DarkRoom/core/dr-film/src/lib.rs

//! TRACES: FR-DEV-3f
//! Spectral film simulation — what a stock would have done with this light.
//!
//! # What this is, and what it is not
//!
//! Not a look-up table someone graded by eye. Each stock here is its
//! manufacturer's own measurements — spectral sensitivity, characteristic
//! curves, dye densities — run through the physics: light exposes three
//! emulsion layers, the layers develop to densities, the densities are dyes
//! that absorb, and what is left is what reaches the eye. A colour negative
//! comes out orange and upside down because that is what a colour negative is;
//! it becomes a photograph when [`bake::Recipe`] prints it on paper.
//!
//! What that buys over a LUT is that the *parameters are physical*. Exposing a
//! stop over moves the picture along the film's real characteristic curve,
//! shoulder and all, rather than scaling a number someone baked at one
//! exposure. And the data cost is the other way round from a LUT collection: a
//! stock is about 17 kB of published measurements, where one HaldCLUT is 800 kB
//! of one person's grade.
//!
//! # The shape of the crate
//!
//! - [`profile`] — a stock, as measured. Data, contributable as a file.
//! - [`spectrum`] — the fixed colour science: observer, illuminants, basis.
//! - [`bake`] — the reduction to three tables a shader can run.
//!
//! No wgpu dependency, deliberately: what comes out is plain `f32` with a
//! documented layout, and every property worth asserting about the model is
//! asserted on the CPU.
//!
//! # Provenance
//!
//! The shipped profiles are converted from **spektrafilm** by Andrea Volpato
//! (<https://github.com/andreavolpato/spektrafilm>), licensed CC BY-SA 4.0 and
//! modified for DarkRoom — see `profiles/LICENSE-PROFILES.txt` and
//! `profiles/CHANGELOG.txt`. The conversion is reproducible from
//! `tools/film-profiles/convert.py` rather than pasted, so what changed is
//! auditable. The sRGB reflectance basis is Mallett & Yuksel (2019).

pub mod bake;
mod built_in;
pub mod grain;
pub mod profile;
pub mod spectrum;
pub mod tables;

pub use bake::{bake, Baked, Recipe};
pub use grain::Grain;
pub use profile::{Kind, Profile, Stage, Support};

use built_in::BUILT_IN;

/// Every stock that is compiled in, parsed on first use.
pub fn built_in() -> &'static [Profile] {
    use std::sync::OnceLock;
    static PARSED: OnceLock<Vec<Profile>> = OnceLock::new();
    PARSED.get_or_init(|| {
        BUILT_IN
            .iter()
            .filter_map(|(stock, yaml)| match Profile::parse(yaml) {
                Ok(p) => Some(p),
                Err(e) => {
                    // A compiled-in profile that does not parse is a build
                    // mistake, but refusing to start over one would take the
                    // whole application down for a stock nobody asked for.
                    log::error!("built-in film profile {stock} is malformed: {e}");
                    None
                }
            })
            .collect()
    })
}

/// The stocks a photographer can put in a camera.
///
/// Filtered on [`Stage`], not on [`Support`], and the difference is not
/// pedantry: Kodak 2383 is a *film* that a negative is printed onto, so a
/// picker built on `Support` would offer a projection print stock as something
/// to shoot on.
pub fn camera_stocks() -> impl Iterator<Item = &'static Profile> {
    built_in().iter().filter(|p| p.stage == Stage::Filming)
}

/// Find a compiled-in stock by its identifier.
pub fn find(stock: &str) -> Option<&'static Profile> {
    built_in().iter().find(|p| p.stock == stock)
}

/// The paper a stock should be printed on, if it names one and we have it.
///
/// A reversal stock names none, and needs none: it is the picture already.
pub fn default_print(film: &Profile) -> Option<&'static Profile> {
    film.target_print.as_deref().and_then(find)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn every_built_in_profile_parses() {
        assert_eq!(
            built_in().len(),
            BUILT_IN.len(),
            "a built-in profile failed to parse"
        );
    }

    #[test]
    fn a_negative_finds_the_paper_it_names() {
        let portra = find("kodak_portra_400").unwrap();
        assert_eq!(default_print(portra).unwrap().stock, "kodak_portra_endura");
    }

    #[test]
    fn every_shipped_stock_bakes() {
        // Cheap and worth it: a profile can parse and still be unusable — a
        // curve that never reaches the density its paper needs, a sensitivity
        // table that is all sentinel. Baking every one is the only check that
        // says the whole database renders.
        for film in camera_stocks() {
            let baked = bake::bake(&bake::Recipe::new(film, default_print(film)));
            let mid = baked.apply([bake::MID_GREY; 3]);
            assert!(
                mid.iter().all(|c| c.is_finite()),
                "{} rendered mid-grey as {mid:?}",
                film.stock
            );

            // And that it is a *photograph*. Finiteness alone passes for a
            // stock that renders every frame black or blown, which is the way
            // a constructed profile fails: the curve parses, bakes, and sits
            // entirely off one end of its own exposure range.
            let luma = (mid[0] + mid[1] + mid[2]) / 3.0;
            assert!(
                (0.02..0.75).contains(&luma),
                "{} put mid-grey at {luma:.3}, which is not a rendering of it",
                film.stock
            );

            // Monotone, too. A stock that darkens as it is exposed is either
            // inverted or broken, and either way is not what the picker
            // offered.
            let shadow = baked.apply([0.02; 3]);
            let highlight = baked.apply([0.8; 3]);
            let rising = highlight[1] > shadow[1];
            let inverted = film.kind == Kind::Negative && default_print(film).is_none();
            assert_eq!(
                rising, !inverted,
                "{}: shadow {:.3} highlight {:.3} runs the wrong way",
                film.stock, shadow[1], highlight[1]
            );
        }
    }

    #[test]
    fn a_projection_print_stock_is_not_offered_as_a_camera_film() {
        // Kodak 2383 is `support: film` and is nevertheless the Vision3
        // stocks' paper. Filtering on support alone would put it in the picker.
        assert!(find("kodak_2383").is_some(), "the profile is shipped");
        assert!(
            !camera_stocks().any(|p| p.stock == "kodak_2383"),
            "a projection print film is being offered as a camera stock"
        );
    }

    #[test]
    fn a_reversal_stock_names_no_paper() {
        let k64 = find("kodak_kodachrome_64").unwrap();
        assert_eq!(k64.kind, Kind::Positive);
        assert!(default_print(k64).is_none());
    }
}