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

Fixes #1586.

crates/engine/src/cmd/sketch.rs

@@ -29,7 +29,7 @@ pub static COMMANDS: &[CommandSpec] = &[
     CommandSpec::new("sketch.create", "Create Sketch", create_sketch)
         .at("SOLID", "CREATE")
         .icon("sketch")
-        .params("plane: XY|XZ|YZ | construction plane name or id | {origin, x_dir, y_dir} | {face: [world x,y,z], body?: exact world body, index?: face index, path?: complete occurrence ids, fixed?: bool}; new face attachments require a unique certified source (a flat face of a sheet metal body attaches to its panel instead, in the flat pattern's coordinates), fixed:true explicitly freezes the selected plane; offset?: expr, project_edges?: bool, name?"),
+        .params("plane: XY|XZ|YZ | construction plane name or id | {origin, x_dir, y_dir} | {face: [world x,y,z], body?: exact world body, index?: face index, path?: complete occurrence ids, fixed?: bool}; new face attachments require a unique certified source (a flat face of a sheet metal body attaches to its panel instead, in the flat pattern's coordinates), fixed:true explicitly freezes the selected plane; offset?: expr, flip?: bool | \"x\" | \"y\" (not on a face: sketch the plane from its other side, so the normal and the default extrude direction are reversed; a half turn about the plane's own x or y axis, true picks the one keeping the sketch's y axis upward; offset still moves along the unflipped normal), project_edges?: bool, name?"),
     CommandSpec::new("sketch.edit", "Edit Sketch", edit_sketch).noundo().icon("sketch").params("sketch: id or name"),
     CommandSpec::new("sketch.finish", "Finish Sketch", finish_sketch).at("SKETCH", "FINISH SKETCH").icon("finish").enabled(in_sketch).noundo(),
     CommandSpec::new("sketch.line", "Line", draw_line)
@@ -543,7 +543,7 @@ pub(crate) fn author_path_for_rebind(s: &Session, feature: &solidcraft_doc::Feat
         let mut plane = plane;
         loop {
             match plane {
-                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } => plane = base,
+                PlaneRef::Offset { base, .. } | PlaneRef::AtAngle { base, .. } | PlaneRef::Flipped { base, .. } => plane = base,
                 PlaneRef::Face { anchor: Some(anchor), .. } => {
                     saved = Some(anchor.author_path.clone());
                     break;
@@ -635,7 +635,31 @@ fn create_sketch(s: &mut Session, p: &Value) -> Result<Value> {
             return Err(bad(cmd, format!("`{key}` belongs inside `plane`: {example}")));
         }
     }
-    create_sketch_with_plane(s, p, plane_ref(s, p, cmd)?, None)
+    let mut plane = plane_ref(s, p, cmd)?;
+    if let Some(about_y) = flip_param(s, p, &plane, cmd)? {
+        plane = PlaneRef::Flipped { base: Box::new(plane), about_y };
+    }
+    create_sketch_with_plane(s, p, plane, None)
+}
+
+/// `flip` of `sketch.create`: `None` keeps the plane's own side, otherwise the axis to turn
+/// about (`true`: about y when the plane's y axis points upward, so it stays up, else about x).
+fn flip_param(s: &Session, p: &Value, plane: &PlaneRef, cmd: &str) -> Result<Option<bool>> {
+    let about_y = match p.get("flip") {
+        None | Some(Value::Null) | Some(Value::Bool(false)) => return Ok(None),
+        Some(Value::String(a)) if a.eq_ignore_ascii_case("x") => false,
+        Some(Value::String(a)) if a.eq_ignore_ascii_case("y") => true,
+        Some(Value::Bool(true)) => {
+            let (vals, _) = s.doc.param_values();
+            let resolved = s.doc.resolve_plane_in(&vals, plane, 0, Some(&s.model.state()))?;
+            resolved.y.z > 1e-9
+        }
+        Some(_) => return Err(bad(cmd, "`flip` must be true, false, \"x\" or \"y\"")),
+    };
+    if face::is_face(p) {
+        return Err(bad(cmd, "`flip` is for origin, construction and explicit planes; a face already faces outward"));
+    }
+    Ok(Some(about_y))
 }
 
 /// Used only by face Extrude/Revolve/PressPull, preserving pre-certificate linked profiles.
@@ -2493,6 +2517,10 @@ fn solve_cmd(s: &mut Session, p: &Value) -> Result<Value> {
     Ok(json!({"status": ss.report.status, "dof": ss.report.dof, "failing": ss.report.failing, "profiles": ss.profiles.len()}))
 }
 
+#[cfg(test)]
+#[path = "sketch_flip_tests.rs"]
+mod flip_tests;
+
 #[cfg(test)]
 #[path = "sketch_rect_tests.rs"]
 mod rect_tests;