Refuse a clipped highlight as a neutral

Sampling the overcast sky on a Canon 6D frame set tint to -100 and
temperature to -15 for a patch the canvas showed as pure white. A clipped
photosite is sensor white, not a colour: every channel stopped counting,
so what the tap hands back is the as-shot multipliers themselves, which
are strongly magenta, and the solver dutifully drove green to its stop.
The display shader already fades such a pixel to a neutral of the same
brightness before any operation runs, so the picker was balancing against
something the photographer could not see.

The probe now refuses a sample with any channel at or above the onset the
shader fades from, the way the solver already refuses black. The threshold
is one constant, CLIP_ONSET, formatted into the shader and read by the
probe, so the two cannot drift apart.

core/dr-pipeline/src/lib.rs

@@ -65,7 +65,8 @@ pub use history::{Edit, Entry as HistoryEntry, History, Step};
 pub use lens::{compose_warps, ComposedWarp, LensProfile, Tca, Warp};
 pub use operation::{
     compose, compose_with_framing, Affects, ComposedShader, Helper, Invalidation, Operation,
-    OutputMode, Uniform, BASE_CURVE_POINTS, BASE_CURVE_UNIFORM_OFFSET, RESERVED_UNIFORM_FIELDS,
+    OutputMode, Uniform, BASE_CURVE_POINTS, BASE_CURVE_UNIFORM_OFFSET, CLIP_ONSET,
+    RESERVED_UNIFORM_FIELDS,
 };
 pub use preset::{LibraryParseError, NameError, Preset, PresetLibrary, Scope};
 pub use sidecar::{Sidecar, Version};

core/dr-pipeline/src/neutral.rs

@@ -69,11 +69,13 @@ const ROUNDS: u32 = 3;
 
 /// Below this a channel carries no ratio worth balancing.
 ///
-/// A sample in the deep shadows, or one taken on a blown highlight where a
+/// A sample in the deep shadows has no white balance in it: the logarithms
-/// channel has already clipped to nothing, has no white balance in it: the
+/// below would run away and the picker would slam a slider to its stop.
-/// logarithms below would run away and the picker would slam a slider to its
+/// Refusing is the honest answer, and the caller reports that the point was
-/// stop. Refusing is the honest answer, and the caller reports that the point
+/// not usable rather than moving the photograph. The other end — a blown
-/// was not usable rather than moving the photograph.
+/// highlight, where every channel has stopped counting — is refused by the
+/// caller before the sample is taken, because only the caller can see the
+/// sensor value; see [`crate::operation::CLIP_ONSET`].
 const FLOOR: f32 = 1e-4;
 
 /// TRACES: FR-DEV-3

core/dr-pipeline/src/operation.rs

@@ -492,6 +492,19 @@ pub const BASE_CURVE_UNIFORM_OFFSET: usize = 16;
 /// selection in [`compose_full`].
 pub const BASE_CURVE_POINTS: usize = 5;
 
+/// Where the highlight desaturation begins: the fraction of the white level
+/// above which a photosite is treated as clipped.
+///
+/// A photosite this close to saturation has stopped counting, so its ratio
+/// to its neighbours is not a colour. The generated prologue fades a pixel
+/// above this toward a neutral of the same brightness before any operation
+/// runs, and the white balance probe refuses to sample one: a blown sky is
+/// sensor white, which the as-shot multipliers make magenta, and a solve
+/// over that slams tint to its stop. One number, so the two cannot drift
+/// apart — a probe that accepted what the shader had already desaturated
+/// would be balancing against a pixel the photographer cannot see.
+pub const CLIP_ONSET: f32 = 0.985;
+
 /// Where an operation's own uniforms begin in the generated block.
 ///
 /// The base fields, then framing's. Exported because `dr-gpu` writes the
@@ -1016,6 +1029,9 @@ fn compose_inner(
         .to_string()
     };
 
+    // Formatted with Rust's `Display` so the shader reads the same threshold
+    // the probe checks against; see `CLIP_ONSET`.
+    let clip_onset = CLIP_ONSET;
     let source = format!(
         "// GENERATED — do not edit.
 //
@@ -1086,7 +1102,7 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {{
     // A photosite at its white level carries no colour information — every
     // channel simply stopped counting — so the balance below must not be
     // allowed to tint it.
-    let clipped = smoothstep(0.985, 1.0, max(c.r, max(c.g, c.b)));
+    let clipped = smoothstep({clip_onset}, 1.0, max(c.r, max(c.g, c.b)));
 
     c = c * u.as_shot_wb.rgb;
 

ui/dr-ui/src/develop.rs

@@ -4502,6 +4502,15 @@ impl DevelopSession {
         if pixel[3] < 0.5 {
             return None;
         }
+        // Nor in a clipped one. A blown sky reads as sensor white, and sensor
+        // white with the as-shot balance on is strongly magenta — a solve over
+        // it drives tint to its stop for a pixel that, on the canvas, the
+        // shader has already desaturated to neutral. The same threshold the
+        // shader fades from, so what is refused here is what it would have
+        // hidden there.
+        if pixel[..3].iter().any(|c| *c >= dr_pipeline::CLIP_ONSET) {
+            return None;
+        }
 
         // The tap is the sensor's numbers with the profile filled neutral;
         // the operation multiplies them *after* the camera's own balance, so
@@ -6745,6 +6754,52 @@ mod tests {
         );
     }
 
+    /// TRACES: FR-DEV-3
+    /// A blown highlight is refused, the way black is.
+    ///
+    /// Sensor white is not a colour: every channel stopped counting, so the
+    /// ratio between them is the as-shot multipliers and nothing about the
+    /// scene. Sampling the overcast sky on a Canon 6D frame drove tint to
+    /// -100 and temperature to -15 for a patch the canvas showed as pure
+    /// white, which is the picker being wrong rather than the point being a
+    /// poor choice. Refused, nothing moves and no step is taken.
+    #[test]
+    fn sampling_a_blown_highlight_moves_nothing() {
+        let Some(ctx) = headless() else { return };
+        let size = 16u32;
+        let raw = RawImage {
+            width: size,
+            height: size,
+            data: vec![65535; (size * size * 3) as usize],
+            cfa_pattern: dr_decode::CfaPattern::Rggb,
+            black_level: [0; 4],
+            white_level: 65535,
+            wb_coeffs: [1.9, 1.0, 1.7, 0.0],
+            color_matrix: None,
+            base_curve: dr_decode::BaseCurve::IDENTITY,
+            samples_per_pixel: 3,
+            profile: None,
+            make: String::new(),
+            model: String::new(),
+            crop: dr_decode::CropRect {
+                x: 0,
+                y: 0,
+                width: size,
+                height: size,
+            },
+        };
+        let mut session =
+            DevelopSession::open(&ctx, &raw, dr_types::Orientation::NORMAL).expect("session");
+        let steps = session.history_rows().len();
+
+        assert!(
+            !session.sample_neutral(0.5, 0.5),
+            "a clipped photosite has no balance in it"
+        );
+        assert!(session.is_neutral(), "and so nothing was corrected");
+        assert_eq!(session.history_rows().len(), steps);
+    }
+
     /// TRACES: FR-DEV-7 | FR-DEV-5
     /// A held comparison hands the edit straight back.
     ///