SparkJS ExtSplats

ExtSplats is the Spark 2.0 preview extended splat container using 32 bytes per splat (vs 16 for PackedSplats), stored across two Uint32Array

Source: https://sparkjs.dev/2.0.0-preview/docs/ext-splats/ Version: Spark 2.0 preview

ExtSplats is the Spark 2.0 preview extended splat container using 32 bytes per splat (vs 16 for PackedSplats), stored across two Uint32Arrays.

Byte Layout (32 bytes = 2 × uvec4 = 8 × uint32)

extArrays[0] (bytes 0–15):

Offset	Field	Size	Description
0–3	center.x	4B	float32 bits (uintBitsToFloat)
4–7	center.y	4B	float32 bits
8–11	center.z	4B	float32 bits
12–13	opacity	2B	float16
14–15	reserved	2B	unused

extArrays[1] (bytes 16–31):

Offset	Field	Size	Description
16–17	color.r	2B	float16
18–19	color.g	2B	float16
20–21	color.b	2B	float16
22–23	ln(scale.x)	2B	float16 (decoded with exp)
24–25	ln(scale.y)	2B	float16
26–27	ln(scale.z)	2B	float16
28–31	quaternion	4B	packed oct+angle (10/10/12 bits)

Total: 32 bytes/splat

Key differences vs PackedSplats

Property	PackedSplats	ExtSplats
Size	16 B/splat	32 B/splat
center	float16	float32 (higher precision)
opacity	uint8	float16
RGB color	uint8 (sRGB)	float16
scales	uint8 log (e^-12..e^9)	float16 log
quaternion	oct88 + u8 angle	oct10/10 + 12-bit angle

Creating

const extSplats = new ExtSplats({
  url?: string,           // .ply, .spz, .splat, .ksplat, .rad
  fileBytes?: Uint8Array,
  extArrays?: [Uint32Array, Uint32Array],
  numSplats?: number,
  construct?: (splats) => void,
  lod?: boolean | number,
  lodSplats?: ExtSplats,
});

Encoding / Decoding

import { utils } from "@sparkjsdev/spark";

utils.encodeExtSplat(extSplats.extArrays, index, x, y, z, sx, sy, sz, qx, qy, qz, qw, opacity, r, g, b);
const { center, scales, quaternion, color, opacity } = utils.decodeExtSplat(extSplats.extArrays, index);

extSplats.forEachSplat((index, center, scales, quaternion, opacity, color) => { ... });

SH Layout (extra property)

SH3 split into sh3a + sh3b (different from PackedSplats):

Buffer	Words/splat	Coefficients
sh1	4 (16B)	sh1_0, sh1_1, sh1_2, sh2_0 (4th word reused by SH2)
sh2	4 (16B)	sh2_1..sh2_4
sh3a	4 (16B)	sh3_0..sh3_3
sh3b	4 (16B)	sh3_4..sh3_6

Each RGB coefficient: 8-bit magnitude per channel + 5-bit shared exponent + 3 sign bits = 4 bytes/coefficient.