dtourolleandClaude Opus 5
ce6458547a
Pushing was not a thing to simulate. It was measured data being thrown away: Double-X and 2302 each ship five characteristic curves, one per development time, and this shipped the 6.5-minute column and discarded four. All five now ship and interpolate. The axis is real. Double-X runs 4 to 12 minutes, and across it the average gradient goes 0.472 to 1.034 while Dmax goes 1.19 to 2.56. The control is in stops, because that is what a photographer means, and one stop is a factor of about 1.41 in time. That mapping is checked rather than assumed: against Double-X's own axis it lands within 2% of the 9-minute column for +1, and near 12 minutes for +2, which are the times the datasheet gives for exactly that. There is a test. **Pushing must not recover shadow detail, and this does not.** Across the whole measured range the speed point moves about a third of a stop while the gradient doubles; three stops under mid-grey, density goes from 0.008 to 0.035, which is still nothing. Developing longer multiplies what was already recorded and cannot record what never hit the film. A push built as added exposure or global contrast brightens those shadows instead and looks convincing until someone who shoots film sees it, so that property has a test of its own. Interpolated in *log* time, because development is multiplicative: 4 to 5 minutes is the same amount of push as 9 to 12, and interpolating linearly would bunch the control at one end. Clamped at both ends, because past the published range there is no data and extrapolating a contrast curve invents an emulsion nobody tested. A stock measured at one process ignores the control entirely rather than inventing a curve for it -- Portra 800's pushes are separate *measured* profiles, which is the honest way to offer those. Costs nothing per pixel and changes no shader. The curves are a per-stock table, so the interpolation happens on the CPU at bake time, where choosing a stock and moving its sliders already rebakes. The Vulkan shader is untouched. 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°.