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A layer offered the film's sliders and they moved nothing: its copy of the node was never given the stock, so it stayed inactive. Film now works in a layer the way the other adjustments do, as offsets to the photograph's settings, but blended as settings rather than as results, since a film is a rendering and cross-fading two developments is not what a region on a pushed film looks like. - dr-film bakes no slider. Exposure is a gain in the shader; push interpolates the stock's measured processes, one curve row each; the print is split at the paper's log exposure, so print exposure is an addition between two lookups and exact at any setting. The enlarger stays balanced at the photograph's exposure. - film_sim reads all four settings as uniforms, format one-hot over a grain count per format, so every uniform is linear in what it does. - Operation::blends_settings lets the composer average each overlapping layer's uniforms with the global ones by mask weight, the global setting taking whatever weight the layers leave, and run the fragment once. Three layers at full weight give the mean of their settings. - The stock picker is hidden on a layer. Only the photograph's exposure re-solves the print balance; push, print exposure and format need no rebake at all now.
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°.