The HDF5 library does not implement the scale-offset filter's floating-point
E-scale mode. When asked for it (cd_values[0] = 1) it stores the chunk raw
(no minbits/minval header) and sets the chunk filter mask to skip the filter,
so such datasets read back verbatim purely by honoring the per-chunk filter
mask — no E-scale decoder is required.
Add a fixture written via the HDF5 low-level API (exact-representable values)
and a test asserting it reads back verbatim, locking in the filter-mask path.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
