The header-only path added in #755 answers for a Fuji RAF, whose preview pointer sits in the first
bytes. A Canon CR3 keeps its preview in a JPEG track, and a TIFF raw (CR2, ARW, DNG, RW2, 3FR …) can
keep its preview tag in an IFD the prefix cuts short: both fell back to `std::fs::read` of the whole
file — tens of megabytes for a JPEG of a few, on the format of the largest bucket in gaps.md (Canon,
33 % of photographers).
`preview_window` now asks `lightcraft_raw::sparse::preview_plan` — the module #961 added for the
import path, which had no production caller — which ranges `embedded_preview` reads, and reads
exactly those: `planned_preview` fills a buffer of the file's length, reads the ranges the plan names
and asks again until it says `Ready`. The buffer is the file's length, which the memory gate counted
already, and its untouched pages stay zero, so what this path costs is the bytes read — on macOS a
freed buffer of that size may sit in the allocator's cache rather than go back to the OS (measured on
a 38k-photo import, #832). The loop gives up past 60 % of the file, the bound step 2's partial read
uses, and a plan that does not fit, a file past 1 GiB or a window that does not decode all end in the
whole-file fallback.
The quick path hands the JPEG to the decoder alone, so two container facts the whole-file path applies
have to be visible in the bytes it is handed, or the file declines and the whole file keeps the
preview:
- a preview colour space of Adobe RGB (`decode_raw_preview` applies it; a JPEG-only decode has the
JPEG's own space, else sRGB). A container that says sRGB does not decline — that is what the decode
assumes anyway, and declining it took the fast path away from a real Sony ARW;
- the file's own EXIF orientation, when the preview JPEG carries none of its own: the plan's header is
a bounded 64 KiB (the header route grows its prefix to the preview's declared extent instead), and
an orientation tag it cannot see would show the preview unrotated.
Measured over the whole CC0 corpus (111 raws): 52 files take this route, reading a median 7.8 % of
their bytes (0.9-27.7 %) — RW2 3.3 %, Leica RWL 4.1 %, ARW 7.5 %, CR3 8.4 %, DNG 9.7 %, CR2 14.3 % —
while the 21 RAFs stay on the header route (median 4.5 %). The other 38 read whole: NEF, NRW, ORF, PEF
and SRW because the planner does not describe them (an SRW plans to the whole file, which the 60 %
budget then declines), plus the ARWs, CR2s, CR3s and DNGs that declare Adobe RGB or whose plan wants
more than that budget — the bytes they read are the ones the plan would have read anyway. Across all
111 files the plan handed on the same preview as the whole file, with the skipped bytes zeroed and
again 0xFF. `LIGHTCRAFT_PROFILE=1` prints
[profile] preview …/arw-sony-a7rm4a-compressed.arw: read 688128 of 61832192 bytes (plan, window 483441)
Tests: a synthetic TIFF/DNG whose preview only its tags locate (identical pixels to the whole-file
path, the CFA strip unread), the same structure without the tag that makes it a raw, a file whose
orientation only the whole file shows, and a file that states Adobe RGB against one that states sRGB;
the opt-in real-file test reports which path answered.
Co-Authored-By: DeepSeek V3.2 <noreply@deepseek.com>
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