Merge branch 'worktree-agent-a89309a856c8f4947' into integration

core/dr-gpu/tests/tone_curve.rs

@@ -0,0 +1,139 @@
+//! TRACES: FR-DEV-3
+//! The tone curve's four curves, on a device.
+//!
+//! `dr-pipeline` asserts that the right WGSL is generated and `dr-gpu`'s other
+//! tests assert that a shader runs; neither notices a fragment that says
+//! exactly what it should and does not compile, or one that compiles and puts
+//! the red curve's uniforms into the blue slot. So this renders flat grey
+//! through each curve and looks at what came out.
+//!
+//! Flat grey because it makes every assertion a comparison between the three
+//! components of one pixel: a curve that is meant to be chromatic must move
+//! them apart, and one that is meant to be tonal must not.
+
+use dr_gpu::{AdjustPass, DemosaicedImage, GpuContext};
+use dr_pipeline::ops::curve::{self, Axis, Channel};
+use dr_pipeline::EditGraph;
+
+const SIZE: u32 = 8;
+
+fn ctx() -> Option<GpuContext> {
+    pollster::block_on(GpuContext::new_headless()).ok()
+}
+
+/// The centre pixel's red, green and blue, after `graph` has run over flat
+/// mid-grey.
+fn rendered(ctx: &GpuContext, graph: &EditGraph) -> (u8, u8, u8) {
+    let data: Vec<u8> = (0..SIZE * SIZE).flat_map(|_| [128, 128, 128, 255]).collect();
+    let source = DemosaicedImage::from_rgba8(ctx, &data, SIZE, SIZE).expect("upload");
+
+    // Composed the way the display path composes it. A curve that generates
+    // invalid WGSL fails at `render` below, which is the point of running this
+    // on a device at all.
+    let shader = graph.compose();
+
+    let mut adjust = AdjustPass::new(ctx);
+    adjust.render(&source, &shader, SIZE, SIZE).expect("render");
+    let pixels = adjust.export_pixels().expect("readback").0;
+
+    let at = ((SIZE / 2 * SIZE + SIZE / 2) * 4) as usize;
+    (pixels[at], pixels[at + 1], pixels[at + 2])
+}
+
+/// A curve with its mid-point lifted — the simplest edit that is unmistakably
+/// an edit.
+fn lifted(channel: Channel) -> EditGraph {
+    let mut graph = EditGraph::default_chain();
+    graph.set_param(
+        curve::ID,
+        curve::coordinate(channel, 2, Axis::Y),
+        0.75,
+    );
+    graph
+}
+
+#[test]
+fn the_master_curve_lifts_every_component_together() {
+    let Some(ctx) = ctx() else {
+        eprintln!("no adapter; skipping");
+        return;
+    };
+
+    let (r0, g0, b0) = rendered(&ctx, &EditGraph::default_chain());
+    let (r, g, b) = rendered(&ctx, &lifted(Channel::Master));
+
+    assert!(r > r0, "the master curve did not lift the image: {r} vs {r0}");
+    // Grey in, grey out: the master curve is applied as a ratio over
+    // luminance, so it changes tone and not hue. A tolerance of one code
+    // value, because the components travel through the ratio separately and
+    // the result is quantised to eight bits.
+    assert!(
+        r.abs_diff(g) <= 1 && g.abs_diff(b) <= 1,
+        "the master curve tinted a neutral pixel: {r},{g},{b}"
+    );
+    assert_eq!((g0, b0), (r0, r0), "the unedited image is neutral");
+}
+
+#[test]
+fn a_channel_curve_lifts_only_its_own_component() {
+    let Some(ctx) = ctx() else {
+        eprintln!("no adapter; skipping");
+        return;
+    };
+
+    let (r0, g0, b0) = rendered(&ctx, &EditGraph::default_chain());
+    for (channel, name) in [
+        (Channel::Red, "red"),
+        (Channel::Green, "green"),
+        (Channel::Blue, "blue"),
+    ] {
+        let (r, g, b) = rendered(&ctx, &lifted(channel));
+        // The component the curve names moves; the other two stay exactly
+        // where they were. This is what catches a fragment whose uniforms are
+        // wired to the wrong curve — it would still lift *something*.
+        let (moved, still) = match channel {
+            Channel::Red => (r > r0, g == g0 && b == b0),
+            Channel::Green => (g > g0, r == r0 && b == b0),
+            _ => (b > b0, r == r0 && g == g0),
+        };
+        assert!(moved, "the {name} curve changed nothing: {r},{g},{b}");
+        assert!(
+            still,
+            "the {name} curve moved a component that was not its own: \
+             {r},{g},{b} from {r0},{g0},{b0}"
+        );
+    }
+}
+
+#[test]
+fn the_master_and_the_channels_compose_in_one_pass() {
+    // All four curves at once: the case where the generated fragment is
+    // longest, every helper is present, and forty uniforms are in the block.
+    // Mostly a compile check, which is why the assertion is only that the
+    // result is a colour and not the one we started with.
+    let Some(ctx) = ctx() else {
+        eprintln!("no adapter; skipping");
+        return;
+    };
+
+    let mut graph = EditGraph::default_chain();
+    graph.set_param(curve::ID, curve::P1_Y, 0.15);
+    graph.set_param(curve::ID, curve::P3_Y, 0.85);
+    for (channel, y) in [
+        (Channel::Red, 0.55),
+        (Channel::Green, 0.5),
+        (Channel::Blue, 0.62),
+    ] {
+        graph.set_param(curve::ID, curve::coordinate(channel, 2, Axis::Y), y);
+    }
+
+    let (r0, _, _) = rendered(&ctx, &EditGraph::default_chain());
+    let (r, g, b) = rendered(&ctx, &graph);
+    assert!(
+        (r, g, b) != (r0, r0, r0),
+        "four active curves left the image untouched"
+    );
+    // Red and blue were pushed apart from green, which is the chromatic half
+    // doing its work on top of the tonal one.
+    assert!(b > g, "blue was lifted above green: {r},{g},{b}");
+}

core/dr-pipeline/ops/README.md

@@ -209,9 +209,10 @@ They still belong in this directory, because the pipeline's **order** is the
 one thing a reader comes here to learn, and an order written half in YAML and
 half in Rust would be worse than either alone.
 
-Currently hand-written: `tone_curve` (a curve widget over five interpolated
-points), `colour_mixer` (thirty-six faceted parameters from twelve computed hue
-bands). `vignetting` is hand-written too but is not in the develop chain — it
+Currently hand-written: `tone_curve` (one widget over four curves of five
+interpolated points — master, red, green, blue — each reaching the shader only
+when it has been moved), `colour_mixer` (thirty-six faceted parameters from
+twelve computed hue bands). `vignetting` is hand-written too but is not in the develop chain — it
 carries lens-profile coefficients that are not parameters. `distortion` and
 `aberration` are `Warp`s rather than operations: they rewrite coordinates
 before sampling rather than transforming a colour after it.

core/dr-pipeline/ops/tone_curve.yaml

@@ -16,13 +16,26 @@ attributes: [tone, colour]
 rust: ToneCurve
 
 why_rust: |
-  Five control points presented as one curve widget, with an interpolator and
-  a monotonicity guarantee behind it. Its neutral is a *relationship* between
-  parameters rather than a set of values — the identity diagonal — which is
-  not something the declarative `active:` rule can express, and its
-  `presentation()` spans parameters rather than describing one.
+  Four curves — master, red, green, blue — of five control points each,
+  presented as one widget, with an interpolator and a monotonicity guarantee
+  behind them. Its neutral is a *relationship* between parameters rather than
+  a set of values — the identity diagonal, on every channel — which is not
+  something the declarative `active:` rule can express; its `presentation()`
+  spans forty parameters rather than describing one; and its fragment is
+  *assembled* rather than written, because each curve reaches the shader only
+  when it has been moved off the diagonal. A declared node's `wgsl:` is one
+  fixed block of text, which is the right shape for nearly everything here and
+  the wrong one for a node whose cost has to follow what the photographer
+  actually touched.
 
 placement: |
   After the fixed-weight region controls, so the curve is the final word on
   tone: a photographer reaches for it to fix what those controls could not
   place exactly.
+
+  Within the node, the master curve runs before the per-channel ones. Both
+  orders are visibly different images and the reasons for this one are written
+  out in `src/ops/curve.rs`: the master is tonal and hue-preserving, the
+  per-channel curves are the chromatic grade over the tones it produced, and a
+  point placed on a channel curve should act on the tone the photographer can
+  see rather than on the one the master is about to move.

core/dr-pipeline/src/sidecar.rs

@@ -1088,7 +1088,7 @@ impl std::error::Error for ParseError {}
 mod tests {
     use super::*;
     use crate::framing;
-    use crate::ops::{exposure, saturation, white_balance};
+    use crate::ops::{curve, exposure, saturation, white_balance};
 
     fn edited() -> EditGraph {
         let mut g = EditGraph::default_chain();
@@ -1273,6 +1273,90 @@ mod tests {
         assert_eq!(once, twice);
     }
 
+    /// TRACES: FR-DEV-3 | FR-CAT-8
+    /// A file written before the tone curve had per-channel curves.
+    ///
+    /// Spelled out as literal text rather than produced by `to_text`, because
+    /// the claim is about *those bytes*: a sidecar generated by this build
+    /// would agree with this build by construction, and would go on agreeing
+    /// with it through a rename that broke every file on disk.
+    #[test]
+    fn a_sidecar_from_before_the_channel_curves_still_names_the_master() {
+        let text = "drsc 1\n\n[version u1]\nname = Default\nrevision = 4\nmodified = 9\n\
+                    tone_curve.p1_y = 0.15\ntone_curve.p3_y = 0.85\n";
+        let parsed = Sidecar::parse(text).expect("valid");
+        let mut g = EditGraph::default_chain();
+        parsed.default_version().expect("a version").apply(&mut g);
+
+        // The S-curve the file describes, on the master curve and nowhere
+        // else.
+        assert_eq!(g.param(curve::ID, curve::P1_Y), Some(0.15));
+        assert_eq!(g.param(curve::ID, curve::P3_Y), Some(0.85));
+        for channel in [curve::Channel::Red, curve::Channel::Green, curve::Channel::Blue] {
+            for point in 0..curve::POINTS {
+                for axis in [curve::Axis::X, curve::Axis::Y] {
+                    let id = curve::coordinate(channel, point, axis);
+                    let expected = g
+                        .capabilities()
+                        .iter()
+                        .find(|c| c.id == curve::ID)
+                        .and_then(|c| c.params.iter().find(|p| p.id == id))
+                        .map(|p| p.default);
+                    assert_eq!(
+                        g.param(curve::ID, id),
+                        expected,
+                        "{id} moved, and no line in the file mentions it"
+                    );
+                }
+            }
+        }
+
+        // And writing it back produces the same two lines: the curves the file
+        // never mentioned are still at their defaults, so they are still
+        // absent (`only_non_default_values_are_written`).
+        let written = Sidecar::parse(&Sidecar::parse(text).expect("valid").to_text())
+            .expect("valid")
+            .to_text();
+        assert!(written.contains("tone_curve.p1_y = 0.15"), "{written}");
+        assert!(written.contains("tone_curve.p3_y = 0.85"), "{written}");
+        assert!(
+            !written.contains("tone_curve.r_"),
+            "an untouched channel curve was written out:\n{written}"
+        );
+    }
+
+    /// TRACES: FR-DEV-3
+    /// The other direction: the new curves persist like any other parameter.
+    #[test]
+    fn a_per_channel_curve_survives_the_round_trip() {
+        let mut g = EditGraph::default_chain();
+        // A faded shadow: blue lifted at the black point, red pulled down.
+        let blue = curve::coordinate(curve::Channel::Blue, 0, curve::Axis::Y);
+        let red = curve::coordinate(curve::Channel::Red, 4, curve::Axis::Y);
+        g.set_param(curve::ID, blue, 0.08);
+        g.set_param(curve::ID, red, 0.92);
+        g.set_param(curve::ID, curve::P2_Y, 0.55);
+
+        let mut sidecar = Sidecar::new();
+        sidecar.put(version_of(&g));
+        let text = sidecar.to_text();
+        // Keyed by the channel-prefixed id, which is what makes the master's
+        // unprefixed ones safe to leave alone.
+        assert!(text.contains("tone_curve.b_p0_y = 0.08"), "{text}");
+
+        let parsed = Sidecar::parse(&text).expect("valid");
+        let mut restored = EditGraph::default_chain();
+        parsed
+            .default_version()
+            .expect("a version")
+            .apply(&mut restored);
+
+        assert_eq!(restored.param(curve::ID, blue), Some(0.08));
+        assert_eq!(restored.param(curve::ID, red), Some(0.92));
+        assert_eq!(restored.param(curve::ID, curve::P2_Y), Some(0.55));
+        assert!(!restored.is_neutral());
+    }
+
     #[test]
     fn an_unknown_operation_survives_a_round_trip() {
         // The data-loss case that matters: a device running an older build

core/dr-pipeline/tests/tone_curve.rs

@@ -0,0 +1,202 @@
+//! TRACES: FR-DEV-3
+//! The tone curve's four curves, seen from outside the crate.
+//!
+//! The unit tests beside the operation assert what one `ToneCurve` does. These
+//! assert the two properties that only show up once it is a node in a chain
+//! with a sidecar under it: that a file written before the per-channel curves
+//! existed still describes the edit it described, and that the three new
+//! curves cost a photograph that does not use them precisely nothing.
+
+use dr_pipeline::ops::curve::{self, Axis, Channel};
+use dr_pipeline::{EditGraph, Sidecar};
+
+/// A version block carrying `params`, in the on-disk spelling.
+fn sidecar_with(params: &[(&str, f32)]) -> String {
+    let mut text = String::from("drsc 1\n\n[version u1]\nname = Default\nrevision = 2\nmodified = 0\n");
+    for (key, value) in params {
+        text.push_str(&format!("{key} = {value}\n"));
+    }
+    text
+}
+
+fn apply(text: &str) -> EditGraph {
+    let sidecar = Sidecar::parse(text).expect("a valid sidecar");
+    let mut graph = EditGraph::default_chain();
+    sidecar
+        .default_version()
+        .expect("a default version")
+        .apply(&mut graph);
+    graph
+}
+
+/// TRACES: FR-CAT-8
+/// **The compatibility guarantee, end to end.**
+///
+/// An edit made before this build existed has to produce the same image now.
+/// Asserted on the generated shader rather than on the parameter values,
+/// because that is what the photograph is actually made of: same source, same
+/// uniforms, same picture.
+#[test]
+fn an_edit_written_before_the_channel_curves_renders_as_it_did() {
+    let from_file = apply(&sidecar_with(&[
+        ("tone_curve.p1_y", 0.15),
+        ("tone_curve.p3_y", 0.85),
+    ]));
+
+    let mut by_hand = EditGraph::default_chain();
+    by_hand.set_param(curve::ID, curve::P1_Y, 0.15);
+    by_hand.set_param(curve::ID, curve::P3_Y, 0.85);
+
+    let restored = from_file.compose();
+    let expected = by_hand.compose();
+    assert_eq!(restored.source, expected.source);
+    assert_eq!(restored.uniforms, expected.uniforms);
+    assert_eq!(restored.structure_hash, expected.structure_hash);
+}
+
+/// **Neutral means absent, and stays absent with four times as much to be
+/// neutral about.**
+///
+/// The curve carries forty parameters now. An image edited with an S-curve and
+/// nothing else must generate the shader it generated when it carried ten:
+/// three untouched curves are not three identity evaluations, they are nothing
+/// at all.
+#[test]
+fn three_untouched_curves_cost_nothing() {
+    let mut graph = EditGraph::default_chain();
+    graph.set_param(curve::ID, curve::P2_Y, 0.62);
+    let shader = graph.compose();
+
+    assert!(
+        shader.source.contains("---- tone_curve ----"),
+        "the master curve must reach the shader"
+    );
+    assert!(
+        !shader.source.contains("channel_curve"),
+        "an untouched channel curve reached the shader:\n{}",
+        shader.source
+    );
+    for prefix in ["r_", "g_", "b_"] {
+        assert!(
+            !shader.source.contains(&format!("tone_curve_{prefix}")),
+            "an untouched channel curve declared uniforms:\n{}",
+            shader.source
+        );
+    }
+}
+
+/// And with none of them touched, the operation is not there at all — the
+/// property `a_fresh_graph_is_neutral` asserts for the chain, restated for the
+/// node that just quadrupled in size.
+#[test]
+fn a_curve_at_its_defaults_is_absent_from_the_shader() {
+    let graph = EditGraph::default_chain();
+    assert!(graph.is_neutral());
+    assert!(!graph.compose().source.contains("tone_curve"));
+
+    // Including when every one of the forty parameters has been explicitly
+    // written to its own default, which is what a sidecar round trip through
+    // a build with a different idea of "default" would produce.
+    let mut written = EditGraph::default_chain();
+    for cap in written.capabilities() {
+        if cap.id != curve::ID {
+            continue;
+        }
+        for p in &cap.params {
+            written.set_param(curve::ID, p.id, p.default);
+        }
+    }
+    assert!(written.is_neutral());
+}
+
+/// A grade with no tonal work is a real edit, and it must not drag the
+/// luminance path in behind it.
+#[test]
+fn a_channel_curve_reaches_the_shader_on_its_own() {
+    let mut graph = EditGraph::default_chain();
+    graph.set_param(
+        curve::ID,
+        curve::coordinate(Channel::Blue, 0, Axis::Y),
+        0.08,
+    );
+
+    let shader = graph.compose();
+    assert!(shader.source.contains("c.b = channel_curve(c.b,"));
+    assert!(
+        !shader.source.contains("apply_tone_gain"),
+        "the identity master curve reached the shader:\n{}",
+        shader.source
+    );
+    // Blue's ten points, and nothing else: the other two curves are at the
+    // identity and contribute no slot to the uniform block.
+    for prefix in ["r_", "g_"] {
+        assert!(
+            !shader.source.contains(&format!("tone_curve_{prefix}")),
+            "an untouched channel curve declared uniforms:\n{}",
+            shader.source
+        );
+    }
+    for point in 0..curve::POINTS {
+        assert!(
+            shader.source.contains(&format!("tone_curve_b_x{point}")),
+            "blue's point {point} is missing from the uniform block"
+        );
+    }
+}
+
+/// Each curve is its own shader, so the pipeline cache cannot hand the red
+/// curve's compiled program to an edit that moved the green one.
+#[test]
+fn every_curve_generates_a_distinct_shader() {
+    let mut hashes: Vec<u64> = Vec::new();
+    for channel in Channel::ALL {
+        let mut graph = EditGraph::default_chain();
+        graph.set_param(curve::ID, curve::coordinate(channel, 2, Axis::Y), 0.62);
+        hashes.push(graph.compose().structure_hash);
+    }
+    let before = hashes.len();
+    hashes.sort_unstable();
+    hashes.dedup();
+    assert_eq!(before, hashes.len(), "two curves share a compiled shader");
+}
+
+/// The forty parameters all persist, and the file names each one once.
+#[test]
+fn every_point_of_every_curve_round_trips_through_a_sidecar() {
+    let mut graph = EditGraph::default_chain();
+    // A different y per coordinate, so a point wired to the wrong channel
+    // cannot pass by holding the value it was supposed to hold anyway.
+    // Thirty-secondths because they survive both the decimal the file is
+    // written in and the binary32 it is read back into exactly — this test is
+    // about which parameter a value lands in, and a rounding difference here
+    // would fail it for an unrelated reason.
+    let mut step = 1;
+    for channel in Channel::ALL {
+        for point in 0..curve::POINTS {
+            graph.set_param(
+                curve::ID,
+                curve::coordinate(channel, point, Axis::Y),
+                step as f32 / 32.0,
+            );
+            step += 1;
+        }
+    }
+
+    let mut sidecar = Sidecar::new();
+    sidecar.put(dr_pipeline::sidecar::Version::from_graph(
+        "u1", "Default", &graph,
+    ));
+    let restored = apply(&sidecar.to_text());
+
+    for channel in Channel::ALL {
+        for point in 0..curve::POINTS {
+            let id = curve::coordinate(channel, point, Axis::Y);
+            assert_eq!(
+                restored.param(curve::ID, id),
+                graph.param(curve::ID, id),
+                "{id} did not survive the sidecar"
+            );
+        }
+    }
+}
+

ui/dr-ui/src/develop.rs

@@ -273,6 +273,20 @@ pub struct DevelopSession {
     /// attributes leaves it at — the tabs are the interface's idea, not the
     /// core's, and nothing breaks without them (ARCH §4.3a).
     active_tab: Option<dr_pipeline::Attribute>,
+    /// TRACES: FR-DEV-3
+    /// Which of the curve widget's subjects the panel is plotting.
+    ///
+    /// The tone curve is four curves — one over tone and one per colour
+    /// channel — and one square plot draws one of them at a time. The index
+    /// is into the subjects the operation's parameters are faceted on, in the
+    /// order it declares them, so nothing here knows that "red" exists.
+    ///
+    /// **Interface state, not part of the edit.** It changes no pixel, so it
+    /// is not a parameter, it is not in the graph, it is not in the sidecar
+    /// and it is not on the undo stack — the same standing as which tab is
+    /// open. One value rather than one per operation, for the same reason
+    /// `curve_samples` is one polyline: the panel draws one curve.
+    curve_channel: usize,
 }
 
 impl DevelopSession {
@@ -339,6 +353,7 @@ impl DevelopSession {
             active_mask: None,
             show_overlay: false,
             active_tab: None,
+            curve_channel: 0,
         }
     }
 
@@ -349,8 +364,12 @@ impl DevelopSession {
     pub fn rows(&self) -> Vec<ParamRow> {
         let caps = self.scoped_capabilities();
         match self.active_tab {
-            Some(attribute) => rows_filtered(&caps, |op| op.attributes.contains(&attribute)),
-            None => rows_from(&caps),
+            Some(attribute) => rows_filtered(
+                &caps,
+                |op| op.attributes.contains(&attribute),
+                self.curve_channel,
+            ),
+            None => rows_filtered(&caps, |_| true, self.curve_channel),
         }
     }
 
@@ -384,8 +403,10 @@ impl DevelopSession {
             .into_iter()
             .filter(|a| *a != Attribute::Geometry)
             .filter(|a| {
-                caps.iter()
-                    .any(|c| c.attributes.contains(a) && !rows_filtered(&caps, |o| o.attributes.contains(a)).is_empty())
+                caps.iter().any(|c| {
+                    c.attributes.contains(a)
+                        && !rows_filtered(&caps, |o| o.attributes.contains(a), 0).is_empty()
+                })
             })
             .map(|a| (a, crate::labels::resolve(a.label().0)))
             .collect()
@@ -494,8 +515,13 @@ pub(crate) fn supported(widget: WidgetKind) -> bool {
 /// FR-DEV-3c acceptance test asks for — an operation the frontend has never
 /// heard of appearing in a generated panel — and it cannot be asserted at all
 /// if generating a row requires a device.
+///
+/// `#[cfg(test)]` since the panel began passing the selected curve down: the
+/// session always has one to pass, and a wrapper that quietly picked the first
+/// would be a second answer to a question the session already answers.
+#[cfg(test)]
 pub(crate) fn rows_from(caps: &[OpCapability]) -> Vec<ParamRow> {
-    rows_filtered(caps, |_| true)
+    rows_filtered(caps, |_| true, 0)
 }
 
 /// The panel model for the capabilities `keep` accepts.
@@ -508,9 +534,16 @@ pub(crate) fn rows_from(caps: &[OpCapability]) -> Vec<ParamRow> {
 ///
 /// Getting that backwards is how a slider ends up driving a different
 /// operation, which is the kind of fault that looks like a rendering bug.
+///
+/// `curve_channel` is which subject a multi-subject widget is showing — the
+/// tone curve's four curves are one plot with a selector over it. It is passed
+/// in rather than read from anywhere because this function is deliberately
+/// free-standing: the descriptor-to-panel path has to be exercisable against a
+/// hand-built capability list with no session behind it.
 pub(crate) fn rows_filtered(
     caps: &[OpCapability],
     keep: impl Fn(&OpCapability) -> bool,
+    curve_channel: usize,
 ) -> Vec<ParamRow> {
     let mut rows = Vec::new();
     for (op_index, op) in caps.iter().enumerate() {
@@ -558,7 +591,9 @@ pub(crate) fn rows_filtered(
                 // to the core stops this compiling until someone has decided,
                 // here, whether the panel draws it.
                 let row = match widget {
-                    WidgetKind::ToneCurve => curve_row(op_index, group_head, op, presentation),
+                    WidgetKind::ToneCurve => {
+                        curve_row(op_index, group_head, op, presentation, curve_channel)
+                    }
                     // Canvas-hosted kinds returned above; the rest are not
                     // implemented and reached sliders via `choose`.
                     WidgetKind::ColourWheel
@@ -671,8 +706,76 @@ pub(crate) fn rows_filtered(
     rows
 }
 
+/// One run of a curve widget's parameters: the points of a single curve.
+///
+/// A widget may span several curves — the tone curve is one plot over a master
+/// curve and three colour channels — and it says so the way the colour mixer
+/// says it has twelve bands: by faceting each parameter with the *subject* it
+/// acts on. Consecutive parameters sharing a subject are one curve.
+struct CurveRun {
+    /// The subject's localisation key, or `None` where the widget's parameters
+    /// carry no facet at all and are therefore a single unnamed curve.
+    subject: Option<&'static str>,
+    /// Where this run's points begin in the operation's parameter list. What
+    /// a drag routes back through, so it must be a position in `op.params`
+    /// and not in the presentation's list.
+    base: usize,
+    /// How many coordinates it holds.
+    len: usize,
+}
+
+/// TRACES: FR-DEV-3a
+/// The curves a curve widget spans, in the order the operation declares them.
+///
+/// **This is the whole of the panel's knowledge of colour channels: none.** It
+/// groups by whatever subject the parameters carry, so an operation offering a
+/// master curve and three channels gets a four-way selector, one offering a
+/// single unfaceted curve gets no selector at all, and one that grows a fifth
+/// curve tomorrow needs no change here.
+///
+/// Returns `None` where the parameters do not look like point coordinates —
+/// an odd count, a run that is not contiguous in the capability list — in
+/// which case the caller falls back to sliders rather than drawing a widget
+/// over a layout it has guessed at.
+fn curve_runs(op: &OpCapability, presentation: &Presentation) -> Option<Vec<CurveRun>> {
+    // Points are x/y pairs, so an odd count means the operation and this code
+    // disagree about the layout.
+    if presentation.params.len() < 2 || !presentation.params.len().is_multiple_of(2) {
+        log::warn!("{}: curve widget needs an even parameter count", op.id);
+        return None;
+    }
+
+    let mut runs: Vec<CurveRun> = Vec::new();
+    for id in presentation.params {
+        // The widget addresses points by offset from the first of its run, so
+        // a run has to be contiguous in the capability list.
+        let at = op.params.iter().position(|p| p.id == *id)?;
+        let subject = op.params[at].facet.as_ref().map(|f| f.subject.0);
+
+        match runs.last_mut() {
+            Some(run) if run.subject == subject && run.base + run.len == at => run.len += 1,
+            _ => runs.push(CurveRun {
+                subject,
+                base: at,
+                len: 1,
+            }),
+        }
+    }
+
+    if runs.iter().any(|r| !r.len.is_multiple_of(2)) {
+        log::warn!("{}: a curve's points are not contiguous", op.id);
+        return None;
+    }
+    Some(runs)
+}
+
 /// One row standing for a whole curve.
 ///
+/// `channel` picks which of the widget's curves is plotted; it is clamped
+/// rather than validated, because the selection is interface state that
+/// outlives a change of photograph and the new image's operation may have
+/// fewer curves than the old one's.
+///
 /// Returns `None` if the operation's parameters do not look like point
 /// coordinates, in which case the caller falls back to sliders rather than
 /// rendering a broken widget.
@@ -681,38 +784,21 @@ fn curve_row(
     group_head: usize,
     op: &OpCapability,
     presentation: &Presentation,
+    channel: usize,
 ) -> Option<ParamRow> {
-    // Points are x/y pairs, so an odd count means the operation and this
-    // code disagree about the layout.
-    if presentation.params.len() < 2 || !presentation.params.len().is_multiple_of(2) {
-        log::warn!("{}: curve widget needs an even parameter count", op.id);
-        return None;
-    }
+    let runs = curve_runs(op, presentation)?;
+    let run = runs.get(channel.min(runs.len().saturating_sub(1)))?;
 
-    // The widget addresses points by offset from the first, so they must
-    // be contiguous in the capability list.
-    let base = op
-        .params
+    let points: Vec<f32> = op.params[run.base..run.base + run.len]
         .iter()
-        .position(|p| p.id == presentation.params[0])?;
-    for (i, id) in presentation.params.iter().enumerate() {
-        if op.params.get(base + i).map(|p| p.id) != Some(*id) {
-            log::warn!("{}: curve parameters are not contiguous", op.id);
-            return None;
-        }
-    }
-
-    let points: Vec<f32> = presentation
-        .params
-        .iter()
-        .filter_map(|id| op.params.iter().find(|p| p.id == *id))
         .map(|p| p.value)
         .collect();
 
     Some(ParamRow {
         op_index: op_index as i32,
-        // The first point parameter; the widget offsets from here.
-        param_index: base as i32,
+        // The first point parameter *of the curve on show*; the widget offsets
+        // from here, so switching curve is what re-points the drag.
+        param_index: run.base as i32,
         op_label: labels::resolve(op.label.0).into(),
         param_label: String::new().into(),
         // A widget spanning a whole operation is not a row in anyone's
@@ -733,21 +819,97 @@ fn curve_row(
         precision: 4,
         unit: String::new().into(),
         points: slint::ModelRc::new(slint::VecModel::from(points)),
-        // A curve is not a choice between named alternatives.
+        // A curve is not a choice between named alternatives. The curves it
+        // can switch between are named on the panel rather than on the row —
+        // see `DevelopSession::curve_channels` for why they cannot ride here.
         choices: no_choices(),
     })
 }
 
 impl DevelopSession {
-    /// The curve's shape, sampled for drawing.
+    /// TRACES: FR-DEV-3
+    /// The names of the curves the widget can switch between.
+    ///
+    /// Empty where there is only one, which is also the answer for a frontend
+    /// with no curve at all: a selector over a single choice is a row of
+    /// nothing.
+    ///
+    /// **Derived from the facets, so nothing here names a colour channel.**
+    /// The operation says its forty points are one control applied to four
+    /// subjects and publishes a localisation key for each; this resolves the
+    /// keys and hands over four words. An operation that grew a fifth curve
+    /// would appear here on its own.
+    ///
+    /// A panel property rather than a field on the curve's `ParamRow`, and the
+    /// reason is Slint's: a row's models are compared by identity, so a fresh
+    /// list of names built on every parameter event would make the row look
+    /// changed every time, and rewriting a row rebuilds the repeater item
+    /// underneath it — destroying the `TouchArea` holding the drag in
+    /// progress. The same hazard `rows`'s in-place point update exists to
+    /// avoid. Nothing in this list is a drag target, so up here it is safe to
+    /// replace wholesale, exactly as [`Self::curve_samples`] is.
+    pub fn curve_channels(&self) -> Vec<String> {
+        for op in &self.scoped_capabilities() {
+            let Some(presentation) = &op.presentation else {
+                continue;
+            };
+            if presentation.choose(supported) != Some(WidgetKind::ToneCurve) {
+                continue;
+            }
+            let Some(runs) = curve_runs(op, presentation) else {
+                continue;
+            };
+            if runs.len() < 2 {
+                continue;
+            }
+            return runs
+                .iter()
+                .map(|r| r.subject.map(labels::resolve).unwrap_or_default())
+                .collect();
+        }
+        Vec::new()
+    }
+
+    /// Which curve the widget is plotting, as an index into
+    /// [`Self::curve_channels`].
+    pub fn curve_channel(&self) -> i32 {
+        self.curve_channel as i32
+    }
+
+    /// Plot a different one of the operation's curves.
+    ///
+    /// Out-of-range indices are ignored rather than clamped: the only thing
+    /// that can send one is a stale interface event, and quietly moving the
+    /// selection somewhere the user did not point is worse than doing nothing.
+    pub fn set_curve_channel(&mut self, index: i32) {
+        let Ok(index) = usize::try_from(index) else {
+            return;
+        };
+        if index < self.curve_channels().len() {
+            self.curve_channel = index;
+        }
+    }
+
+    /// The plotted curve's shape, sampled for drawing.
     ///
     /// Evaluated with `dr_pipeline`'s own spline, so the line the user drags
     /// is the line the shader applies. The alternative — reading the curve
     /// back off the GPU — is the round-trip ARCH §6.1 forbids, to draw a
     /// polyline.
+    ///
+    /// The line drawn is the *selected* curve's own shape, not the composition
+    /// of it with the master. Two curves overlaid on one grid is a plot of two
+    /// things, and the one being dragged has to be the one whose points are
+    /// under the pointer.
     pub fn curve_samples(&self) -> Vec<f32> {
         const SAMPLES: usize = 96;
 
+        // The selection is an index over the subjects the panel found, which
+        // for this operation is its channel order. Clamped rather than
+        // trusted: a selection made on one photograph outlives the change to
+        // the next.
+        let channel = curve::Channel::ALL[self.curve_channel.min(curve::CHANNELS - 1)];
+
         let mut xs = [0.0f32; curve::POINTS];
         let mut ys = [0.0f32; curve::POINTS];
         let mut found = false;
@@ -757,16 +919,18 @@ impl DevelopSession {
                 continue;
             }
             found = true;
-            for (i, p) in cap.params.iter().enumerate() {
-                let point = i / 2;
-                if point >= curve::POINTS {
-                    break;
-                }
-                if i % 2 == 0 {
-                    xs[point] = p.value;
-                } else {
-                    ys[point] = p.value;
-                }
+            // By id rather than by position, so which curve is plotted is
+            // decided by naming it and not by arithmetic over the parameter
+            // list.
+            let value = |id| {
+                cap.params
+                    .iter()
+                    .find(|p| p.id == id)
+                    .map_or(0.0, |p| p.value)
+            };
+            for i in 0..curve::POINTS {
+                xs[i] = value(curve::coordinate(channel, i, curve::Axis::X));
+                ys[i] = value(curve::coordinate(channel, i, curve::Axis::Y));
             }
         }
         if !found {
@@ -3419,9 +3583,9 @@ mod tests {
 
     #[test]
     fn the_curve_collapses_to_a_single_row() {
-        // Ten point parameters must appear as one curve control, not ten
-        // sliders — otherwise the widget and the sliders both render and the
-        // panel shows the same values twice.
+        // Every point parameter — all four curves' worth — must appear as one
+        // curve control, not as forty sliders. Otherwise the widget and the
+        // sliders both render and the panel shows the same values twice.
         let graph = EditGraph::default_chain();
         let curve_cap = graph
             .capabilities()
@@ -3429,7 +3593,11 @@ mod tests {
             .find(|c| c.id == curve::ID)
             .expect("the chain includes a tone curve");
 
-        assert_eq!(curve_cap.params.len(), curve::POINTS * 2);
+        assert_eq!(
+            curve_cap.params.len(),
+            curve::CHANNELS * curve::POINTS * 2,
+            "a master curve and one per colour channel"
+        );
         let presentation = curve_cap
             .presentation
             .as_ref()
@@ -3469,6 +3637,144 @@ mod tests {
         }
     }
 
+    /// The panel's whole knowledge of colour channels, asserted to be none.
+    ///
+    /// It groups the widget's parameters by the subject the *operation* put on
+    /// them and finds four curves; nothing below says "red", and an operation
+    /// that grew a fifth curve would arrive here on its own.
+    #[test]
+    fn a_curve_widget_offers_one_run_per_subject() {
+        let graph = EditGraph::default_chain();
+        let cap = graph
+            .capabilities()
+            .into_iter()
+            .find(|c| c.id == curve::ID)
+            .expect("tone curve present");
+        let presentation = cap.presentation.as_ref().expect("declares a widget");
+
+        let runs = curve_runs(&cap, presentation).expect("a curve-shaped operation");
+        assert_eq!(runs.len(), curve::CHANNELS);
+        for (i, run) in runs.iter().enumerate() {
+            assert_eq!(run.len, curve::POINTS * 2, "run {i} is not five points");
+            assert_eq!(run.base, i * curve::POINTS * 2);
+            assert!(run.subject.is_some(), "run {i} is unnamed");
+        }
+    }
+
+    #[test]
+    fn switching_curve_repoints_the_row() {
+        use slint::Model as _;
+
+        // What a drag routes through. The row's `param_index` is the base of
+        // the curve *on show*, so picking a different one must move it — if it
+        // did not, dragging a point on the red curve would write to the
+        // master's.
+        let graph = EditGraph::default_chain();
+        let caps = graph.capabilities();
+        let curve_at = caps
+            .iter()
+            .position(|c| c.id == curve::ID)
+            .expect("tone curve present");
+
+        let mut bases = Vec::new();
+        for channel in 0..curve::CHANNELS {
+            let rows = rows_filtered(&caps, |_| true, channel);
+            let row = rows
+                .iter()
+                .find(|r| r.op_index as usize == curve_at)
+                .expect("the curve has a row");
+            assert_eq!(row.kind, "curve");
+            assert_eq!(
+                row.points.row_count(),
+                curve::POINTS * 2,
+                "one curve's points, not all four curves'"
+            );
+            bases.push(row.param_index);
+        }
+
+        assert_eq!(
+            bases,
+            (0..curve::CHANNELS)
+                .map(|i| (i * curve::POINTS * 2) as i32)
+                .collect::<Vec<_>>()
+        );
+    }
+
+    #[test]
+    fn a_selection_the_operation_cannot_honour_falls_back_to_its_last_curve() {
+        // The selection outlives the photograph it was made on, and the next
+        // image's operation may offer fewer curves. Clamping keeps a plot on
+        // the grid; the alternative is a curve row that vanishes, which reads
+        // as the tone curve having disappeared from the panel.
+        let graph = EditGraph::default_chain();
+        let caps = graph.capabilities();
+        let rows = rows_filtered(&caps, |_| true, 99);
+        let row = rows
+            .iter()
+            .find(|r| r.kind == "curve")
+            .expect("the curve still has a row");
+        assert_eq!(
+            row.param_index,
+            ((curve::CHANNELS - 1) * curve::POINTS * 2) as i32
+        );
+    }
+
+    #[test]
+    fn an_operation_whose_points_are_unfaceted_is_one_curve() {
+        // A curve widget that spans a single unnamed curve — which is what
+        // this operation was before the channels arrived, and what any other
+        // node declaring a `tone_curve` widget over ten scalars would be.
+        // It must draw, and it must offer no choice.
+        use dr_pipeline::{LocalizedKey, ParamCapability, WidgetDemand};
+        use slint::Model as _;
+
+        static IDS: [ParamId; 4] = [
+            ParamId("p0_x"),
+            ParamId("p0_y"),
+            ParamId("p1_x"),
+            ParamId("p1_y"),
+        ];
+        let param = |id: ParamId| ParamCapability {
+            id,
+            label: LocalizedKey("param.point"),
+            kind: ParamKind::Scalar {
+                min: 0.0,
+                max: 1.0,
+                scale: dr_pipeline::Scale::Linear,
+                unit: Unit::None,
+                precision: 4,
+            },
+            default: 0.0,
+            value: 0.0,
+            facet: None,
+        };
+        let plain = OpCapability {
+            id: OpId("invented_curve"),
+            label: LocalizedKey("op.invented_curve"),
+            active: false,
+            presentation: Some(Presentation {
+                widgets: &[WidgetKind::ToneCurve],
+                demand: WidgetDemand {
+                    two_dimensional: true,
+                    precise_pointing: true,
+                },
+                params: &IDS,
+            }),
+            params: IDS.iter().map(|id| param(*id)).collect(),
+            attributes: &[dr_pipeline::Attribute::Tone],
+        };
+
+        let presentation = plain.presentation.as_ref().expect("declares a widget");
+        let runs = curve_runs(&plain, presentation).expect("curve-shaped");
+        assert_eq!(runs.len(), 1, "one unnamed curve");
+        assert_eq!(runs[0].subject, None);
+
+        let rows = rows_from(&[plain]);
+        assert_eq!(rows.len(), 1);
+        assert_eq!(rows[0].kind, "curve");
+        assert_eq!(rows[0].points.row_count(), IDS.len());
+    }
+
     #[test]
     fn curve_samples_start_on_the_diagonal() {
         // A fresh curve is the identity, so the drawn line must be the 45°

ui/dr-ui/src/labels.rs

@@ -54,6 +54,19 @@ pub fn resolve(key: &str) -> String {
         "param.channel.sat" => "Saturation".into(),
         "param.channel.lum" => "Luminance".into(),
 
+        // The tone curve's four curves, which its points are *subject* to.
+        //
+        // Catalogued rather than derived because the master curve's key would
+        // otherwise read "Rgb": these are the terms of a four-way choice, and
+        // one of them miscapitalised is the one the eye goes to. The three
+        // colours would derive correctly and are written out beside it anyway,
+        // since a list where one entry is translated and three are guessed is
+        // the shape a half-finished translation takes.
+        "channel.rgb" => "RGB".into(),
+        "channel.red" => "Red".into(),
+        "channel.green" => "Green".into(),
+        "channel.blue" => "Blue".into(),
+
         // The hue bands, which a faceted row is *subject* to.
         //
         // Catalogued even where `derive` would produce the same word, because

ui/dr-ui/src/lib.rs

@@ -583,8 +583,24 @@ pub(crate) fn sync_rows(
         // curve must be drawn whatever shape it is in.
         rows.set_vec(current);
         curve_moved = true;
+
+        // The curves the widget can switch between, named. They can only
+        // change with the operation set, which is what this branch means, so
+        // the walk that derives them is not on the parameter-event path.
+        let channels: Vec<slint::SharedString> = match session.borrow().as_ref() {
+            Some(s) => s.curve_channels().into_iter().map(Into::into).collect(),
+            None => Vec::new(),
+        };
+        window.set_curve_channels(slint::ModelRc::new(slint::VecModel::from(channels)));
     }
 
+    // Which curve is plotted, on every pass. Picking one that happens to be
+    // shaped like the last — two untouched curves are both the diagonal —
+    // moves no point, so this cannot ride on the resample below: the chips
+    // would go on highlighting the curve the user just navigated away from.
+    let channel = session.borrow().as_ref().map_or(0, |s| s.curve_channel());
+    window.set_curve_channel(channel);
+
     if !curve_moved {
         return;
     }
@@ -1804,6 +1820,24 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
             redraw(&w);
         });
     }
+    {
+        // Which of the curve's curves the plot is showing. **No redraw**, and
+        // that is the whole character of this control: it changes no
+        // parameter, so the photograph is already correct on screen and
+        // recomputing it would be a frame spent to produce the same pixels.
+        // For the same reason it records no history step — there is nothing
+        // to undo — and the sidecar never hears about it.
+        let weak = window.as_weak();
+        let session = session.clone();
+        let rows = rows.clone();
+        window.on_curve_channel_picked(move |index| {
+            let Some(w) = weak.upgrade() else { return };
+            if let Some(s) = session.borrow_mut().as_mut() {
+                s.set_curve_channel(index);
+            }
+            sync_rows(&w, &rows, &session);
+        });
+    }
 
     // ---- undo and redo (FR-DEV-5) ---------------------------------------
     //

ui/dr-ui/ui/adjust.slint

@@ -365,10 +365,17 @@ component ParamControl inherits Rectangle {
     in property <ParamRow> data;
     /// Curve rows only; ignored by every other kind.
     in property <[float]> curve-samples;
+    /// The curves this widget can plot, named. Empty, or one entry, where
+    /// there is nothing to choose between — see `curve-channel-picked`.
+    in property <[string]> curve-channels;
+    /// Which of `curve-channels` is on the grid.
+    in property <int> curve-channel;
 
     callback param-changed(int, int, float);
     callback param-reset(int, int);
     callback curve-reset(int);
+    /// Plot a different one of the operation's curves.
+    callback curve-channel-picked(int);
     callback drag-changed(bool);
 
     height: layout.preferred-height;
@@ -420,20 +427,49 @@ component ParamControl inherits Rectangle {
             }
         }
 
-        if root.data.kind == "curve": CurveEditor {
-            points: root.data.points;
-            samples: root.curve-samples;
-            drag-changed(on) => { root.drag-changed(on); }
-            // A point carries two parameters, so the parameter index is the
-            // row's base plus the point's offset. This component still knows
-            // nothing about which operation it belongs to.
-            point-moved(point, x, y) => {
-                root.param-changed(
-                    root.data.op-index, root.data.param-index + point * 2, x);
-                root.param-changed(
-                    root.data.op-index, root.data.param-index + point * 2 + 1, y);
+        // A curve, and — where the operation offers more than one — the choice
+        // of which curve is on the grid.
+        //
+        // One plot rather than four stacked ones: the curves are read against
+        // the diagonal and against each other, which needs the grid large, and
+        // four grids at a quarter of the width would each be too small to
+        // place a point in. So the selector switches the subject of a single
+        // plot, and the names in it come from the core — this file does not
+        // know that a colour channel is what is being chosen between, only
+        // that the widget said it spans several named things.
+        if root.data.kind == "curve": VerticalLayout {
+            spacing: Theme.gap-sm;
+
+            if root.curve-channels.length > 1: Segmented {
+                // The operation's own name, which nothing else in this row
+                // draws: a curve row heads no group, so without this the plot
+                // would sit in the panel unlabelled.
+                label: root.data.op-label;
+                options: root.curve-channels;
+                selected: root.curve-channel;
+                picked(i) => { root.curve-channel-picked(i); }
+            }
+
+            CurveEditor {
+                points: root.data.points;
+                samples: root.curve-samples;
+                drag-changed(on) => { root.drag-changed(on); }
+                // A point carries two parameters, so the parameter index is
+                // the row's base plus the point's offset. The base is the
+                // first point of the curve *on show*, so switching curve
+                // re-points the drag and this component still knows nothing
+                // about which operation — or which curve — it is drawing.
+                point-moved(point, x, y) => {
+                    root.param-changed(
+                        root.data.op-index, root.data.param-index + point * 2, x);
+                    root.param-changed(
+                        root.data.op-index, root.data.param-index + point * 2 + 1, y);
+                }
+                // Resetting a curve resets the operation, which is all four of
+                // them — a photographer who double-clicks to start again means
+                // the control, not the curve that happens to be on show.
+                reset => { root.curve-reset(root.data.op-index); }
             }
-            reset => { root.curve-reset(root.data.op-index); }
         }
     }
 }
@@ -825,9 +861,16 @@ export component AdjustPanel inherits Rectangle {
     /// The tone curve's sampled shape, evaluated in Rust by the same spline
     /// the shader runs so the drawn line cannot disagree with the applied one.
     in property <[float]> curve-samples;
+    /// The curves the tone curve widget can plot, named by the core. Fewer
+    /// than two of them means there is nothing to choose and no selector.
+    in property <[string]> curve-channels;
+    /// Which of them `curve-samples` and the row's points describe.
+    in property <int> curve-channel;
     callback param-changed(int, int, float);
     callback param-reset(int, int);
     callback curve-reset(int);
+    /// Plot a different one of the curve's curves.
+    callback curve-channel-picked(int);
     /// Return every parameter of one operation to its default — the reset on
     /// a section's own header, beside the panel-wide one.
     callback op-reset(int);
@@ -986,12 +1029,15 @@ export component AdjustPanel inherits Rectangle {
                     ParamControl {
                         data: row;
                         curve-samples: root.curve-samples;
+                        curve-channels: root.curve-channels;
+                        curve-channel: root.curve-channel;
                         drag-changed(on) => { root.slider-dragging = on; }
                         param-changed(op, param, v) => {
                             root.param-changed(op, param, v);
                         }
                         param-reset(op, param) => { root.param-reset(op, param); }
                         curve-reset(op) => { root.curve-reset(op); }
+                        curve-channel-picked(i) => { root.curve-channel-picked(i); }
                     }
                 }
             }

ui/dr-ui/ui/app.slint

@@ -716,9 +716,14 @@ export component AppWindow inherits Window {
     // The tone curve's sampled shape, evaluated by the core so the drawn
     // line and the applied one cannot disagree.
     in property <[float]> curve-samples;
+    // The curves that widget can plot, named by the core, and which of them
+    // `curve-samples` describes. Fewer than two means nothing to choose.
+    in property <[string]> curve-channels;
+    in property <int> curve-channel;
     callback param-changed(int, int, float);
     callback param-reset(int, int);
     callback curve-reset(int);
+    callback curve-channel-picked(int);
     callback reset-all();
 
     // --- copying settings between photographs (FR-DEV-6) ---
@@ -2177,6 +2182,11 @@ in property <bool> panel-visible: true;
                                 enabled: root.adjust-enabled;
                                 scope: root.adjust-scope;
                                 curve-samples: root.curve-samples;
+                                curve-channels: root.curve-channels;
+                                curve-channel: root.curve-channel;
+                                curve-channel-picked(i) => {
+                                    root.curve-channel-picked(i);
+                                }
                                 param-changed(op, param, value) => {
                                     root.param-changed(op, param, value);
                                 }