3. renderAtmosphereOverlay → translucent full-sky atmosphere (fades at night)

Celestial Textures

Body textures are organized by category, auto-discovered by celestial.json atlas:

assets/<namespace>/textures/environment/celestial/
├── luminous/<name>.png       # Luminous bodies (single image, no phases)
├── non-luminous/<name>/      # Non-luminous bodies (8 moon phase PNGs)
└── halo.png                  # Glow texture (grayscale radial falloff)

Luminous bodies need one PNG in luminous/. Non-luminous bodies need 8 moon phase PNGs in non-luminous/<name>/. readstar:luminous/white_sun.png is a placeholder — replace with your own.

Resource Pack

Star Catalog

Path: assets/readstar/custom/stars/stars.json

Data Sources

Star catalogs are auto-generated by scripts in .data/:

Color pipeline: Prefer Gaia GSP-Phot effective temperature → Planckian blackbody → sRGB; fallback to bp_rp color-index mapping. 12 brightest stars (Sirius, Vega, etc.) retain BSC5 data due to Gaia detector saturation.

JSON Format

{
  "Stars": [
    {
      "name": "Sirius",
      "position": [-0.1875, -0.2876, 0.9392],
      "Vmag": -1.46,
      "color": 4294967295
    }
  ]
}

Custom Moon Textures

Core feature: add 8 moon phases to any celestial body by simply placing PNG files — zero code.

Directory

assets/<namespace>/textures/environment/celestial/non-luminous/<body-name>/

• <body-name> must match system.json name in lowercase
• Requires exactly 8 PNGs with fixed filenames:

Requirements

Example: Jupiter

assets/readstar/textures/environment/celestial/non-luminous/jupiter/
├── full_moon.png
├── waning_gibbous.png
├── third_quarter.png
├── waning_crescent.png
├── new_moon.png
├── waxing_crescent.png
├── first_quarter.png
└── waxing_gibbous.png

Also define Jupiter's orbit in system.json (see example above).

How It Works

1. PNGs under textures/environment/celestial/ auto-discovered by Minecraft's atlas system
2. ReadstarSkyRenderer scans moons/ subdirectories and builds GPU buffers by group
3. Runtime matching by body.name

Other mods can override textures via resource pack priority.

Sprites & Atlas

Star Atlas

assets/readstar/atlases/star.json declares readstar:star source. At runtime, all color values from stars.json tint base stencils per-pixel:

Each color → 4 sprites: color_{c}, glow_low_{c}, glow_med_{c}, glow_high_{c}.

Requirements: RGBA 32-bit, 32×32, black edges, glow brightness suppressed ×0.35.

Celestial Atlas

assets/readstar/atlases/celestial.json uses minecraft:directory source, auto-scanning all namespaces under textures/environment/celestial/.

Network Sync

Server loads data/readstar/celestial/*.json and broadcasts via readstar:planet_system packet. Client rebuilds the celestial tree in CelestialBodyManager.initializeFromJson().

Build & Troubleshooting

./gradlew build          # Build JAR
./gradlew runData        # Data generation (atlas config)
./gradlew runClient      # Run client
./gradlew runServer      # Run server

Output: build/libs/readstar-*.jar

© 2026 FrozenStream. Licensed under MIT.