Starlight for Godot 4.1

(Planet rendered using Atmosphere Shader v0.4)

Starlight is a Godot addon that renders 100 000 stars in realtime, with low performance cost. It's an alternative to using a skybox, and also may be relevant to anyone making a space game.

Check out the demo in your web browser: https://tiffnix.com/starlight-demo/

Features

• Stars are rendered positionally in 3D, allowing you to fly around and see stars go by.
• Exact position, luminosity, and temperature of each star can be configured by you.
• Physically based light model: Using a Point Spread Function (PSF), rather than a texture that grows or shrinks with distance/brightness.
• Based on MultiMeshInstance3D for performance.
• Uses a trick to avoid being clipped by the far plane, to let stars be very far away.
• Works with Forward+, Mobile, and Compatibility renderers.
• Comes with a random star generator based on main sequence stars (classes M through O).

Usage Guide

To get started, insert Stars.tscn into your scene. By default, nothing will be visible. You can attach the script StarGenerator.gd to randomly generate stars. The default will be 10 000.

StarGenerator

This script procedurally generates stars in a ball centered on the origin. It has these properties:

• size: Stars are spawned inside of a sphere of this radius.
• star_count: The number of stars to create.
• rng_seed: The random seed to use when generating stars. Incrementing this will give you a different sky.
• generate_at_origin: If checked, one extra G-type star will be generated exactly at 0, 0, 0. This can be useful for representing the Sun in your scene.

For further customization of the star generator, I recommend editing the script directly.

StarManager / Stars.tscn

There is only 1 script property exported, which is which shader to use. The default points to Star.gdshader. You can fork the shader easily by changing this.

The script also forwards the shader parameters which you can edit directly.

If you want to load your own star catalog, or use a custom random star generator, you will need to call set_star_list() with an array of Star.

The Star constructor takes 3 arguments:

1. position: 3D position in model space units.
2. luminosity: Luminosity in solar luminosity. Approximately 1.0 for Sol.
3. temperature: Effective temperature in Kelvin. Approximately 5778 for Sol.

Star.gdshader

The following visual properties are exposed:

• emission_energy - Multiplier for how bright stars should be. Generally this is some extremely large number like 500000000 - you'll need to add or remove zeros until it looks right.
• color_gamma - How strongly colors should show through. A value of 1 should be close to real life, while a very high value of 5-10 resembles what you see in false-color images from telescopes. A value of 3 or 4 is a good balance.
• billboard_size_deg - This controls how much of the screen the PSF texture takes up, in degrees. For the default JWST PSF I recommend a value of around 90.
• meters_per_lightyear - This is a scaling setting, you'll need to set it depending on how far away you want your stars to be.
• luminosity_cap - This is the maximum brightness a star can have. The main usage of this is to prevent the PSF texture from being blown out and showing as a white square. This mainly happens when very close to stars.
• texture_emission - This is the actual PSF texture. The default one is the PSF from the James Webb Space Telescope, because it looks cool. There are a few others in the psf-textures folder which can be used instead.
• clamp_output - Clamps the output from 0 to 1 when enabled. Can be useful depending on how your HDR is setup.

In order to have good performance, the PSF texture needs to be cropped depending on how bright the star is on screen. The majority of stars only appear as a couple of pixels in size, as opposed to covering almost the entire screen for a star you're very close to. This cropping behavior directly affects how much overdraw there is, which can massively impact performance. You will need to adjust these properties if you change the PSF texture from the default JWST one. To control this behavior, the shader has these properties:

• min_size_ratio - This is the minimum size that a star can render at. This corresponds to the innermost bright spot of the PSF texture, and is usually a very small value like 0.003.
• max_luminosity - This is the point at which the cropping stops and the full PSF texture is used. To set this value, try to find the point at which the PSF texture is fully visible, specifically things like diffraction spikes reach the edge of the texture. Then adjust max_luminosity until it's just below that point.
• scaling_gamma - Diffraction spikes usually fall in brightness according to distance^2 from the center of the texture, which means a value of 0.5 is ideal. You may need to use other values depending on your PSF texture. For a perfect airy disk in particular, the falloff is faster than quadratic.
• debug_show_rects - This can be useful while tweaking any of these values. It helps visualize whether any stars are being over-cropped, or if there is too much overdraw in your scene.

Be careful when tweaking these values. You may want to reduce the star count to something more manageable like 1000 or 10,000 while doing this. Setting scaling_gamma to 0 by accident, for example, can crash Godot, your graphics drivers, or even your entire PC.

The shader also requires knowledge of the camera FOV to work correctly. StarManager tries to automatically find the Camera3D of the current viewport, but in some cases this auto-detection may be wrong, and you will need to edit StarManager.gd for your needs. It also does not display correctly in the editor due to it using different FOV settings from the scene.

Credit

Code is released under MIT license.

The default PSF texture, jwst.exr, is based on FITS data obtained from here. Code for cropping, downscaling, and converting to OpenEXR is located in docs/fits2exr.py.

The alternative PSF textures hst.exr, hex_aperture.exr, and airy_disk.exr were created using Poppy based on examples in the documentation. Code is located in docs/poppy psfs.ipynb.