Concepts

The vocabulary every document and every SDK in this project uses. Where a language's conventions differ in casing or style, follow the language; do not rename the concept.

Gaussian — one splat: a position, scale, rotation, colour and opacity, plus the temporal fields that say when it exists and how it moves. The atom of the format.

Scene — everything one .4dgs file describes: its gaussians, its timeline, and optionally its audio and camera.

Scene clock — time in seconds from 0, the single timeline everything in the file refers to. When the file has audio, the audio track is the clock's master.

Validity window — the half-open interval [win_lo, win_hi) during which a gaussian exists at all. The format's only hard temporal gate: outside its window a gaussian is not faded, it is absent.

Marginal — the soft temporal weight of a gaussian at a time, from its birth time mu_t and temporal extent sigma_t. Multiplies opacity; a gaussian whose marginal falls below the file's cutoff is not drawn.

Chunk — the gaussians of one time interval, stored together and independently decodable. The unit of seeking and of transfer.

Chunk index — the map from time to chunk byte ranges. What makes an instant cheap to display without reading the file.

Attribute stream — one attribute of one chunk, stored structure-of-arrays: all the positions, then all the scales, and so on. The unit of compression.

Record — a length-prefixed, opcode-tagged top-level structure. Everything in the file is one, which is why unknown ones can be skipped.

Profile — a named promise about what a file contains beyond the base format, so a consumer can reject an unsuitable file before decoding it.

Bounds — the maximum deviation a decoder may observe for each attribute, declared in the file by the encoder that verified them.