dtourolleandClaude Opus 5
e14bc34a9e
A crystal is a fixed size in micrometres. How grainy a photograph looks is therefore not a property of the emulsion alone -- it is film size against output size, and the frame is the half a digital file cannot supply. This assumed 35 mm for everything. The same emulsion on 4x5 averages about 3,800 crystals into the pixel that holds 300 on 35 mm, so it renders roughly 3.5 times smoother at the same print; every large-format photograph was being rendered as grainy as a half-frame. `Format` now carries the real image widths -- the gate, not the nominal inches, since a "4x5" exposes about 121 mm -- and the film node asks for it. It is a genuinely fixed list, unlike the stocks, so it is a declared `enum` parameter and gets its control, its sidecar entry and its undo step for nothing. It is also the first enum in the develop chain, and it broke two tests by being one. A row has to compare equal to itself across two builds or `sync_rows` replaces it on every parameter event -- destroying the elements built from it, including whichever TouchArea holds the current gesture, so the format picker would have fought every slider drag in the panel. `ModelRc` compares by identity and the row built a fresh choices model each call. `no_choices` already shares one empty model for exactly this reason, and the build site already said "see no_choices for why the identity matters". The fix follows it: memoise the model per variant list. Curve rows solve the same problem the other way, writing values through the existing model, which is not needed here -- a variant list is fixed at compile time, so one model can serve forever. 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°.