BufferGeometry offsets and indices
buffer-geometry, indices, javascript, three.js
Solution
There are two ways to defining geometries:
Non-Indexed
"vertices": [ 0, 1, 2, 3, 4, 5, 6, 7, 8, ... ],
"normals": [ 0, 1, 2, 3, 4, 5, 6, 7, 8, ... ]
In this mode every triangle position is as defined and you can't reuse data.
triangle 0: [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]
Indexed
"indices": [ 0, 1, 2, ... ],
"vertices": [ 0, 1, 2, 3, 4, 5, 6, 7, 8, ... ],
"normals": [ 0, 1, 2, 3, 4, 5, 6, 7, 8, ... ]
In this mode, indices define the order of the data. The first triangle is using indices `0`, `1`, and `2`. These indices will be used to fetch the `vertices` and `normals` data:
triangle 0: [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]
The main benefit of indexed is the possibility of reusing data and uploading less data to the GPU:
"indices": [ 0, 0, 0, ... ],
"vertices": [ 0, 1, 2, 3, 4, 5, 6, 7, 8, ... ]
triangle 0: [ 0, 1, 2, 0, 1, 2, 0, 1, 2 ]
As per offsets...
With offsets you can render specific ranges of your geometry. Instead of drawing from `triangle 0` to `triangle.length`, you can draw from `triangle 200` to `triangle 400`.
Problem
I was just wondering for a while now what exactly "offsets" and "indices / index" are. Offsets are e.g. mentioned in https://github.com/mrdoob/three.js/blob/dev/src/core/BufferGeometry.js and indices were mentioned in IndexedGeometry, however I can't currently find it anymore in the dev tree. Although indices seem rather obvious and although I could dive into the code to figure out some maybe-correct answer for myself I'd love to hear an "official" statement :) Thanks!