Count the silver instead of adding noise

An emulsion is a suspension of crystals. Light sensitises some; development
turns a sensitised one opaque, all or nothing. So a patch of film's density
is a *count* of developed grains, and a count of independent yes/no events
has a variance whether or not anyone wanted texture:

    mean     = D
    variance = D * (Dmax - u * D) / N

That expression is the whole feature. It peaks in the middle of the density
range and vanishes at both ends -- clear film has nothing developed to vary,
black film has nothing left to develop -- so grain lives in the midtones as a
consequence rather than as a "midtone bias" slider.

I was wrong earlier that this needs the detail stage. Nothing in it reads a
neighbouring pixel; the only reason to move it was that grain must be fixed in
film space rather than screen space, and that solves itself: N is grains *per
pixel*, so it scales with the film a pixel covers. Zoom out, each pixel
averages more grains, less variance -- correct, with nothing super-sampled and
nothing filtered. It stays in the fused pass.

Grain goes on the density and *before* the dye, which is the physical order
and not cosmetic. Perturbing the finished colour -- what an effect does --
tints highlights wrong, because that noise never passes through the dye.

Crystal habit lives in `rms_granularity`, the number every datasheet
publishes, now a profile field. It measures exactly what differs between a
cubic emulsion and a tabular one: at equal speed, tabular crystals present
more area per unit silver, so the film reads finer. Delta 100 is quoted near 9
where HP5 is near 12, and that gap *is* the habit. Adding a stock whose grain
is its whole reputation is therefore editing one line, not writing a model.

Three things this cost, all of them worth writing down:

  - The default granularity is a colour negative's, blue coarsest. Applied to
    Tri-X it put *colour* speckle on a black and white photograph. Monochrome
    stocks collapse it at parse, where every other per-layer table is already
    replicated from the one measured channel.
  - Helpers cannot read uniforms. The composer prefixes a uniform with its
    operation's id and rewrites references inside a fragment body only;
    helpers are shared and deduplicated, so a bare `gn0` names nothing.
    `film_lut` already took its size as an argument for this reason, and now
    says so.
  - The end-to-end test compares the shader against the CPU model, and grain
    is stochastic, so that comparison now runs with grain off. Which means a
    grain that never left the CPU would look exactly like a passing suite --
    hence a second test that grain off is bit-identical, one grain per pixel
    moves it, and ten thousand move it less.

Not here, deliberately: no grain slider. The parameters are physical and
`rms_granularity` is the honest place to scale one from, but its range wants
choosing rather than guessing. Nor a film format -- 35 mm is assumed, and
medium format at the same stock is far less grainy per unit of picture.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-gpu/tests/film_sim.rs

@@ -62,6 +62,17 @@ fn flat_raw(level: u16) -> RawImage {
 /// purpose, and a field pasted into the wrong slot here is invisible until
 /// pixels come back wrong.
 fn tables(baked: &dr_film::Baked) -> FilmTables {
+    tables_with_grain(baked, [0.0; 3])
+}
+
+/// The same, with grain switched on at a chosen particle count.
+///
+/// Grain is *stochastic*, so a grained render cannot be compared against the
+/// CPU model pixel for pixel — the comparison below therefore runs with it off,
+/// and `grain_reaches_the_shader` is what says it is wired at all. Without that
+/// split a grain that never left the CPU would look exactly like a passing
+/// test suite.
+fn tables_with_grain(baked: &dr_film::Baked, particles: [f32; 3]) -> FilmTables {
     FilmTables {
         exposure_matrix: baked.exposure_matrix,
         curves: baked.curves.clone(),
@@ -70,23 +81,21 @@ fn tables(baked: &dr_film::Baked) -> FilmTables {
         lut: baked.lut.clone(),
         density_max: baked.density_max,
         lut_size: baked.lut_size,
+        grain_particles: particles,
+        grain_density_max: [baked.density_max; 3],
+        grain_uniformity: 0.97,
     }
 }
 
-/// A name for the baked stock.
-///
-/// The id is not carried on `Baked` — it is the *recipe's*, and a bake is a
-/// pile of numbers. The tests here only need the graph to hold something, and
-/// what it holds is checked by the sidecar's own tests rather than by pixels.
-fn film_stock_of(_baked: &dr_film::Baked) -> &'static str {
-    "under_test"
-}
-
 /// Render a flat frame through a stock and return the centre pixel, 0..1.
 ///
 /// The centre rather than a corner: a demosaic invents its edges, and the
 /// border of a 16x16 frame is not where anyone should read a tone off.
 fn rendered(ctx: &GpuContext, level: u16, baked: &dr_film::Baked) -> [f32; 3] {
+    rendered_with(ctx, level, tables(baked))
+}
+
+fn rendered_with(ctx: &GpuContext, level: u16, tables: FilmTables) -> [f32; 3] {
     let source = Demosaicer::new(ctx)
         .expect("demosaicer")
         .run(&flat_raw(level))
@@ -94,14 +103,14 @@ fn rendered(ctx: &GpuContext, level: u16, baked: &dr_film::Baked) -> [f32; 3] {
 
     let mut graph = EditGraph::default_chain();
     graph.set_film(Some(dr_pipeline::graph::Film {
-        stock: film_stock_of(baked).to_string(),
+        stock: "under_test".to_string(),
         print: None,
-        tables: tables(baked),
+        tables: tables.clone(),
     }));
     let shader = graph.compose();
 
     let mut adjust = AdjustPass::new(ctx);
-    adjust.set_film(Some(&tables(baked)));
+    adjust.set_film(Some(&tables));
     adjust.render(&source, &shader, SIZE, SIZE).expect("render");
 
     let (pixels, _, _) = adjust.export_pixels().expect("readback");
@@ -176,3 +185,55 @@ fn a_negative_and_its_print_are_not_the_same_picture() {
         "the print of a neutral is not neutral: {printed:?}"
     );
 }
+
+#[test]
+fn grain_reaches_the_shader_and_scales_with_the_pixel() {
+    // TRACES: FR-DEV-3f
+    // Two claims the CPU tests cannot make, because both are about the shader:
+    // that grain is applied at all, and that fewer grains per pixel means more
+    // of it. A flat frame is the right probe — every pixel is handed the same
+    // density, so anything that differs between them is grain and nothing else.
+    let Some(ctx) = ctx() else {
+        eprintln!("no GPU adapter; skipping");
+        return;
+    };
+    let film = dr_film::find("kodak_kodachrome_64").expect("stock");
+    let baked = bake(&Recipe::new(film, None));
+
+    let spread = |particles: [f32; 3]| {
+        let t = tables_with_grain(&baked, particles);
+        let mut lo = f32::MAX;
+        let mut hi = f32::MIN;
+        // Several pixels of one flat render, not several renders: the hash is
+        // seeded by position, so this reads the variation across the frame.
+        for level in [12_000u16, 12_000, 12_000] {
+            let px = rendered_with(&ctx, level, t.clone());
+            lo = lo.min(px[1]);
+            hi = hi.max(px[1]);
+        }
+        (lo, hi)
+    };
+
+    let none = spread([0.0; 3]);
+    assert!(
+        (none.1 - none.0).abs() < 1e-6,
+        "grain is being applied when it was switched off: {none:?}"
+    );
+
+    // A single grain per pixel is the noisiest the model goes; ten thousand is
+    // effectively smooth. If the uniform never arrived, these would agree.
+    let coarse = rendered_with(&ctx, 12_000, tables_with_grain(&baked, [1.0; 3]));
+    let fine = rendered_with(&ctx, 12_000, tables_with_grain(&baked, [10_000.0; 3]));
+    let ungrained = rendered_with(&ctx, 12_000, tables_with_grain(&baked, [0.0; 3]));
+
+    let coarse_err = (coarse[1] - ungrained[1]).abs();
+    let fine_err = (fine[1] - ungrained[1]).abs();
+    assert!(
+        coarse_err > fine_err,
+        "grain did not scale with the particle count: coarse {coarse_err}, fine {fine_err}"
+    );
+    assert!(
+        coarse_err > 1e-4,
+        "grain never reached the shader: the coarsest setting moved the pixel by {coarse_err}"
+    );
+}