# Film stocks

One file per stock in [`profiles/`](profiles/). Adding a stock is adding a
file — no code change, no shader, no new operation — because 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`](src/bake.rs) for the argument:

1. **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.
2. **Three 1D curves**, log exposure to density, sampled at 256 points — one
   row per development time the datasheet measures. Push picks between the
   rows, and interpolating them is exact, because density is linear in push
   between two measured processes.
3. **One 32³ lookup**, density to linear sRGB — dye absorption, the viewing
   illuminant and the chromatic adaptation, all of which take exactly three
   numbers in. A printed negative is two: the film's cube ends at the paper's
   log exposure through the negative, the enlarger's exposure is added there,
   and the paper's own curve row and cube take it to linear sRGB.

The stock is the last thing that happens to the picture. It runs in the view
transform's place (D19): handed linear sRGB, scene-referred, after every other
adjustment and after sharpening and noise reduction, and handing back the
rendering the output transform encodes. So every other slider decides the
exposure the negative receives, and the default tone mapping is not applied
on top.

Per pixel that is a matrix multiply, a handful of curve taps and one texture
fetch — two for a print. 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.

**No slider is baked.** Camera exposure is a gain before the matrix, push
chooses between curve rows, print exposure is the addition between the two
cubes, and format sets the grain; each reaches the shader as a uniform that is
linear in what it does. That is what lets a mask layer hold its own film
settings, and a pixel under several layers take the weighted average of them.
Only the enlarger's filtration is solved at bake time, against the
photograph's exposure — an enlarger has one filtration for the whole print —
so the film's Exposure, set on the whole photograph, is the one slider that
rebakes. The stock and its
paper are the photograph's; a layer has no picker.

## Adding a stock

If spektrafilm has it, add its name to `STOCKS` in
[`tools/film-profiles/convert.py`](../../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`](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](https://github.com/andreavolpato/spektrafilm) by Andrea Volpato,
licensed CC BY-SA 4.0. See [`profiles/LICENSE-PROFILES.txt`](profiles/LICENSE-PROFILES.txt)
for the licence and [`profiles/CHANGELOG.txt`](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°.