Render raws through the DNG reference curve by default, at contrast 1.5

Decides D21 by measurement. The photo gallery holds Lightroom 6 exports of
raws in the library, each carrying its Camera Raw settings; clustered by
those settings, 663 had no look applied. On 60 of them with their raws,
a third held out, the held-out MSE against Lightroom's JPEG was about 1200
for 0.20.0's sigmoid (0.7 EV darker and flatter), 224 for the DNG reference curve
after baseline exposure, and about 140 once its input is bent by 1.5/1.4
about grey.

So the curve choice defaults to the DNG reference, keeping its index (sidecars
record it), and the default contrast is 1.5. Contrast under the DNG reference curve is
now a power relative to REFERENCE_CONTRAST (1.4), where the table is
untouched; the sigmoid at that contrast still matches the retired base
curve. JPEGs are unaffected: the view transform skips a rendered source.

core/dr-pipeline/src/camera_raw.rs

@@ -19,7 +19,7 @@
 //! shader to it.
 
 use crate::ops::camera_profile::{mul, working_prophoto};
-use crate::view::{DEFAULT_CONTRAST, DEFAULT_WHITE, SCENE_GREY};
+use crate::view::{DEFAULT_CONTRAST, DEFAULT_WHITE, REFERENCE_CONTRAST, SCENE_GREY};
 
 /// The input scale for a white point: 1 at the default, so sensor white is
 /// display white as in the SDK's reference; each stop of `white` above it halves the
@@ -28,10 +28,14 @@ pub fn input_scale(white: f32) -> f32 {
     (DEFAULT_WHITE - white).exp2()
 }
 
-/// The power the input is bent by about middle grey: 1 at the default
-/// contrast, so the curve is the reference's untouched.
+/// The power the input is bent by about middle grey: 1 at
+/// [`REFERENCE_CONTRAST`], where the curve is the reference's untouched.
+///
+/// The default contrast sits above it, so a photograph out of the camera is
+/// bent by `DEFAULT_CONTRAST / REFERENCE_CONTRAST` — the extra contrast
+/// Lightroom's exports showed over the bare reference curve (D21 addendum).
 pub fn contrast_power(contrast: f32) -> f32 {
-    contrast / DEFAULT_CONTRAST
+    contrast / REFERENCE_CONTRAST
 }
 
 /// The curve, its scale and its contrast applied to one ProPhoto colour.
@@ -103,15 +107,23 @@ mod tests {
     }
 
     #[test]
-    fn at_its_defaults_the_input_is_untouched() {
+    fn at_the_reference_the_input_is_untouched() {
         assert_eq!(input_scale(DEFAULT_WHITE), 1.0);
-        assert_eq!(contrast_power(DEFAULT_CONTRAST), 1.0);
+        assert_eq!(contrast_power(REFERENCE_CONTRAST), 1.0);
+    }
+
+    #[test]
+    fn the_default_adds_the_contrast_lightroom_showed() {
+        // Fitted on Lightroom exports with neutral settings (D21 addendum):
+        // the bare reference curve is a little flat against them.
+        let p = contrast_power(DEFAULT_CONTRAST);
+        assert!((1.05..1.12).contains(&p), "{p}");
     }
 
     #[test]
     fn grey_goes_through_the_curve_and_stays_grey() {
         for v in [0.02, 0.13, 0.5] {
-            let out = apply_reference(&ACR3_DEFAULT, [v; 3], DEFAULT_CONTRAST, DEFAULT_WHITE);
+            let out = apply_reference(&ACR3_DEFAULT, [v; 3], REFERENCE_CONTRAST, DEFAULT_WHITE);
             let want = evaluate(&ACR3_DEFAULT, v);
             assert!(
                 out.iter().all(|o| (o - want).abs() < 1e-4),
@@ -123,7 +135,7 @@ mod tests {
     #[test]
     fn an_identity_curve_changes_nothing_inside_the_range() {
         let c = [0.4, 0.2, 0.1];
-        let out = apply_reference(&identity(), c, DEFAULT_CONTRAST, DEFAULT_WHITE);
+        let out = apply_reference(&identity(), c, REFERENCE_CONTRAST, DEFAULT_WHITE);
         assert!(
             out.iter().zip(c).all(|(o, c)| (o - c).abs() < 1e-4),
             "{out:?}"

core/dr-pipeline/src/descriptor.rs

@@ -464,6 +464,15 @@ impl ParamDescriptor {
         }
     }
 
+    /// The same choice with another variant as its default.
+    ///
+    /// For a choice whose variants were numbered before its default was
+    /// settled: a sidecar records the index, so reordering the variants to
+    /// put the default first would change what saved edits mean.
+    pub fn with_default(self, default: f32) -> Self {
+        Self { default, ..self }
+    }
+
     /// A 0…1 fraction — a proportion of something, rather than an amount.
     ///
     /// Its own constructor because the crop rect needs four of them and the

core/dr-pipeline/src/ops/view_transform.rs

@@ -34,10 +34,13 @@ pub const ID: OpId = OpId("view_transform");
 pub const CONTRAST: ParamId = ParamId("contrast");
 pub const WHITE: ParamId = ParamId("white");
 /// TRACES: FR-DEV-3j
-/// Which curve renders: D19's sigmoid (index 0, the default) or Camera
-/// Raw's (D21). Which of the two should be the default is open — the
-/// measurement work decides it — so the default is what 0.20.0
-/// rendered.
+/// Which curve renders: Camera Raw's (D21, the default) or D19's sigmoid.
+///
+/// Camera Raw's because it is the one that matches what the photographs were
+/// first developed with: on 60 Lightroom exports with neutral settings it
+/// renders their raws within MSE ~150 of Lightroom's own JPEGs at the default
+/// contrast, where 0.20.0's sigmoid was ~1200 (darker by about 0.7 EV and
+/// flatter). The sigmoid keeps index 0 because sidecars record the index.
 pub const CURVE: ParamId = ParamId("curve");
 
 /// [`CURVE`]'s values, in the order of its variants.
@@ -93,7 +96,8 @@ static DESCRIPTOR: LazyLock<Arc<OpDescriptor>> = LazyLock::new(|| {
                     LocalizedKey("param.view_transform.curve.sigmoid"),
                     LocalizedKey("param.view_transform.curve.camera_raw"),
                 ],
-            ),
+            )
+            .with_default(CAMERA_RAW),
         ],
     })
 });
@@ -110,7 +114,7 @@ impl Default for ViewTransform {
         Self {
             contrast: DEFAULT_CONTRAST,
             white: DEFAULT_WHITE,
-            curve: SIGMOID,
+            curve: CAMERA_RAW,
         }
     }
 }
@@ -150,7 +154,7 @@ impl Operation for ViewTransform {
     }
 
     fn is_active(&self) -> bool {
-        self.contrast != DEFAULT_CONTRAST || self.white != DEFAULT_WHITE || self.curve != SIGMOID
+        self.contrast != DEFAULT_CONTRAST || self.white != DEFAULT_WHITE || self.curve != CAMERA_RAW
     }
 
     fn stage(&self) -> Stage {

core/dr-pipeline/src/view.rs

@@ -51,8 +51,19 @@ pub const SCENE_GREY: f32 = 0.13;
 /// Display-linear middle grey — what a camera JPEG shows a grey card as.
 pub const DISPLAY_GREY: f32 = 0.18;
 
-/// The default contrast, the sigmoid's log-log slope parameter `n`.
-pub const DEFAULT_CONTRAST: f32 = 1.4;
+/// The default contrast: the sigmoid's log-log slope `n`, and for Camera
+/// Raw's curve a power of `DEFAULT_CONTRAST / REFERENCE_CONTRAST` about grey.
+///
+/// Fitted, not chosen: on Lightroom 6 exports whose look settings were neutral,
+/// Camera Raw's curve matched the exports best with the input bent by about
+/// 1.08 (held-out MSE 224 at 1.4, ~150 at 1.5). The sigmoid, which is no longer
+/// the default, fitted best near 1.7 and is better at 1.5 than at 1.4.
+pub const DEFAULT_CONTRAST: f32 = 1.5;
+
+/// The contrast at which each curve is its own reference: the sigmoid's match
+/// to the retired base curve (see the module note), and Camera Raw's table
+/// untouched.
+pub const REFERENCE_CONTRAST: f32 = 1.4;
 
 /// The default white point, in stops above [`SCENE_GREY`].
 pub const DEFAULT_WHITE: f32 = 4.0;
@@ -223,11 +234,13 @@ mod tests {
     }
 
     #[test]
-    fn the_default_stays_close_to_the_retired_curve() {
+    fn the_reference_stays_close_to_the_retired_curve() {
         // TRACES: FR-DEV-3j | FR-DEV-3e
-        // D19's promise to every existing photograph: the midtones do not
-        // move by more than a third of a stop.
-        let s = Sigmoid::default_curve();
+        // D19's promise, now kept by the sigmoid at its reference contrast
+        // rather than by the default (D21 moved the default to Camera Raw's
+        // curve): the midtones do not move by more than a third of a stop
+        // from the retired base curve.
+        let s = Sigmoid::new(REFERENCE_CONTRAST, DEFAULT_WHITE);
         let mut x = 0.03_f32;
         while x <= 1.0 {
             let ev = (s.channel(x) / retired_default(x)).log2();