ucs: dynamic UCS on planar solid faces

With dynamic UCS on (UCSDETECT 1, toggled with F6), a command's first
point picked on a planar face of a solid sets the UCS to that face for
the rest of the command, so 2D commands draw on the face (#1711). An
unsnapped first point lands on the face itself; a snapped point off the
face's plane belongs to other geometry and leaves the UCS alone. The
previous UCS comes back when the command ends. UCS commands themselves
are left out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

locales/ar-SA/opencadstudio.ftl

@@ -1567,6 +1567,8 @@ common =
     .nav-style-cad = CAD: ‏Shift + الزر الأوسط للدوران
     .nav-style-blender = Blender: الزر الأوسط للدوران، Shift للتحريك
     .nav-style-catia = CATIA: الزر الأوسط + الأيسر للدوران
+    .dynamic-ucs-on = نظام الإحداثيات الديناميكي: تشغيل
+    .dynamic-ucs-off = نظام الإحداثيات الديناميكي: إيقاف
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = لوحة اللمس: التمرير يحرك العرض، Ctrl+تمرير أو القرص يكبّر
     .interface-scale = مقياس الواجهة
     .zoom-factor = معامل التكبير

locales/bg-BG/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Shift + среден бутон завърта
     .nav-style-blender = Blender: среден бутон завърта, Shift премества
     .nav-style-catia = CATIA: среден + ляв бутон завърта
+    .dynamic-ucs-on = Динамична ПКС: вкл.
+    .dynamic-ucs-off = Динамична ПКС: изкл.
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Тъчпад: превъртането мести изгледа, Ctrl+превъртане или щипване мащабира
     .interface-scale = Мащаб на интерфейса
     .zoom-factor = Коефициент на мащабиране

locales/cs-CZ/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Shift + prostřední tlačítko otáčí
     .nav-style-blender = Blender: prostřední tlačítko otáčí, Shift posouvá
     .nav-style-catia = CATIA: prostřední + levé tlačítko otáčí
+    .dynamic-ucs-on = Dynamický USS: zapnuto
+    .dynamic-ucs-off = Dynamický USS: vypnuto
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: posun posouvá pohled, Ctrl+posun nebo sevření zvětšuje
     .interface-scale = Měřítko rozhraní
     .zoom-factor = Faktor zoomu

locales/de-DE/opencadstudio.ftl

@@ -1555,6 +1555,8 @@ common =
     .nav-style-cad = CAD: Umschalt + mittlere Taste dreht
     .nav-style-blender = Blender: mittlere Taste dreht, Umschalt verschiebt
     .nav-style-catia = CATIA: mittlere + linke Taste dreht
+    .dynamic-ucs-on = Dynamisches BKS: ein
+    .dynamic-ucs-off = Dynamisches BKS: aus
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: Scrollen verschiebt, Strg+Scrollen oder Zusammenziehen zoomt
     .interface-scale = Oberflächenskalierung
     .zoom-factor = Zoomfaktor

locales/el-GR/opencadstudio.ftl

@@ -1556,6 +1556,8 @@ common =
     .nav-style-cad = CAD: Shift + μεσαίο πλήκτρο περιστρέφει
     .nav-style-blender = Blender: μεσαίο πλήκτρο περιστρέφει, Shift μετακινεί
     .nav-style-catia = CATIA: μεσαίο + αριστερό πλήκτρο περιστρέφει
+    .dynamic-ucs-on = Δυναμικό UCS: ενεργό
+    .dynamic-ucs-off = Δυναμικό UCS: ανενεργό
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Επιφάνεια αφής: η κύλιση μετακινεί, Ctrl+κύλιση ή τσίμπημα κάνει ζουμ
     .interface-scale = Κλίμακα διεπαφής
     .zoom-factor = Συντελεστής ζουμ

locales/en-US/opencadstudio.ftl

@@ -1555,6 +1555,8 @@ common =
     .nav-style-cad = CAD: Shift + middle button orbits
     .nav-style-blender = Blender: middle button orbits, Shift pans
     .nav-style-catia = CATIA: middle + left button orbits
+    .dynamic-ucs-on = Dynamic UCS: on
+    .dynamic-ucs-off = Dynamic UCS: off
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: scroll pans, Ctrl+scroll or pinch zooms
     .interface-scale = Interface scale
     .zoom-factor = Zoom factor

locales/es-ES/opencadstudio.ftl

@@ -1555,6 +1555,8 @@ common =
     .nav-style-cad = CAD: Mayús + botón central orbita
     .nav-style-blender = Blender: botón central orbita, Mayús desplaza
     .nav-style-catia = CATIA: botón central + izquierdo orbita
+    .dynamic-ucs-on = SCP dinámico: activado
+    .dynamic-ucs-off = SCP dinámico: desactivado
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Panel táctil: el desplazamiento encuadra, Ctrl+desplazamiento o pellizco hace zoom
     .interface-scale = Escala de la interfaz
     .zoom-factor = Factor de zoom

locales/fi-FI/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Vaihto + keskipainike kiertää
     .nav-style-blender = Blender: keskipainike kiertää, Vaihto panoroi
     .nav-style-catia = CATIA: keski- + vasen painike kiertää
+    .dynamic-ucs-on = Dynaaminen UCS: päällä
+    .dynamic-ucs-off = Dynaaminen UCS: pois
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Kosketuslevy: vieritys panoroi, Ctrl+vieritys tai nipistys zoomaa
     .interface-scale = Käyttöliittymän skaalaus
     .zoom-factor = Zoomauskerroin

locales/fr-FR/opencadstudio.ftl

@@ -1555,6 +1555,8 @@ common =
     .nav-style-cad = CAO : Maj + bouton du milieu fait pivoter
     .nav-style-blender = Blender : bouton du milieu fait pivoter, Maj déplace
     .nav-style-catia = CATIA : boutons du milieu + gauche font pivoter
+    .dynamic-ucs-on = SCU dynamique: activé
+    .dynamic-ucs-off = SCU dynamique: désactivé
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Pavé tactile : le défilement déplace, Ctrl+défilement ou pincement zoome
     .interface-scale = Échelle de l'interface
     .zoom-factor = Facteur de zoom

locales/hi-IN/opencadstudio.ftl

@@ -1543,6 +1543,8 @@ common =
     .nav-style-cad = CAD: Shift + मध्य बटन घुमाता है
     .nav-style-blender = Blender: मध्य बटन घुमाता है, Shift खिसकाता है
     .nav-style-catia = CATIA: मध्य + बायाँ बटन घुमाता है
+    .dynamic-ucs-on = डायनामिक UCS: चालू
+    .dynamic-ucs-off = डायनामिक UCS: बंद
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = टचपैड: स्क्रॉल पैन करता है, Ctrl+स्क्रॉल या पिंच ज़ूम करता है
     .interface-scale = इंटरफ़ेस स्केल
     .zoom-factor = ज़ूम गुणक

locales/hu-HU/opencadstudio.ftl

@@ -1560,6 +1560,8 @@ common =
     .nav-style-cad = CAD: Shift + középső gomb forgat
     .nav-style-blender = Blender: középső gomb forgat, Shift mozgat
     .nav-style-catia = CATIA: középső + bal gomb forgat
+    .dynamic-ucs-on = Dinamikus FKR: be
+    .dynamic-ucs-off = Dinamikus FKR: ki
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Érintőpad: a görgetés mozgat, a Ctrl+görgetés vagy csippentés nagyít
     .interface-scale = Felület mérete
     .zoom-factor = Nagyítási tényező

locales/it-IT/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Maiusc + tasto centrale ruota
     .nav-style-blender = Blender: tasto centrale ruota, Maiusc sposta
     .nav-style-catia = CATIA: tasto centrale + sinistro ruota
+    .dynamic-ucs-on = UCS dinamico: attivo
+    .dynamic-ucs-off = UCS dinamico: disattivo
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: lo scorrimento sposta la vista, Ctrl+scorrimento o pizzico esegue lo zoom
     .interface-scale = Scala dell'interfaccia
     .zoom-factor = Fattore di zoom

locales/ja-JP/opencadstudio.ftl

@@ -1545,6 +1545,8 @@ common =
     .nav-style-cad = CAD: Shift + 中ボタンで回転
     .nav-style-blender = Blender: 中ボタンで回転、Shift で画面移動
     .nav-style-catia = CATIA: 中 + 左ボタンで回転
+    .dynamic-ucs-on = ダイナミック UCS: オン
+    .dynamic-ucs-off = ダイナミック UCS: オフ
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = タッチパッド: スクロールで画面移動、Ctrl+スクロールまたはピンチでズーム
     .interface-scale = インターフェースの拡大率
     .zoom-factor = ズーム倍率

locales/ko-KR/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Shift + 가운데 버튼으로 회전
     .nav-style-blender = Blender: 가운데 버튼으로 회전, Shift로 이동
     .nav-style-catia = CATIA: 가운데 + 왼쪽 버튼으로 회전
+    .dynamic-ucs-on = 동적 UCS: 켜짐
+    .dynamic-ucs-off = 동적 UCS: 꺼짐
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = 터치패드: 스크롤은 화면 이동, Ctrl+스크롤 또는 핀치는 확대/축소
     .interface-scale = 인터페이스 배율
     .zoom-factor = 줌 배율

locales/lo-LA/opencadstudio.ftl

@@ -1560,6 +1560,8 @@ common =
     .nav-style-cad = CAD: Shift + ປຸ່ມກາງ ໝຸນ
     .nav-style-blender = Blender: ປຸ່ມກາງ ໝຸນ, Shift ເລື່ອນ
     .nav-style-catia = CATIA: ປຸ່ມກາງ + ຊ້າຍ ໝຸນ
+    .dynamic-ucs-on = UCS ແບບໄດນາມິກ: ເປີດ
+    .dynamic-ucs-off = UCS ແບບໄດນາມິກ: ປິດ
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = ທັດແພດ: ເລື່ອນເພື່ອຍ້າຍມຸມມອງ, Ctrl+ເລື່ອນ ຫຼື ບີບເພື່ອຊູມ
     .interface-scale = ຂະໜາດສ່ວນຕິດຕໍ່
     .zoom-factor = ຕົວຄູນການຊູມ

locales/nl-NL/opencadstudio.ftl

@@ -1553,6 +1553,8 @@ common =
     .nav-style-cad = CAD: Shift + middelste knop draait
     .nav-style-blender = Blender: middelste knop draait, Shift verschuift
     .nav-style-catia = CATIA: middelste + linkerknop draait
+    .dynamic-ucs-on = Dynamisch GCS: aan
+    .dynamic-ucs-off = Dynamisch GCS: uit
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: scrollen verschuift, Ctrl+scrollen of knijpen zoomt
     .interface-scale = Interfaceschaal
     .zoom-factor = Zoomfactor

locales/pl-PL/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD: Shift + środkowy przycisk obraca
     .nav-style-blender = Blender: środkowy przycisk obraca, Shift przesuwa
     .nav-style-catia = CATIA: środkowy + lewy przycisk obraca
+    .dynamic-ucs-on = Dynamiczny LUW: wł.
+    .dynamic-ucs-off = Dynamiczny LUW: wył.
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: przewijanie przesuwa widok, Ctrl+przewijanie lub szczypanie powiększa
     .interface-scale = Skala interfejsu
     .zoom-factor = Współczynnik powiększenia

locales/pt-BR/opencadstudio.ftl

@@ -1555,6 +1555,8 @@ common =
     .nav-style-cad = CAD: Shift + botão do meio orbita
     .nav-style-blender = Blender: botão do meio orbita, Shift desloca
     .nav-style-catia = CATIA: botão do meio + esquerdo orbita
+    .dynamic-ucs-on = SCU dinâmico: ativado
+    .dynamic-ucs-off = SCU dinâmico: desativado
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Touchpad: rolar move a vista, Ctrl+rolar ou pinçar aplica zoom
     .interface-scale = Escala da interface
     .zoom-factor = Fator de zoom

locales/ru-RU/opencadstudio.ftl

@@ -1558,6 +1558,8 @@ common =
     .nav-style-cad = CAD: Shift + средняя кнопка — вращение
     .nav-style-blender = Blender: средняя кнопка — вращение, Shift — сдвиг
     .nav-style-catia = CATIA: средняя + левая кнопка — вращение
+    .dynamic-ucs-on = Динамическая ПСК: вкл.
+    .dynamic-ucs-off = Динамическая ПСК: выкл.
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Тачпад: прокрутка панорамирует, Ctrl+прокрутка или щипок масштабирует
     .interface-scale = Масштаб интерфейса
     .zoom-factor = Коэффициент зумирования

locales/tr-TR/opencadstudio.ftl

@@ -1541,6 +1541,8 @@ common =
     .nav-style-cad = CAD: Shift + orta tuş döndürür
     .nav-style-blender = Blender: orta tuş döndürür, Shift kaydırır
     .nav-style-catia = CATIA: orta + sol tuş döndürür
+    .dynamic-ucs-on = Dinamik UCS: açık
+    .dynamic-ucs-off = Dinamik UCS: kapalı
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = Dokunmatik yüzey: kaydırma kaydırır, Ctrl+kaydırma veya sıkıştırma yakınlaştırır
     .interface-scale = Arayüz ölçeği
     .zoom-factor = Yakınlaştırma çarpanı

locales/zh-CN/opencadstudio.ftl

@@ -1542,6 +1542,8 @@ common =
     .nav-style-cad = CAD：Shift + 中键旋转
     .nav-style-blender = Blender：中键旋转，Shift 平移
     .nav-style-catia = CATIA：中键 + 左键旋转
+    .dynamic-ucs-on = 动态 UCS：开
+    .dynamic-ucs-off = 动态 UCS：关
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = 触控板：滚动平移，Ctrl+滚动或捏合缩放
     .interface-scale = 界面缩放
     .zoom-factor = 缩放系数

locales/zh-TW/opencadstudio.ftl

@@ -1554,6 +1554,8 @@ common =
     .nav-style-cad = CAD：Shift + 中鍵旋轉
     .nav-style-blender = Blender：中鍵旋轉，Shift 平移
     .nav-style-catia = CATIA：中鍵 + 左鍵旋轉
+    .dynamic-ucs-on = 動態 UCS：開
+    .dynamic-ucs-off = 動態 UCS：關
     .touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms = 觸控板：捲動平移，Ctrl+捲動或捏合縮放
     .interface-scale = 介面縮放
     .zoom-factor = 縮放係數

src/app/command_driver/utilities.rs

@@ -624,6 +624,48 @@ impl OpenCADStudio {
         Some(key)
     }
 
+    /// Dynamic UCS: a command's first point picked on a planar solid face sets
+    /// the UCS to that face for the rest of the command. Returns the picked
+    /// point on the face. A snapped point (`snapped`) off the face's plane
+    /// belongs to other geometry, so it leaves the UCS alone.
+    pub(in crate::app) fn apply_dynamic_ucs(
+        &mut self,
+        snapped: Option<glam::DVec3>,
+    ) -> Option<glam::DVec3> {
+        let i = self.active_tab;
+        if !self.dynamic_ucs
+            || self.dynamic_ucs_restore.is_some()
+            || self.last_point.is_some()
+            || !self.tabs[i].editing_model_space()
+            || self.tabs[i]
+                .active_cmd
+                .as_ref()
+                .is_none_or(|command| command.name().starts_with("UCS"))
+        {
+            return None;
+        }
+        let cursor = self.tabs[i].scene.selection.borrow().input.last_move_pos?;
+        let (vw, vh) = self.tabs[i].scene.selection.borrow().view.vp_size;
+        let bounds = self.tabs[i].scene.active_model_tile_bounds(vw, vh);
+        let local = iced::Point::new(cursor.x - bounds.x, cursor.y - bounds.y);
+        let (ucs, pick) = self.tabs[i].scene.planar_face_ucs_at(bounds, local)?;
+        if let Some(point) = snapped {
+            let normal = glam::DVec3::new(ucs.x_axis.x, ucs.x_axis.y, ucs.x_axis.z)
+                .cross(glam::DVec3::new(ucs.y_axis.x, ucs.y_axis.y, ucs.y_axis.z))
+                .normalize_or_zero();
+            let off_plane = (point - pick).dot(normal).abs();
+            if off_plane > 1.0e-6 * (1.0 + point.length().max(pick.length())) {
+                return None;
+            }
+        }
+        let saved = self.tabs[i].active_ucs.take();
+        self.dynamic_ucs_restore = Some((self.tabs[i].id, saved));
+        self.tabs[i].active_ucs = Some(ucs);
+        self.tabs[i].sync_ucs_to_scene();
+        self.push_ucs_to_cmd(i);
+        Some(pick)
+    }
+
     /// `feed_command`, also reporting whether the step actually took the input.
     /// Only a `Text` token can come back unclaimed (`on_text_input` returning
     /// `None`), which is what lets the caller read it as something else — a
@@ -632,6 +674,9 @@ impl OpenCADStudio {
         &mut self,
         input: StepInput,
     ) -> (Task<Message>, bool) {
+        if matches!(input, StepInput::Point(_)) {
+            let _ = self.apply_dynamic_ucs(None);
+        }
         let gathering = self.report_selection_found(&input);
         // A command that takes associative dimensions only keeps just those:
         // the rest leave the selection and the count of what went is reported,

src/app/commands/mod.rs

@@ -768,6 +768,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "FILEDIA",
         "TRIMEXTENDMODE",
         "SELECTIONCYCLING",
+        "UCSDETECT",
         "SURFACEASSOCIATIVITY",
         "LAYOUTCREATEVIEWPORT",
         "FIELDEVAL",

src/app/commands/styleprops.rs

@@ -1216,6 +1216,7 @@ impl OpenCADStudio {
                     | "FILEDIA"
                     | "TRIMEXTENDMODE"
                     | "SELECTIONCYCLING"
+                    | "UCSDETECT"
                     | "SURFACEASSOCIATIVITY"
                     | "UCSORTHO"
                     | "UCSVIEW"
@@ -1656,6 +1657,30 @@ impl OpenCADStudio {
                         }
                         return Some(self.finish_dispatch(cmd));
                     }
+                    if name == "UCSDETECT" {
+                        let current = i16::from(self.dynamic_ucs);
+                        let prompt = format!("Enter new value for UCSDETECT <{current}>:");
+                        match value
+                            .as_deref()
+                            .map(|v| v.trim().parse::<i16>().ok().filter(|v| (0..=1).contains(v)))
+                        {
+                            Some(Some(mode)) => {
+                                self.dynamic_ucs = mode == 1;
+                                self.persist_settings_if_changed();
+                            }
+                            Some(None) => {
+                                self.command_line
+                                    .push_error(crate::t!("Requires 0 or 1 only.").as_ref());
+                                self.command_line.push_output(&prompt);
+                                self.pending_setvar = Some(name.clone());
+                            }
+                            None => {
+                                self.command_line.push_output(&prompt);
+                                self.pending_setvar = Some(name.clone());
+                            }
+                        }
+                        return Some(self.finish_dispatch(cmd));
+                    }
                     if name == "SELECTIONCYCLING" {
                         // 1 and 2 both turn cycling on; the candidate list
                         // always shows.

src/app/mod.rs

@@ -655,6 +655,12 @@ pub(super) struct OpenCADStudio {
     /// CATIA navigation: the left button went down while the middle one was
     /// held, so the middle drag orbits until it is released.
     nav_left_with_middle: bool,
+    /// UCSDETECT: a command's first point on a planar solid face sets the
+    /// UCS to that face until the command ends.
+    dynamic_ucs: bool,
+    /// The UCS a dynamic UCS replaced, by tab id, restored when that tab's
+    /// command ends.
+    dynamic_ucs_restore: Option<(u64, Option<codec::tables::Ucs>)>,
     /// Scroll pans and Ctrl+scroll zooms (touchpads); see `scroll_intent`.
     touchpad_pan: bool,
     /// Multi-finger gestures on a touch screen.
@@ -2663,6 +2669,8 @@ pub enum Message {
     /// Toggle reversing the mouse-wheel zoom direction (ZOOMWHEEL).
     ZoomWheelReversedChanged(bool),
     NavigationStyleChanged(crate::app::settings::NavigationStyle),
+    /// F6 / UCSDETECT: toggle the dynamic UCS.
+    ToggleDynamicUcs,
     TouchpadPanChanged(bool),
     /// Options: show the pull-down menus (MENUBAR).
     MenuBarChanged(bool),
@@ -4640,6 +4648,8 @@ impl OpenCADStudio {
             zoom_wheel_reversed: false,
             navigation_style: Default::default(),
             nav_left_with_middle: false,
+            dynamic_ucs: true,
+            dynamic_ucs_restore: None,
             touchpad_pan: false,
             touch: Default::default(),
             touch_offset: None,

src/app/settings.rs

@@ -416,6 +416,9 @@ pub struct UserSettings {
     pub polar_increment_deg: f32,
     pub zoom_wheel_reversed: bool,
     pub navigation_style: NavigationStyle,
+    /// UCSDETECT: a command's first point on a planar solid face sets the
+    /// UCS to that face until the command ends.
+    pub dynamic_ucs: bool,
     /// Scrolling pans and Ctrl+scroll (a touchpad pinch) zooms, for a
     /// touchpad that reports two-finger drags as wheel scrolls.
     pub touchpad_pan: bool,
@@ -920,6 +923,7 @@ impl Default for UserSettings {
             polar_increment_deg: 45.0,
             zoom_wheel_reversed: false,
             navigation_style: NavigationStyle::Cad,
+            dynamic_ucs: true,
             touchpad_pan: false,
             ui_scale_percent: 100,
             zoom_factor: 60,

src/app/shortcuts.rs

@@ -41,6 +41,7 @@ pub(super) fn default_bindings() -> BTreeMap<String, String> {
         ("F3".to_string(), "TOGGLEOSNAP"),
         ("F4".to_string(), "TOGGLE3DOSNAP"),
         ("F5".to_string(), "ISOPLANE"),
+        ("F6".to_string(), "TOGGLEDUCS"),
         ("F7".to_string(), "GRID"),
         ("F8".to_string(), "ORTHO"),
         ("F9".to_string(), "SNAP"),
@@ -185,6 +186,7 @@ pub(super) const INPUT_ACTIONS: &[&str] = &[
     "COMMANDHISTORY",
     "TOGGLEOSNAP",
     "TOGGLE3DOSNAP",
+    "TOGGLEDUCS",
     "OTRACK",
     "DYNINPUT",
     "SELECTALL",
@@ -343,6 +345,7 @@ impl OpenCADStudio {
             "COMMANDHISTORY" => Message::CommandHistoryToggle,
             "TOGGLEOSNAP" => Message::ToggleSnapEnabled,
             "TOGGLE3DOSNAP" => Message::ToggleSnap3dEnabled,
+            "TOGGLEDUCS" => Message::ToggleDynamicUcs,
             "OTRACK" => Message::ToggleOTrack,
             "DYNINPUT" => Message::ToggleDynInput,
             "SELECTALL" => Message::SelectAllShortcut,

src/app/update/file.rs

@@ -779,6 +779,7 @@ impl OpenCADStudio {
             polar_increment_deg: self.polar_increment_deg,
             zoom_wheel_reversed: self.zoom_wheel_reversed,
             navigation_style: self.navigation_style,
+            dynamic_ucs: self.dynamic_ucs,
             touchpad_pan: self.touchpad_pan,
             ui_scale_percent: self.ui_scale_percent,
             zoom_factor: self.zoom_factor,
@@ -915,6 +916,7 @@ impl OpenCADStudio {
         self.polar_increment_deg = s.polar_increment_deg;
         self.zoom_wheel_reversed = s.zoom_wheel_reversed;
         self.navigation_style = s.navigation_style;
+        self.dynamic_ucs = s.dynamic_ucs;
         self.touchpad_pan = s.touchpad_pan;
         self.ui_scale_percent = crate::app::settings::clamp_ui_scale(s.ui_scale_percent);
         self.zoom_factor = crate::app::settings::clamp_zoom_factor(s.zoom_factor);

src/app/update/mod.rs

@@ -374,6 +374,7 @@ impl OpenCADStudio {
         // document's selection.
         let before = self.tabs.get(self.active_tab).map(|tab| tab.id);
         let task = self.update_message(msg);
+        self.restore_dynamic_ucs_after_command();
         if self.tabs.get(self.active_tab).map(|tab| tab.id) != before {
             self.sync_underlay_tab();
         }
@@ -396,6 +397,25 @@ impl OpenCADStudio {
         task
     }
 
+    /// A dynamic UCS lasts for the command that set it: once that tab's
+    /// command has ended, the UCS it replaced comes back.
+    fn restore_dynamic_ucs_after_command(&mut self) {
+        let Some((tab_id, _)) = self.dynamic_ucs_restore else {
+            return;
+        };
+        let Some(i) = self.tabs.iter().position(|tab| tab.id == tab_id) else {
+            self.dynamic_ucs_restore = None;
+            return;
+        };
+        if self.tabs[i].active_cmd.is_some() {
+            return;
+        }
+        if let Some((_, saved)) = self.dynamic_ucs_restore.take() {
+            self.tabs[i].active_ucs = saved;
+            self.tabs[i].sync_ucs_to_scene();
+        }
+    }
+
     /// The Frames list on the ribbon shows the active drawing's frame state
     /// (FRAME), however it was changed: the list, a command, undo or another
     /// drawing.
@@ -9245,6 +9265,18 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::ToggleDynamicUcs => {
+                self.dynamic_ucs = !self.dynamic_ucs;
+                self.command_line.push_output(if self.dynamic_ucs {
+                    crate::t!("Dynamic UCS: on")
+                } else {
+                    crate::t!("Dynamic UCS: off")
+                }
+                .as_ref());
+                self.persist_settings_if_changed();
+                Task::none()
+            }
+
             Message::NavigationStyleChanged(style) => {
                 self.navigation_style = style;
                 self.persist_settings_if_changed();

src/app/update/viewport.rs

@@ -4325,6 +4325,7 @@ impl OpenCADStudio {
             };
 
             let snap_taken = self.tabs[i].snap_result.take();
+            let snapped = snap_taken.is_some();
             let tangent_obj_at_click = snap_taken.and_then(|s| s.tangent_obj);
 
             let world_pt = {
@@ -5019,6 +5020,11 @@ impl OpenCADStudio {
                     self.finish_command_click(i);
                     return Task::none();
                 }
+                // An unsnapped first point on a solid face lands on the face.
+                let world_pt = match self.apply_dynamic_ucs(snapped.then_some(world_pt)) {
+                    Some(on_face) if !snapped => on_face,
+                    _ => world_pt,
+                };
                 self.last_point = Some(world_pt);
                 // The one-shot snap override is spent by this pick —
                 // restore the running osnap configuration (#337).

src/locale_catalog.rs

@@ -5074,6 +5074,8 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "CAD: Shift + middle button orbits" => Some(("common", "nav-style-cad")),
         "Blender: middle button orbits, Shift pans" => Some(("common", "nav-style-blender")),
         "CATIA: middle + left button orbits" => Some(("common", "nav-style-catia")),
+        "Dynamic UCS: on" => Some(("common", "dynamic-ucs-on")),
+        "Dynamic UCS: off" => Some(("common", "dynamic-ucs-off")),
         "Touchpad: scroll pans, Ctrl+scroll or pinch zooms" => Some(("common", "touchpad-scroll-pans-ctrl-scroll-or-pinch-zooms")),
         "Interface scale" => Some(("common", "interface-scale")),
         "Zoom factor" => Some(("common", "zoom-factor")),

src/scene/mod.rs

@@ -11363,6 +11363,25 @@ impl Scene {
         )
     }
 
+    /// The UCS of the planar solid face under `local` (relative to `bounds`),
+    /// Z along the face normal, and the picked point on the face.
+    pub fn planar_face_ucs_at(
+        &mut self,
+        bounds: iced::Rectangle,
+        local: iced::Point,
+    ) -> Option<(codec::tables::Ucs, glam::DVec3)> {
+        let camera = self.camera.borrow();
+        let view_rot = camera.view_proj_rte(bounds);
+        let eye = camera.eye();
+        drop(camera);
+        let (handle, pick) = self.solid_click_hit_point(local, view_rot, eye, bounds)?;
+        self.restore_solid_models(&[handle]);
+        let body = self.solid_models.get(&handle)?;
+        let face = crate::scene::model::solid_model::nearest_planar_face(body, pick.to_array())?;
+        crate::scene::model::solid_model::planar_face_ucs(body, face, pick.to_array())
+            .map(|ucs| (ucs, pick))
+    }
+
     /// The nearest solid or mesh surface under `local` (relative to `bounds`).
     pub fn surface_pivot_at(
         &self,