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

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.

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.

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

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.

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/.

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

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?

© 2026 FrozenStream. Licensed under MIT.