Section Analysis (FusionHalfSectionViewCommand, INSPECT): cut the view by an origin plane, construction plane or planar face with an offset (drag arrow) and flip; the inside of cut bodies shows a flat cap colour; live while the dialog is open, Remove Section on the right-click menu; viewport shader validated in a test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/mod.rs

@@ -12,6 +12,7 @@ pub use features::auto_operation;
 mod file;
 mod inspect;
 mod measure_sel;
+mod section;
 pub(crate) use measure_sel::measure_items;
 mod parameters;
 mod sketch;
@@ -129,6 +130,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(parameters::COMMANDS.iter());
     v.extend(file::COMMANDS.iter());
     v.extend(inspect::COMMANDS.iter());
+    v.extend(section::COMMANDS.iter());
     v
 }
 

crates/engine/src/cmd/section.rs

@@ -0,0 +1,48 @@
+//! Section Analysis: a view-only cut through the model by a plane (what is on the plane's
+//! positive side is hidden). It changes no geometry and is not an undo step.
+
+use serde_json::{Value, json};
+use solvecraft_geom::{Plane, Vec3};
+
+use super::CommandSpec;
+use crate::params::{bad, bool_, expr, vec3};
+use crate::{Result, Session};
+
+pub static COMMANDS: &[CommandSpec] = &[CommandSpec::new("FusionHalfSectionViewCommand", "Section Analysis", section)
+    .at("SOLID", "INSPECT")
+    .icon("section")
+    .noundo()
+    .params("plane: XY|XZ|YZ | {origin, normal}; offset?: expr along the normal; flip?: bool; or clear: true")];
+
+fn section(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionHalfSectionViewCommand";
+    if bool_(p, "clear").unwrap_or(false) {
+        s.section = None;
+        s.revision += 1;
+        return Ok(json!({"section": null}));
+    }
+    let (origin, normal) = match p.get("plane") {
+        Some(Value::String(n)) => {
+            let pl = Plane::named(n).ok_or_else(|| bad(cmd, format!("unknown plane `{n}` (XY, XZ, YZ)")))?;
+            (pl.origin, pl.normal())
+        }
+        Some(o @ Value::Object(_)) => {
+            let origin = o.get("origin").and_then(vec3).unwrap_or(Vec3::ZERO);
+            let n = o.get("normal").and_then(vec3).and_then(|n| n.normalized()).ok_or_else(|| bad(cmd, "the plane needs a non-zero `normal`"))?;
+            (origin, n)
+        }
+        _ => return Err(bad(cmd, "`plane` must be XY, XZ, YZ or {origin, normal}")),
+    };
+    let offset = match expr(p, "offset") {
+        Some(e) => s.doc.eval(&e, solvecraft_doc::expr::Kind::Length).map_err(|e| bad(cmd, format!("offset: {e}")))?,
+        None => 0.0,
+    };
+    if !origin.is_finite() || !offset.is_finite() {
+        return Err(bad(cmd, "the plane must be finite"));
+    }
+    let normal = if bool_(p, "flip").unwrap_or(false) { -normal } else { normal };
+    let origin = origin + normal * offset * if bool_(p, "flip").unwrap_or(false) { -1.0 } else { 1.0 };
+    s.section = Some((origin, normal));
+    s.revision += 1;
+    Ok(json!({"section": {"origin": origin, "normal": normal}}))
+}

crates/engine/src/lib.rs

@@ -156,6 +156,8 @@ pub struct Session {
     world_cache: std::sync::Mutex<Option<(u64, Arc<solvecraft_doc::ModelState>)>>,
     /// Features copied with `timeline.copy` (pasted by `timeline.paste`).
     pub clipboard: Vec<solvecraft_doc::Feature>,
+    /// Section Analysis: a view cut by a plane (origin, unit normal; the normal side is hidden).
+    pub section: Option<(Vec3, Vec3)>,
 }
 
 const MAX_UNDO: usize = 200;
@@ -188,6 +190,7 @@ impl Session {
             pending_moves: Default::default(),
             world_cache: Default::default(),
             clipboard: Vec::new(),
+            section: None,
         }
     }
 
@@ -314,6 +317,7 @@ impl Session {
             pending_moves: self.pending_moves.clone(),
             world_cache: Default::default(),
             clipboard: self.clipboard.clone(),
+            section: None,
         }
     }
 

crates/engine/src/tests.rs

@@ -626,6 +626,25 @@ fn measure_items() {
     assert!(s.execute("MeasureCommand", &json!({"items": [{"type": "nonsense"}]})).is_err());
 }
 
+/// Section Analysis sets and clears the view cut without touching the design or history.
+#[test]
+fn section_analysis() {
+    let mut s = Session::default();
+    run(&mut s, "PrimitiveBox", json!({"length": 40, "width": 30, "height": 20}));
+    let (doc, undo) = (s.doc.clone(), s.undo.len());
+    run(&mut s, "FusionHalfSectionViewCommand", json!({"plane": "XZ", "offset": 15}));
+    let (o, n) = s.section.unwrap();
+    assert!((o - n * 15.0).len() < 1e-12 && (n.y.abs() - 1.0).abs() < 1e-12);
+    run(&mut s, "FusionHalfSectionViewCommand", json!({"plane": {"origin": [0, 0, 5], "normal": [0, 0, 2]}, "flip": true}));
+    let (o, n) = s.section.unwrap();
+    assert!((o.z - 5.0).abs() < 1e-12 && (n.z + 1.0).abs() < 1e-12);
+    assert!(s.execute("FusionHalfSectionViewCommand", &json!({"plane": {"normal": [0, 0, 0]}})).is_err());
+    run(&mut s, "FusionHalfSectionViewCommand", json!({"clear": true}));
+    assert!(s.section.is_none());
+    assert!(Arc::ptr_eq(&doc, &s.doc));
+    assert_eq!(s.undo.len(), undo);
+}
+
 /// Press Pull: positive pulls a face out, negative pushes it in (a cut), edges get a fillet.
 #[test]
 fn press_pull() {