Merge pull request #1561 from AyberkA0/pen-shift-45

fix(viewer): Shift-click with the Pen draws a straight segment (#1544)

crates/ui-egui/src/panels/viewer.rs

@@ -2228,6 +2228,12 @@ fn pen_press(
             });
             return;
         }
+        // Shift keeps the new segment at a multiple of 45° from the vertex before it (#1544).
+        let last = p.sp.vertices.last().map(|v| p.m.apply(gv2(v[0], v[1])));
+        let c = match last {
+            Some(q) if mods.shift => constrain_45([q.x, q.y], c),
+            _ => c,
+        };
         let lp = inv.apply(gv2(c[0], c[1]));
         let lp = [lp.x, lp.y];
         if let Ok(v) = app.session.execute("mask.addVertex", json!({"layer": lid.0, "mask": uid, "point": lp})) {
@@ -2289,6 +2295,20 @@ fn pen_press(
     });
 }
 
+/// `to` moved onto the nearest line through `from` at a multiple of 45° (horizontal, vertical or
+/// diagonal), as far along it as `to` lies: the Pen's Shift-click.
+fn constrain_45(from: [f64; 2], to: [f64; 2]) -> [f64; 2] {
+    let (dx, dy) = (to[0] - from[0], to[1] - from[1]);
+    if !(dx.is_finite() && dy.is_finite()) || (dx == 0.0 && dy == 0.0) {
+        return to;
+    }
+    let step = std::f64::consts::FRAC_PI_4;
+    let a = (dy.atan2(dx) / step).round() * step;
+    let (ux, uy) = (a.cos(), a.sin());
+    let along = dx * ux + dy * uy;
+    [from[0] + ux * along, from[1] + uy * along]
+}
+
 /// Visible, unlocked layers eligible for viewer hit tests, in front-to-back order.
 /// Solo scope matches the renderer: any soloed AV layer activates it at every time (as in After
 /// Effects), even when hidden, locked or outside its in-out range.
@@ -2692,6 +2712,18 @@ mod tests {
     use super::*;
     use effectcraft_engine::geom::{Mat4, vec3};
 
+    /// #1544: Shift-clicking with the Pen keeps the new segment horizontal, vertical or diagonal.
+    #[test]
+    fn pen_shift_click_keeps_the_segment_at_45_degrees() {
+        let near = |a: [f64; 2], b: [f64; 2]| (a[0] - b[0]).abs() < 1e-9 && (a[1] - b[1]).abs() < 1e-9;
+        let from = [100.0, 100.0];
+        assert!(near(constrain_45(from, [104.0, 180.0]), [100.0, 180.0]), "straight down");
+        assert!(near(constrain_45(from, [30.0, 97.0]), [30.0, 100.0]), "left");
+        assert!(near(constrain_45(from, [150.0, 46.0]), [152.0, 48.0]), "up and right, on the diagonal");
+        assert!(near(constrain_45(from, from), from), "on the vertex");
+        assert!(constrain_45(from, [f64::NAN, 3.0])[0].is_nan(), "a non-finite point stays as it is");
+    }
+
     /// The Rotation tool counts past a full turn in both directions, as in After Effects: the
     /// pointer going round the anchor 1.5 turns clockwise gives 540°, then back 2 turns gives
     /// −180°, with no jump where atan2 wraps from 180° to −180°.