Merge PR #5771: Lab: adjustment results are whole samples, as the reference app stores them (lab8 curves passes)

crates/io/tests/corpus.rs: kept main's pass_floor 188 (TAKE-FIX: drop the PR's 186 hunk)
docs/file-format-parity.md: kept main's header and PSD/PSB rows; added the PR's change-log row without its floor figure
docs/gaps.md: kept main's header and evidence line; added the PR's change-log row without its floor figure
docs/scorecard.md: ours

crates/compose/src/native_lab.rs

@@ -89,6 +89,9 @@ fn collect<'a>(layers: &'a [Layer], depth: SampleType, adjustments: bool, items:
 /// A Lab document's visible layers, prepared for compositing on their stored L, a and b.
 pub struct LabStack<'a> {
     items: Vec<Item<'a>>,
+    /// Steps per unit of the document's samples (`None` at 32 bits): adjustment results are
+    /// whole samples, as Photoshop stores them.
+    quantum: Option<f32>,
 }
 
 impl<'a> LabStack<'a> {
@@ -101,7 +104,7 @@ impl<'a> LabStack<'a> {
         }
         let mut items = Vec::new();
         collect(&doc.layers, doc.depth, true, &mut items, 0)?;
-        Some(LabStack { items })
+        Some(LabStack { items, quantum: crate::adjustment_quantum(doc.depth) })
     }
 
     /// Whether the stack has an adjustment layer (the RGB compositor can't match it exactly).
@@ -161,6 +164,9 @@ impl<'a> LabStack<'a> {
                         op(&mut lab);
                         for (c, v) in lab.iter().enumerate() {
                             d[c] += (v - d[c]) * mix;
+                            if let Some(q) = self.quantum {
+                                d[c] = (d[c] * q + 0.5).floor() / q;
+                            }
                         }
                     }
                 }

crates/compose/src/tests.rs

@@ -3217,6 +3217,32 @@ fn lab_adjustments_work_on_the_stored_values() {
     near3([p[0], p[1], p[2]], lab_display([l, byte(218), byte(38)]), 1e-3, "levels on L at half opacity");
 }
 
+// An adjustment's result in a Lab document is whole samples, as Photoshop stores it: Curves
+// taking L 100 to 120 of 255 shows L 120 exactly (the photoshop corpus's lab8 curves.psd is exact
+// so, where the unrounded result left saturated colours a level off on screen), at 8 bits; 16 bits
+// round to 1/32768.
+#[test]
+fn lab_adjustment_results_are_whole_samples() {
+    let pts = |v: &[(f32, f32)]| v.iter().map(|&(input, output)| CurvePoint { input, output }).collect::<Vec<_>>();
+    let curves = |l: &[(f32, f32)]| Adjustment::Curves {
+        master: Vec::new(),
+        per_channel: [pts(l), Vec::new(), Vec::new()],
+        space: ToneSpace::Lab,
+        black: Vec::new(),
+        pencil: false,
+    };
+    // A straight line from (0, 0) through (100, 120.4): L 100 lands between two bytes.
+    let adj = curves(&[(0.0, 0.0), (100.0 / 255.0, 120.4 / 255.0), (1.0, 1.0)]);
+    for (depth, q) in [(SampleType::U8, 255.0f32), (SampleType::U16, 32768.0)] {
+        let doc = lab_stack([100.0 / 255.0, 0.5, 0.5], depth, vec![adj.clone()]);
+        let p = px(&doc, 1, 1);
+        let stack = native_lab::LabStack::new(&doc);
+        assert!(stack.is_some_and(|s| s.adjusts()), "{depth:?}: the native path");
+        let l = photocraft_color::convert::srgb_to_lab([p[0], p[1], p[2]])[0] / 100.0;
+        assert!((l * q - (l * q).round()).abs() < 0.02, "{depth:?}: L {} is between two samples", l * q);
+    }
+}
+
 // Flattening a Lab document keeps its stored values, out-of-gamut colours included (the RGB
 // compositor clipped them on the way through display RGB).
 #[test]

docs/file-format-parity.md

@@ -142,6 +142,7 @@ remaining gaps in #1606 stay tracked with per-sample corpus ceilings.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-14 | minor | Lab documents round each adjustment layer's result on the stored L, a, b to whole samples, as Photoshop stores them (the native Lab path): photoshop lab8 curves.psd passes |
 | 2026-10-15 | minor | PSD smart filters: Clouds, Difference Clouds, Fibers and Lens Flare open as editable filters with their seeds, colours and lens, and save like Photoshop (checked against a Photoshop 27 file); render filters on a smart object work within the object, so a flare sits where Photoshop puts it |
 | 2026-10-15 | minor | DDS textures save (#1327): uncompressed or BC1/BC2/BC3 (DXT1/3/5) from a clean-room block encoder, optional box-filtered mip chain, legacy header; DDS Options in Save As and Save a Copy, `ddsCompression`/`ddsMipmaps` for scripts, the CLI and MCP |
 | 2026-10-16 | minor | PSD smart filters: Perspective Warp (`perspectiveWarpTransform`: `vertices`, `warpedVertices`, `quads` of vertex indices, document pixels) opens as `layer.smartObjects.perspectiveWarp`, one plane per quad, and re-renders warped; save writes the original item with only the geometry rewritten, or a new one in Photoshop's layout; inconsistent descriptors stay verbatim (FILE-215-2, #6268) |

docs/gaps.md

@@ -290,6 +290,7 @@ what an agent can't finish alone.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-14 | minor | G1: Lab adjustment layers' results are whole samples (native Lab path); photoshop lab8 curves passes |
 | 2026-10-15 | minor | G1 / #5972 A1: render only provably constant-RGB `GdFl` noise fills; variable noise synthesis remains open |
 | 2026-10-18 | minor | G9: with no selection, Copy takes fill and adjustment layers whole and Paste recreates them (#2344); UI-217-23 stays partial (a single pixel layer copies its pixels only); estimate unchanged |
 | 2026-10-15 | minor | G2: a cancelled 24 MP Gaussian Blur job stops within 44 ms max (P25, budget 200 ms; was about 400 ms; local runs, baseline not re-run), the uncancelled job on a locked-transparency layer about 40% faster; #6205 |