Realistic star rendering method developed for the Celestia Project to assist with the transition to gamma corrected (sRGB) rendering. It relies on the photopic point source function (PSF) by Spencer et al. 1995. In its original form, it could be used for convolution at each frame, which is slow, or it could be cropped, which would result in jagged edges. An approximating function has been found that does not alter the appearance, but is simple and spatially bounded.
Brightness of a light source (radiance, in terms of dimensionality) is expressed in peak value units. A radiance value of 1 distinguishes between two rendering modes:
- Point mode: a circular linear distribution with a diameter of several pixels.
- Eye PSF mode: an approximation of the actual PSF with pre-calculated exact size.
On other programming languages, the method can be implemented in two separate shaders. Gamma correction is applied at the end.
The interface is provided through class RenderEngine, which is located in CSR.py. Draw a light source on a NumPy array using the draw_source() method. Check out the comments in the code for details.
Vega's color is used as an example. The values were calculated from the spectrum using TrueColorTools.
The radiance increases logarithmically.
The model depends on two parameters: "optimization" and the radius of the point mode. With zero optimization, the model, like the original PSF, is not size-constrained.
The transition peak radiance of 1 is represented by 0 magnitude.
10,000 random light sources render in less than a second on any device (except for potatoes).
Python version 3.11 or higher is required. Works on Windows/macOS/Linux.
- Clone the repository or download the archive using the GitHub web interface;
- Open the console in the project root folder;
- Create a virtual environment with
python3 -m venv .venv; - Install the dependencies with
.venv/bin/pip install -r requirements.txt; - Execute an example script with
.venv/bin/python example_animation.py.
In Celestia, as in many other programs, the rendering of stars was implemented approximately, without checks for physical accuracy. Recently implemented gamma correction enhances the effect, making the star field appear flat.
The problem was already noticed and described by Chris Layrel, 2010 forum discussion is here and led to this branch. But it was "buggy and slow", with squares around bright stars.
To address these issues, I proposed the initial approaches in 2023-2024 (see legacy_algorithms.py). The repository and the method were redesigned in 2026.
Not vibe coded!