Arbitrary rotation starts at the ruler line's straightening angle (#660)

* fix(image): Arbitrary rotation starts at the ruler's straightening angle

Fixes #602

* Arbitrary rotation: ignore a non-finite ruler angle (from #659)

Co-Authored-By: Alex Gorevski <alex.gorevski@live.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Alex Gorevski <alex.gorevski@live.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/analysis_cmds.rs

@@ -629,13 +629,17 @@ fn inscribed(w: f64, h: f64, a: f64) -> (f64, f64) {
     }
 }
 
+/// Clockwise rotation in degrees that turns the ruler line onto its nearest axis.
+pub fn straighten_angle(r: &Ruler) -> f64 {
+    let (dx, dy) = (r.end[0] - r.start[0], r.end[1] - r.start[1]);
+    let a = (-dy).atan2(dx).to_degrees();
+    a - 90.0 * (a / 90.0).round()
+}
+
 fn straighten(s: &mut Session, p: &Value) -> Result<Value> {
     let d = doc(s)?;
     let r = d.measurement.ruler.ok_or_else(|| EngineError::Other("no ruler line".into()))?;
-    let (dx, dy) = (r.end[0] - r.start[0], r.end[1] - r.start[1]);
-    let a = (-dy).atan2(dx).to_degrees();
-    // Straighten to the nearest axis: rotate clockwise by the remaining angle.
-    let rot = a - 90.0 * (a / 90.0).round();
+    let rot = straighten_angle(&r);
     if rot.abs() < 1e-9 {
         return Ok(json!({"angle": 0.0}));
     }

crates/ui-egui/src/analysis_ui.rs

@@ -902,6 +902,23 @@ pub(crate) mod tests {
         }
     }
 
+    #[test]
+    fn arbitrary_rotation_starts_at_the_ruler_angle() {
+        let (mut app, ctx) = app();
+        let open = |app: &mut PhotocraftApp| {
+            let r = crate::menus::invoke(app, &ctx, "image.rotation.arbitrary", json!({})).unwrap();
+            let f = &app.ui.dialog_mut(r["dialog"].as_u64().unwrap()).unwrap().fields;
+            (f["angle"].as_f64().unwrap(), f["direction"].clone())
+        };
+        assert_eq!(open(&mut app), (0.0, json!("cw")));
+        // A line falling to the right straightens counter-clockwise, a near-vertical one to the y axis.
+        for (end, dir) in [([150, 60], "ccw"), ([150, 40], "cw"), ([60, -50], "ccw")] {
+            app.run("image.analysis.rulerTool", json!({"start": [50, 50], "end": end})).unwrap();
+            let (a, d) = open(&mut app);
+            assert!((a - 5.7106).abs() < 1e-3 && d == json!(dir), "{end:?}: {a} {d}");
+        }
+    }
+
     #[test]
     fn ruler_drag_protractor_and_clear() {
         let (mut app, _) = app();

crates/ui-egui/src/filter_dialog.rs

@@ -192,6 +192,9 @@ pub fn open(app: &mut PhotocraftApp, command: &str) -> Option<u64> {
         };
         fields.insert(p.key, v);
     }
+    if command == "image.rotation.arbitrary" {
+        straighten_defaults(app, &mut fields);
+    }
     if parse_spec(spec.params).iter().any(|p| p.kind == Kind::Document) {
         // The document picker lists every open document (params refer to them by index).
         let names: Vec<String> = app.session.documents().iter().map(|d| d.doc.name.clone()).collect();
@@ -201,6 +204,18 @@ pub fn open(app: &mut PhotocraftApp, command: &str) -> Option<u64> {
     Some(id)
 }
 
+/// Arbitrary rotation starts at the angle that straightens the ruler line, when there is one.
+fn straighten_defaults(app: &PhotocraftApp, fields: &mut Map<String, Value>) {
+    let Some(r) = app.session.active().and_then(|d| d.doc.measurement.ruler) else { return };
+    let rot = photocraft_engine::analysis_cmds::straighten_angle(&r);
+    // A ruler read from a damaged file could hold non-finite ends: keep the 0° default then.
+    if !rot.is_finite() {
+        return;
+    }
+    fields.insert("angle".into(), json!(rot.abs()));
+    fields.insert("direction".into(), json!(if rot < 0.0 { "ccw" } else { "cw" }));
+}
+
 /// A filter dialog with live preview for `command` whose parameters follow `spec` (registry
 /// notation) instead of the command's own; `fixed` params (e.g. a plug-in id) are passed through.
 pub fn open_with_spec(app: &mut PhotocraftApp, command: &str, label: &str, spec: &str, fixed: Map<String, Value>) -> u64 {