Sketch on an origin plane faces the side you clicked (#1587)

Fixes #1586.

crates/doc/src/assembly.rs

@@ -145,7 +145,7 @@ fn map_plane_ref(m: &Mat, p: &mut PlaneRef) {
             *plane = map_plane(m, plane);
             *at = apply_point(m, *at);
         }
-        PlaneRef::Offset { base, .. } => map_plane_ref(m, base),
+        PlaneRef::Offset { base, .. } | PlaneRef::Flipped { base, .. } => map_plane_ref(m, base),
         PlaneRef::AtAngle { base, axis_origin, axis_dir, .. } => {
             map_plane_ref(m, base);
             *axis_origin = apply_point(m, *axis_origin);

crates/doc/src/clipboard.rs

@@ -113,7 +113,9 @@ fn remap_plane(
         return Err(crate::DocError::Invalid("clone plane reference depth exceeded".into()));
     }
     match p {
-        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => remap_plane(base, ids, names, bodies, depth + 1)?,
+        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => {
+            remap_plane(base, ids, names, bodies, depth + 1)?
+        }
         PlaneRef::Construction { name } => mapped_name(&mut name.0, names),
         PlaneRef::Face { name, anchor, .. } => {
             if let Some(name) = name {
@@ -283,7 +285,7 @@ fn checked_plane(p: &PlaneRef, depth: usize) -> crate::Result<()> {
         return Err(crate::DocError::Invalid("copy reference nesting exceeds 64".into()));
     }
     match p {
-        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => checked_plane(base, depth + 1)?,
+        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => checked_plane(base, depth + 1)?,
         PlaneRef::Midplane { a, b } | PlaneRef::TwoEdges { a, b } => {
             checked_geo(a, depth + 1)?;
             checked_geo(b, depth + 1)?;

crates/doc/src/construct.rs

@@ -633,7 +633,7 @@ impl PlaneRef {
             PlaneRef::ThreePoints { a, b, c } => vec![a.as_mut(), b.as_mut(), c.as_mut()],
             PlaneRef::Tangent { face, .. } => vec![face.as_mut()],
             PlaneRef::AlongPath { path, .. } => vec![path.as_mut()],
-            PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => base.refs_mut(),
+            PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => base.refs_mut(),
             _ => Vec::new(),
         }
     }

crates/doc/src/document.rs

@@ -58,6 +58,14 @@ pub enum PlaneRef {
     AtAngle { base: Box<PlaneRef>, axis_origin: Vec3, axis_dir: Vec3, angle: Expr },
     /// A construction plane feature (by name).
     Construction { name: FeatureName },
+    /// `base` seen from its other side: the normal reversed by a half turn about the base's
+    /// own x axis (y reversed), or about its y axis (x reversed) when `about_y`. A sketch
+    /// started from behind a plane faces the viewer this way.
+    Flipped {
+        base: Box<PlaneRef>,
+        #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+        about_y: bool,
+    },
     /// Halfway between two planes (or planar faces); bisecting them when they meet at an angle.
     Midplane { a: Box<GeoRef>, b: Box<GeoRef> },
     /// Through two straight edges (lines) that lie in one plane.
@@ -2884,6 +2892,7 @@ impl Document {
                 let o = *axis_origin + rot(b.origin - *axis_origin);
                 Plane::new(o, rot(b.x), rot(b.y)).ok_or_else(|| DocError::Invalid("degenerate plane".into()))
             }
+            PlaneRef::Flipped { base, about_y } => Ok(self.resolve_plane_in(vals, base, depth + 1, st)?.flipped(*about_y)),
             PlaneRef::Construction { name } => {
                 // Evaluated already: as it came out.
                 if let Some(c) = st.and_then(|st| st.construct.iter().find(|c| &c.name == name))

crates/doc/src/eval.rs

@@ -3998,6 +3998,9 @@ pub(crate) fn sheet_face_plane_in_frame(doc: &Document, st: &ModelState, comp: u
             axis_dir: *axis_dir,
             angle: angle.clone(),
         },
+        PlaneRef::Flipped { base, about_y } => {
+            PlaneRef::Flipped { base: Box::new(sheet_face_plane_in_frame(doc, st, comp, base)?), about_y: *about_y }
+        }
         other => other.clone(),
     })
 }

crates/doc/src/face_anchor.rs

@@ -82,7 +82,7 @@ impl FaceAnchor {
 pub fn plane_face_anchor(plane: &PlaneRef) -> Option<&FaceAnchor> {
     match plane {
         PlaneRef::Face { anchor, .. } => anchor.as_deref(),
-        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => plane_face_anchor(base),
+        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => plane_face_anchor(base),
         _ => None,
     }
 }
@@ -90,7 +90,7 @@ pub fn plane_face_anchor(plane: &PlaneRef) -> Option<&FaceAnchor> {
 fn has_face_reference(plane: &PlaneRef) -> bool {
     match plane {
         PlaneRef::Face { .. } => true,
-        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => has_face_reference(base),
+        PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => has_face_reference(base),
         _ => false,
     }
 }
@@ -110,7 +110,9 @@ impl Document {
             }
             match plane {
                 PlaneRef::Face { anchor: Some(anchor), .. } => *plane = PlaneRef::Custom { plane: doc.resolve_face_anchor(anchor, author, state)? },
-                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => replace(doc, base, author, state, depth + 1)?,
+                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => {
+                    replace(doc, base, author, state, depth + 1)?
+                }
                 _ => {}
             }
             Ok(())

crates/doc/src/format/pending/flipped-sketch-plane.rs

@@ -0,0 +1,5 @@
+//! A sketch can sit on a plane seen from its other side: `{"type": "flipped", "base": <plane>}`
+//! (with `"about_y": true` when it turns about the plane's y axis rather than its x axis). New
+//! sketches started from behind an origin or construction plane save this, so they face the
+//! viewer. Older designs have no flipped planes and open unchanged; the last release refuses a
+//! design that has one (it asks for an update) instead of putting the sketch on the wrong side.

crates/doc/src/sheet.rs

@@ -4257,7 +4257,9 @@ pub fn rename_plane_sheet_body(plane: &mut crate::PlaneRef, old: &str, new: &str
                 *name = renamed;
             }
         }
-        crate::PlaneRef::Offset { base, .. } | crate::PlaneRef::AtAngle { base, .. } => rename_plane_sheet_body(base, old, new),
+        crate::PlaneRef::Offset { base, .. } | crate::PlaneRef::AtAngle { base, .. } | crate::PlaneRef::Flipped { base, .. } => {
+            rename_plane_sheet_body(base, old, new)
+        }
         _ => {}
     }
 }
@@ -4267,7 +4269,9 @@ pub fn rename_plane_sheet_body(plane: &mut crate::PlaneRef, old: &str, new: &str
 pub fn plane_sheet_face(plane: &crate::PlaneRef) -> Option<&str> {
     match plane {
         crate::PlaneRef::Face { name: Some(name), anchor: None, .. } if split_sheet_face_name(name).is_some() => Some(name),
-        crate::PlaneRef::Offset { base, .. } | crate::PlaneRef::AtAngle { base, .. } => plane_sheet_face(base),
+        crate::PlaneRef::Offset { base, .. } | crate::PlaneRef::AtAngle { base, .. } | crate::PlaneRef::Flipped { base, .. } => {
+            plane_sheet_face(base)
+        }
         _ => None,
     }
 }

crates/doc/src/walk.rs

@@ -364,7 +364,7 @@ impl Walk for PlaneRef {
         let mut p = self;
         for _ in 0..64 {
             match p {
-                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => p = base,
+                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => p = base,
                 PlaneRef::Construction { name } => return f(Ref::Feature(name)),
                 _ => return,
             }
@@ -374,7 +374,7 @@ impl Walk for PlaneRef {
         let mut p = self;
         for _ in 0..64 {
             match p {
-                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => p = base,
+                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => p = base,
                 PlaneRef::Construction { name } => return f(Ref::Feature(name)),
                 _ => return,
             }