@azohra/meteo.j2k
A JPEG 2000 (ITU-T T.800) decoder in pure TypeScript, scoped to exactly
the codestream subset ECCC's GRIB2 feeds ship, justified by
explainability: every marker, MQ context, and lifting step lives in ten
small modules with its clause of T.800 named. It is the production
codec behind @azohra/meteo.grib/j2k-node, and owning the
internals pays twice: decodeJ2kRegion decodes only the codeblocks a
few requested gridpoints touch (bit-identical to the full decode at
those points, ~16× faster per core on the largest ECCC field), and the
worker pool fans one field's independent EBCOT codeblocks across threads
for full decodes. Benchmarks, with dates and method:
https://meteo.azohra.com/docs/j2k/performance/.
Zero runtime dependencies, no Node APIs in src/.
pnpm add @azohra/meteo.j2k
Decode a real field
The workspace's golden corpus carries real ECCC messages; the codestream is the GRIB section 7 payload (DRT 5.40). This decodes one and reads a sample:
// decode-fixture.mjs — run inside j2k/ after `pnpm build` (and a grib build)
import { readFileSync } from "node:fs";
import { parseFields, splitMessages } from "../grib/dist/index.js";
import { decodeJ2k } from "./dist/index.js";
const bytes = readFileSync("../grib/test/fixtures/geps-orog-m00.grib2");
const [field] = parseFields(splitMessages(bytes)[0]);
const codestream = field.section7.subarray(5); // DRT 5.40: raw J2K after the section header
const { values, width, height, bitsPerSample, isSigned } = decodeJ2k(codestream);
console.log(`${width}x${height} = ${values.length} samples, ${bitsPerSample}-bit ${isSigned ? "signed" : "unsigned"}`);
console.log(`first samples: ${Array.from(values.slice(0, 4)).join(", ")}`);
720x361 = 259920 samples, 12-bit unsigned
first samples: 1166, 1166, 1166, 1166
When only a few gridpoints matter, decodeJ2kRegion(codestream, indices)
returns the same integers decodeJ2k would put at those raster indexes
while entropy-decoding only the codeblocks the points touch.
The decoder covers the measured subset the feeds ship and nothing more
(everything outside it fails loudly with a named UnsupportedJ2kError);
how every configuration is accepted — including region decode's exactness
contract — is at https://meteo.azohra.com/docs/j2k/correctness/.
Documentation
The reference lives in docs/ (the subset, the two-ring
correctness gate, and the measured performance each have a page) and is
served at https://meteo.azohra.com/docs/j2k/.
MIT © Justin Watts