The subset

The decoder covers exactly the codestream shape listed below. The feeds ship two codestream lineages, and every one falls inside that shape. The first is the twelve OpenJPEG-encoded ECCC fields of the twenty-message golden corpus (GDPS, GEPS, HRDPS, RDPS, REPS, RAQDPS; the corpus’s NOAA messages carry no JPEG 2000). The second is the JasPer-encoded shape that a live RDPS CAPE field surfaced on the day this decoder went to production.

• J2K Part 1 raw codestream. There is no JP2 container, because GRIB embeds bare SOC..EOC.
• One tile covering the whole grid, with one tile-part.
• One grayscale component with no subsampling. The feeds show 12/16/20/24-bit unsigned samples. The decoder accepts any depth up to 28 bits, signed or unsigned. The ceiling comes from the int32 coefficient carrier. Tier-1 carries magnitudes doubled, and 28 bits is the most that keeps every subband’s magnitude inside it.
• Reversible 5/3 wavelet with no quantization (QCD style 0). The decoder honours a tile-part QCC that restates style-0 quantization for the one component. The JasPer fields ship one whose exponents differ from the QCD’s.
• One quality layer and DEFAULT precincts. That is the implicit 2^15 grid, which gives one whole-tile precinct for every gridded field and several for JasPer’s one-row bitmapped fields (813275×1 is 25 precincts at r=5).
• Default codeblock style and 64×64 codeblocks.
• 5 decomposition levels. Any level count in [0, 32] is accepted, because the loops are generic.
• No ROI.

Loud failure is the design

Everything outside the subset fails with a named UnsupportedJ2kError that says which feature it met and why it is unsupported. That covers multiple tiles, tile-parts, components, or layers, the 9/7 irrational wavelet, explicit precinct partitions, RGN/COC/POC, main-header QCC, PPM/PPT packed headers, every non-default codeblock style bit by name, SOP/EPH markers, and quantization.

The scope is narrow on purpose. A tripped guard means a feed changed shape, and the decoder should fail loudly rather than fall into a silent slow path. The guards live in src/codestream.ts, and the error vocabulary (UnsupportedJ2kError, J2kFormatError) lives in src/errors.ts.

The progression-order guard is looser than the rest. It accepts the three resolution-major orders interchangeably, because with one layer and one component they emit the same resolution-then-precinct packet sequence. It accepts the position-major orders (PCRL/CPRL) only while every resolution has a single precinct. With one position to walk, all five orders emit the same packet sequence.

The JasPer story

A loud failure caught the JasPer shape on adoption day. Most ECCC fields are OpenJPEG-encoded, but the live RDPS smoke surfaced a CAPE field whose COM marker says “Creator: JasPer”. A guard rejected it, and the subset was then extended to cover it and oracle-verified. It differs in three load-bearing ways from every fixture in the golden corpus.

• Its geometry is bitmapped and one row high. The coded values are flattened to an 813275×1 image (section 6 bitmap; the grid is 1140×1045).
• It has multiple precincts. 813275 exceeds the default 2^15 precinct, so the upper resolutions have several precincts (25 at r=5) and the one-packet-per-resolution degeneracy breaks. This fixture is why packets.ts walks the precinct grid.
• It restates quantization in a tile-part QCC, with different subband exponents than the main QCD. Mis-honoring it mis-decodes every codeblock. Its samples are also 24-bit, deeper than the 20-bit RAQDPS corpus fixture.

The fixture (rdps-cape-sfc-jasper) is committed beside the golden corpus with its own oracle answers. The full provenance is in grib/test/fixtures/README.md, and Two-ring correctness covers how it is gated.