Immersive visualization glossary

Definitions of key terms about Gaussian Splatting, digital twins, photogrammetry and immersive visualization.

Gaussian: 3D particle with position, covariance (shape), color and opacity. The basic unit of representation in Gaussian Splatting.
Covariance: 3×3 matrix defining the shape and orientation of a Gaussian. Controls how it stretches and rotates in 3D space.
SfM (Structure from Motion): Technique for estimating camera positions and 3D geometry from multiple 2D images. COLMAP is the reference tool.
Differentiable rasterization: Process of rendering Gaussians to 2D while allowing gradient propagation back to 3D parameters. Essential for training.
Densification: Iterative process that adds Gaussians where detail is missing and removes redundant ones during training.
Alpha blending: Composition of back-to-front sorted Gaussians using opacity. Each Gaussian contributes to the final color based on its α.
Spherical harmonics (SH): Mathematical functions that allow a Gaussian's color to depend on the viewing angle, enabling reflections and view-dependent effects.
NeRF (Neural Radiance Field): Prior technique that represents 3D scenes as a neural network. Slower in rendering but similar in quality.
Radiance field: Function describing light radiation at every point and direction in space. Both NeRF and Gaussian Splatting are radiance fields.