#1007: Reverse Gradient mirrors off-centre midpoints

Reversal read the midpoints after Gradient::reverse had reordered the stops, so each stop
took the wrong segment's midpoint: red-to-blue with its midpoint at 20% reversed to a 50%
midpoint instead of 80%, moving the transition.

Gradient::reverse now mirrors each segment's midpoint with it, so a reversed gradient is the
same ramp the other way round; paint.editGradient's own (wrong) pass is gone, and the SVG
importer's reflect spread, which had a correct copy of the same mirroring, uses it too.

Tests: two- and three-stop gradients with off-centre midpoints sample the same reversed (t vs
1 - t) and come back after two reversals; paint.editGradient reverse turns a 0.2 midpoint into
0.8.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

# Conflicts:
#	ROADMAP.md

ROADMAP.md

@@ -123,7 +123,7 @@ Ranked; detail in [docs/roadmap.md](docs/roadmap.md) and [docs/gaps.md](docs/gap
 
 | Date | What landed |
 |---|---|
-| 2026-10-11 | Effect › Brush Strokes: Accented Edges, Angled Strokes, Crosshatch, Dark Strokes, Ink Outlines, Spatter, Sprayed Strokes, Sumi-e (G2). Command parameters from agents and files checked before anything changes (paint, place, cuts, dashes, crop, asset ids, anchors, paths, distort; #984–#999); command dialogs keep whole numbers whole, so Create Object Mosaic takes the tiles typed (#1000); stepper arrows take Shift (×10) and Ctrl/Cmd (÷10) like the arrow keys (#991). Exports and Rasterize supersample Art Optimized edges, so shapes meeting edge to edge leave no seam (#983). Character and paragraph styles take leading, kerning, mojikumi and the other attributes left out at their defaults (#1001). Drawing, typing and pasting with no layer shown and unlocked is refused instead of adding art to a hidden or locked layer (#1002). MCP `add_text` gives type on or in a path the colour asked for (#1003). MCP `screenshot` of a connected app renders the artboard asked for (#1004). `vectorcraft-cli run` ends quietly when its reader closes early (#1005). Select › All on Active Artboard selects on the artboard made active, not always the first (#1006). |
+| 2026-10-11 | Effect › Brush Strokes: Accented Edges, Angled Strokes, Crosshatch, Dark Strokes, Ink Outlines, Spatter, Sprayed Strokes, Sumi-e (G2). Command parameters from agents and files checked before anything changes (paint, place, cuts, dashes, crop, asset ids, anchors, paths, distort; #984–#999); command dialogs keep whole numbers whole, so Create Object Mosaic takes the tiles typed (#1000); stepper arrows take Shift (×10) and Ctrl/Cmd (÷10) like the arrow keys (#991). Exports and Rasterize supersample Art Optimized edges, so shapes meeting edge to edge leave no seam (#983). Character and paragraph styles take leading, kerning, mojikumi and the other attributes left out at their defaults (#1001). Drawing, typing and pasting with no layer shown and unlocked is refused instead of adding art to a hidden or locked layer (#1002). MCP `add_text` gives type on or in a path the colour asked for (#1003). MCP `screenshot` of a connected app renders the artboard asked for (#1004). `vectorcraft-cli run` ends quietly when its reader closes early (#1005). Select › All on Active Artboard selects on the artboard made active, not always the first (#1006). Reverse Gradient keeps off-centre midpoints, mirrored (#1007). |
 | 2026-10-10 | Initial live Revolve (Effect › 3D and Materials, #846, #605), in its own `three-d` crate. Progress docs re-measured and restructured to the craftrules standard (this page, `docs/target-app-parity.md`, `gaps.md`, `ui-parity.md`, the format, hardware, localization, effects and type checklists). Same day: v0.8.0; Effect › Distort, Pixelate and Texture filters (13); German interface; Variables (data merge); Graph Type value axes and tick marks; restart when the first frame never reaches the screen (#964) |
 | 2026-10-09 | Radial Blur, Smart Blur, Unsharp Mask (gap 2 begins); 340 commits, the busiest day |
 | 2026-10-08 | v0.5.0–v0.7.0; community contributions: Pen and shape-tool modifiers, Layers, Artboards, Japanese composition, Hebrew/Arabic type, interface languages, saved selections |

crates/color/src/gradient.rs

@@ -131,10 +131,18 @@ impl Gradient {
             std::iter::once((s.offset, s.color, s.opacity, None)).chain(mid)
         })
     }
+    /// The same ramp the other way round: the stops in reverse order at mirrored offsets, each
+    /// segment's midpoint mirrored with it.
     pub fn reverse(&mut self) {
         self.stops.reverse();
-        for s in &mut self.stops {
-            s.offset = 1.0 - s.offset;
+        for i in 0..self.stops.len() {
+            // A midpoint belongs to the segment after its stop: reversed, stop i's segment is the
+            // one the next stop (not yet mirrored) had, measured from the other end.
+            let mid = self.stops.get(i + 1).map_or(0.5, |n| 1.0 - n.midpoint);
+            if let Some(s) = self.stops.get_mut(i) {
+                s.offset = 1.0 - s.offset;
+                s.midpoint = mid;
+            }
         }
     }
     pub fn sort(&mut self) {
@@ -569,6 +577,32 @@ mod tests {
         assert_eq!(g.stops[0].offset, 0.0);
     }
 
+    /// #1007: reversing mirrors off-centre midpoints with their segments, so the ramp is the same
+    /// the other way round.
+    #[test]
+    fn reverse_mirrors_midpoints() {
+        let stop = |offset, hex, midpoint| GradientStop { midpoint, ..GradientStop::new(offset, Color::from_hex(hex).unwrap()) };
+        let two = Gradient { kind: GradientKind::Linear, stops: vec![stop(0.0, "#ff0000", 0.2), stop(1.0, "#0000ff", 0.5)] };
+        let three =
+            Gradient { kind: GradientKind::Linear, stops: vec![stop(0.0, "#ff0000", 0.2), stop(0.3, "#00ff00", 0.8), stop(1.0, "#0000ff", 0.5)] };
+        for g in [two, three] {
+            let mut r = g.clone();
+            r.reverse();
+            let mids: Vec<f32> = r.stops.iter().map(|s| s.midpoint).collect();
+            let want: Vec<f32> = g.stops.iter().rev().skip(1).map(|s| 1.0 - s.midpoint).chain([0.5]).collect();
+            assert!(mids.iter().zip(&want).all(|(a, b)| (a - b).abs() < 1e-6), "{mids:?} {want:?}");
+            for i in 0..=20 {
+                let t = i as f32 / 20.0;
+                let ((a, ao), (b, bo)) = (g.sample(t), r.sample(1.0 - t));
+                let same = a.to_rgba8(1.0).iter().zip(b.to_rgba8(1.0)).all(|(x, y)| x.abs_diff(y) <= 1);
+                assert!(same && (ao - bo).abs() < 1e-6, "t {t}: {a:?} vs {b:?}");
+            }
+            r.reverse();
+            let back = r.stops.iter().zip(&g.stops).all(|(a, b)| (a.offset - b.offset).abs() < 1e-6 && (a.midpoint - b.midpoint).abs() < 1e-6);
+            assert!(back, "reversed twice, it is as it was: {:?}", r.stops);
+        }
+    }
+
     fn close(a: Point, b: Point) -> bool {
         a.distance(b) < 1e-9
     }

crates/engine/src/cmd/gradient.rs

@@ -270,13 +270,8 @@ pub(crate) fn apply_gradient_edit_in(
         gp.swatch = None;
     }
     if bool_or(p, "reverse", false) {
+        // Midpoints mirrored with their segments.
         gp.gradient.reverse();
-        // Midpoints belong to the segment to the right; mirror them.
-        let n = gp.gradient.stops.len();
-        let mids: Vec<f32> = gp.gradient.stops.iter().map(|s| s.midpoint).collect();
-        for i in 0..n {
-            gp.gradient.stops[i].midpoint = if i + 1 < n { 1.0 - mids[n - 2 - i] } else { 0.5 };
-        }
     }
     // Angle and aspect edits keep the focal point in its place in the extent ellipse.
     let before = gp.geom;

crates/engine/src/tests_panelcmds.rs

@@ -56,6 +56,13 @@ fn edit_gradient_angle_reverse_and_errors() {
     s.execute("paint.editGradient", &json!({"stroke": false, "reverse": true})).unwrap();
     let Paint::Gradient(g) = node(&s, id).appearance.fill_paint() else { panic!() };
     assert_eq!(g.gradient.stops[0].color.to_hex(), "#000000");
+    // #1007: an off-centre midpoint is mirrored, not reset.
+    let stops = json!([{"offset": 0, "color": "#ff0000", "midpoint": 0.2}, {"offset": 1, "color": "#0000ff"}]);
+    s.execute("paint.editGradient", &json!({"stroke": false, "stops": stops})).unwrap();
+    s.execute("paint.editGradient", &json!({"stroke": false, "reverse": true})).unwrap();
+    let Paint::Gradient(g) = node(&s, id).appearance.fill_paint() else { panic!() };
+    assert_eq!(g.gradient.stops[0].color.to_hex(), "#0000ff");
+    assert!((g.gradient.stops[0].midpoint - 0.8).abs() < 1e-6, "{:?}", g.gradient.stops);
     assert!(s.execute("paint.editGradient", &json!({"stops": [{"offset": 0, "color": "#fff"}]})).is_err());
     assert!(s.execute("paint.editGradient", &json!({"kind": "conic"})).is_err());
 }

crates/svg/src/import/spread.rs

@@ -2,7 +2,7 @@
 //! repeated (every other period mirrored, for reflect) as many times as the painted area needs,
 //! and the gradient stretched over those periods.
 
-use vectorcraft_color::{GradientGeom, GradientKind, GradientStop};
+use vectorcraft_color::{Gradient, GradientGeom, GradientKind, GradientStop};
 use vectorcraft_geom::{Point, Rect};
 
 /// Most periods a gradient is expanded to.
@@ -80,17 +80,9 @@ fn whole_period(stops: &[GradientStop]) -> Vec<GradientStop> {
 
 /// A period mirrored: the stops in reverse, each midpoint measured from the other side.
 fn mirror(period: &[GradientStop]) -> Vec<GradientStop> {
-    let rev: Vec<&GradientStop> = period.iter().rev().collect();
-    rev.iter()
-        .enumerate()
-        .map(|(i, s)| GradientStop {
-            offset: 1.0 - s.offset,
-            // The midpoint from this stop to the next one is the mirror of the one the next stop
-            // had towards this one.
-            midpoint: rev.get(i + 1).map_or(0.5, |next| 1.0 - next.midpoint),
-            ..(*s).clone()
-        })
-        .collect()
+    let mut g = Gradient { kind: GradientKind::Linear, stops: period.to_vec() };
+    g.reverse();
+    g.stops
 }
 
 #[cfg(test)]