Version 1.21.1 is no longer updated.

ReadStar — NeoForge 26.1 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

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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.

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Configuration

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

Option	Default	Description
starCoreSize	0.648	Star core quad size multiplier
starGlowSize	1.5	Bright star glow quad size multiplier
starFovCompensationStrength	0.8	Star size FOV compensation. 1.0 = full, 0.0 = none
starFovBrightnessStrength	1.0	Star brightness boost when zoomed in. 0.0 = none
celestialApparentSizeFactor	4000.0	Celestial body apparent size factor
celestialApparentSizeMin	1.024	Minimum apparent size clamp

FOV-Aware Rendering

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

Attribute	Meaning	Behavior
Position (vec3)	Celestial sphere coordinate, shared by 4 vertices	Transformed by MVP, naturally affected by FOV
Offset (vec3)	Billboard corner offset, unique per vertex	Multiplied by FovCompensation for size correction

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:

Parameter	Formula	Notes
Alpha	clamp(1 - max(0, Vmag-3)/12, 0.4, 1)	Full brightness at Vmag ≤ 3
RGB	clamp(1 - max(0, Vmag-1)/19, 0.7, 1)	Full brightness at Vmag ≤ 1
Size	clamp(1 - Vmag/12, 0.5, 1) × starCoreSize	Larger Vmag = smaller dot

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

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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

Field	Description
mass	Mass (kg). 0 = fixed at parent position
radius	Radius (m). Apparent size = max(1.024, radius / distance × factor)
luminance	Self-luminosity 0~15. >0 = star; children auto-resolve hostStar upward
unstableDirtySnowball	Comet flag. true renders dual-tail structure
hasAtmosphere	Has atmosphere (controls overlay and glow rendering)
atmosphereHSV	Atmosphere HSV color, packed int (H<<16 | S<<8 | V). H=hue, S=saturation, V=density (0~255)
starHSV	Body's own color, same encoding. Luminous = emission, non-luminous = surface reflection
axis	Rotation axis. Zero vector defaults to (0,0,-1)
orbit.semiMajorAxis	Semi-major axis (m). 0 = no orbit
orbit.eccentricity	Eccentricity. 0 = circular
orbit.inclination	Orbital inclination (radians)
orbit.argumentOfPeriapsis	Argument of periapsis (radians)
orbit.longitudeOfAscendingNode	Longitude of ascending node (radians)
orbit.initialMeanAnomaly	Initial mean anomaly (radians)

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

Component	Bits	Range	Meaning
H (Hue)	16-23	0~255	Hue (0=red → 255 wraps to red)
S (Saturation)	8-15	0~255	Saturation (0=gray, 255=pure color)
V (Value)	0-7	0~255	Density/brightness (atmosphereHSV=density, starHSV=brightness)

Example values:

Body	atmosphereHSV	starHSV	Notes
Sun	0	2172671	No atmosphere, G-type yellow-white
Earth	9738751	9732275	Blue sky (H=0.58, S=0.6, V=1.0)
Mars	1336835	1356697	Faint red-brown (V=0.01, very thin)

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.

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Resource Pack

Star Catalog

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

Data Sources

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

Script	Source	Output
generate_stars.py	BSC5 Bright Star Catalogue	stars_named.json (361 IAU-named) + stars_numbered.json (8043 HR-numbered)
gaia_download.py	Gaia Archive TAP API	gaia_bright_with_teff.vot (with effective temperature)
gaia_to_stars.py	Gaia DR3 + BSC5 fallback	stars_gaia_named.json (361) + stars_gaia_numbered.json (11809)

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
    }
  ]
}

Field	Description
name	Identifier (reserved, no runtime effect)
position	Unit sphere direction [x,y,z], normalized to distance 100. Y=North Celestial Pole
Vmag	Apparent magnitude (Gaia G-band or BSC5 V-band). Determines glow tier and brightness decay
color	ARGB color value; key for atlas sprite generation

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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:

File	Phase
full_moon.png	Full
waning_gibbous.png	Waning Gibbous
third_quarter.png	Third Quarter
waning_crescent.png	Waning Crescent
new_moon.png	New
waxing_crescent.png	Waxing Crescent
first_quarter.png	First Quarter
waxing_gibbous.png	Waxing Gibbous

Requirements

Spec	Value
Format	PNG, RGBA 32-bit
Size	16×16 or 32×32 recommended
Edges	Transparent (alpha=0)
Color	Self-colored; no runtime tinting

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.

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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:

Stencil	Path	Purpose
star_base.png	textures/environment/star/	Core (32×32 RGBA)
star_glow_low.png	same	Low glow
star_glow_med.png	same	Medium glow
star_glow_high.png	same	High glow

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/.

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Network Sync

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

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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

Symptom	Check
Purple/black star textures	Is star_base.png 32-bit RGBA?
Purple moon phase	Missing PNG or filename mismatch
Body not visible	Does it have hostStar (ancestor luminance>0)?
Glow/halo not rendering	Is halo.png under textures/environment/celestial/?
Atmosphere color wrong	Are atmosphereHSV / starHSV packed correctly?
Data pack not applied	/reload or restart
Resource pack not updating	F3+T
FOV compensation not working	starFovCompensationStrength > 0?

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© 2026 FrozenStream. Licensed under MIT.