dtourolleandClaude Opus 5
ff1a3e0e80
Choosing Ilford HP5 Plus showed an inverted grey frame. So did Double-X. They are negatives, and upstream leaves `target_print` null on every monochrome stock, so nothing was ever printed and the scan was all there was. A colour negative at least announces itself -- the orange mask says plainly that you are looking at a negative. A monochrome one just looks broken. They print on Kodak 2302 now, which is a monochrome print film and is what such a negative is actually printed onto; Double-X onto 2302 is the standard cine chain. For the Ilford stocks it stands in for an Ilford paper, which nobody has measured, and is at least the right kind of material. The scan is still reachable through the Scanned/Printed toggle. It is a thing to choose now rather than the only thing on offer. `every_shipped_stock_bakes` did not catch this, and could not: it derives "should this be inverted?" from the stock's kind *and whether it names a paper*, so it looked at an inverted HP5, concluded that was right for an unprinted negative, and passed. The assertion was self-consistent and the situation was still wrong. The new test asserts the thing that actually matters -- a camera negative must name a paper, that paper must be a printing stock, and it must be the same kind of material, so a monochrome negative cannot end up on colour paper. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Film stocks
One file per stock in profiles/. Adding a stock is adding a
file — no code change, no shader, no new operation — for the same reason
dr-decode's base curves work that way: under the GPLv3 a stock should be
contributable without a release.
What a profile is
Three measured tables, all of them published in the manufacturer's datasheet:
| Field | What it decides |
|---|---|
log_sensitivity |
what each emulsion layer sees, per wavelength |
density_curves |
contrast, latitude, and where the stock clips |
dye_density |
what the developed stock looks like, per wavelength |
base_density |
the support: film base, and a colour negative's orange mask |
Plus kind (negative or positive), support (film or paper), and the two
illuminants the data is referenced to. A print paper is a stock like any
other; support exists so an interface can offer papers separately, not
because the renderer treats them differently.
Why it is not a LUT
Because the parameters stay physical. Opening up a stop moves the picture along the film's own characteristic curve — toe, shoulder and all — instead of scaling a number somebody baked at one exposure. A scanned negative comes out orange and inverted because that is what a negative is, and it becomes a photograph when a paper profile prints it, exactly as it would in a darkroom.
The data cost runs the other way from a LUT collection too: a stock is about 17 kB of measurements, where one HaldCLUT is roughly 800 kB of one person's grade.
How it runs
The spectral chain reduces to three tables, and the reduction is exact where it
matters — see src/bake.rs for the argument:
- A 3×3 matrix, linear sRGB to the three layers' exposure. Exact, not an approximation: the reconstructed scene spectrum is linear in the sRGB triple, so the integral collapses into nine numbers.
- Three 1D curves, log exposure to density, sampled at 256 points.
- One 32³ lookup, density to linear sRGB — dye absorption, the print through the negative, the paper, the viewing illuminant and the chromatic adaptation, all of which take exactly three numbers in.
Per pixel that is a matrix multiply, three curve taps and one texture fetch. Splitting 2 from 3, rather than baking one LUT over exposure, is measured rather than assumed: the curve carries all the sharp shape and the dye mixing is smooth, so folding the curve into the 3D lookup would need it three times larger for the same error. At 32³ the worst interpolation error is about 0.003 in linear sRGB, below one 8-bit code value, and there is a test that says so.
Adding a stock
If spektrafilm has it, add its name to STOCKS in
tools/film-profiles/convert.py and
re-run it. Otherwise write the YAML by hand from the datasheet; the loader
validates the table lengths and says which file and field is wrong.
Either way, list it in BUILT_IN in src/lib.rs to compile it
in — or drop it in the profile directory at runtime, which is the path meant
for stocks that ship separately from the binary.
Provenance
The shipped profiles are converted from
spektrafilm by Andrea Volpato,
licensed CC BY-SA 4.0. See profiles/LICENSE-PROFILES.txt
for the licence and profiles/CHANGELOG.txt for what
the conversion changed and what it deliberately did not.
The sRGB reflectance basis is Mallett & Yuksel (2019); the observer is the CIE 1931 2°.