results.deformed: scale 0 draws the undeformed shape

A factor of 0 was refused, so a picture sequence swinging the deformation
out from the undeformed shape had to start at a small factor or switch
`show` off. 0 is now a factor like any other (the view stays deformed), and
the results panel no longer works around the refusal. The MCP show_results
docs say so, beside `deformed: false`.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/engine/src/cmd/results.rs

@@ -11,7 +11,7 @@ use crate::{CommandKind, CommandSpec, EngineError, Result, Session};
 pub const DEFORMED: CommandSpec = CommandSpec {
     id: "results.deformed",
     label: "Deformed Shape",
-    params: r#"{"step": "Load", "show": true, "scale": "auto"} — shows a solved step's deformed shape (`show: false` the undeformed mesh); `scale` is a factor or "auto" (the largest displacement drawn a tenth of the model's size); `step` defaults to the step shown or the first solved one"#,
+    params: r#"{"step": "Load", "show": true, "scale": "auto"} — shows a solved step's deformed shape (`show: false` the undeformed mesh); `scale` is a factor (0 draws the undeformed shape, so a sequence from 0 swings it out) or "auto" (the largest displacement drawn a tenth of the model's size); `step` defaults to the step shown or the first solved one"#,
     kind: CommandKind::Session,
     run: deformed,
 };
@@ -141,8 +141,9 @@ fn deformed(s: &mut Session, p: &Value) -> Result<Value> {
         Some(Value::String(a)) if a == "auto" => Some(None),
         Some(v) => Some(Some(
             v.as_f64()
-                .filter(|x| x.is_finite() && *x > 0.0 && *x <= MAX_SCALE)
-                .ok_or_else(|| EngineError::Params(format!("`scale` must be \"auto\" or a factor above 0 and at most {MAX_SCALE}")))?,
+                .filter(|x| x.is_finite() && (0.0..=MAX_SCALE).contains(x))
+                .map(f64::abs)
+                .ok_or_else(|| EngineError::Params(format!("`scale` must be \"auto\" or a factor from 0 (undeformed) to {MAX_SCALE}")))?,
         )),
     };
     let v = view_of(s, &step)?;
@@ -336,6 +337,9 @@ mod tests {
         assert!(r["contour"]["range"]["max"]["value"].as_f64().unwrap() > 0.0, "{r}");
         let r = s.execute("results.deformed", &json!({"scale": 2.5})).unwrap();
         assert_eq!((r["view"]["deformed"].as_bool(), r["view"]["scale"].as_f64()), (Some(true), Some(2.5)));
+        // Scale 0: the undeformed shape, still the deformed view (an animation's first picture).
+        let r = s.execute("results.deformed", &json!({"scale": 0})).unwrap();
+        assert_eq!((r["view"]["deformed"].as_bool(), r["view"]["scale"].as_f64()), (Some(true), Some(0.0)));
         s.execute("results.deformed", &json!({"scale": "auto", "show": false})).unwrap();
         assert_eq!(s.results_view.as_ref().map(|v| (v.deformed, v.scale)), Some((false, None)));
         // Modes switch to the frequency step (its own first look) and list the frequencies.
@@ -418,6 +422,7 @@ mod tests {
             json!({"field": "von_mises", "bands": 0}),
             json!({"field": "von_mises", "bands": 25}),
             json!({"scale": -1}),
+            json!({"scale": -0.0001}),
             json!({"scale": 1e300}),
             json!({"scale": "big"}),
             json!({"scale": f64::NAN}),

crates/mcp/src/tools.rs

@@ -210,7 +210,7 @@ pub fn tool_definitions() -> Vec<Value> {
             "show_results",
             "Show results",
             "Choose what the results display shows and read it back: the step, the deformed shape (`deformed`, `scale`: \
-             a factor or \"auto\"), contours of a nodal field (`field`: displacement, displacement_x/y/z, von_mises, \
+             a factor, 0 for the undeformed shape, or \"auto\"), contours of a nodal field (`field`: displacement, displacement_x/y/z, von_mises, \
              max/mid/min_principal, stress_xx/yy/zz/xy/xz/yz, or none; `bands` 2–24), a frequency step's mode or an \
              explicit step's frame. The reply has the contour's range: its smallest and largest value and their nodes \
              (field displacement gives the largest displacement). Only `step`: show that step.",
@@ -220,7 +220,7 @@ pub fn tool_definitions() -> Vec<Value> {
                     "field": string("Contour field, or \"none\""),
                     "bands": integer("Contour bands, 2 to 24"),
                     "deformed": {"type": "boolean", "description": "Draw the deformed shape"},
-                    "scale": {"type": ["number", "string"], "description": "Deformation scale factor or \"auto\""},
+                    "scale": {"type": ["number", "string"], "description": "Deformation scale factor (0: undeformed) or \"auto\""},
                     "mode": integer("Mode of a frequency step, from 1"),
                     "frame": integer("Field frame of an explicit step, from 0"),
                     "time": {"type": "number", "description": "Show the explicit step's frame nearest this time"},

crates/ui-egui/src/results_panel.rs

@@ -143,10 +143,10 @@ pub fn controls(app: &mut ModelApp, plot: &Plot, ui: &mut egui::Ui, t: &Tokens)
             }
             let mut auto = plot.view.scale.is_none();
             if ui.checkbox(&mut auto, "Auto").on_hover_text("Scale the largest displacement to a tenth of the model").changed() {
-                run.push(("results.deformed", json!({"scale": if auto { json!("auto") } else { json!(plot.scale.max(f64::MIN_POSITIVE)) }})));
+                run.push(("results.deformed", json!({"scale": if auto { json!("auto") } else { json!(plot.scale) }})));
             }
             let mut scale = plot.scale;
-            let drag = egui::DragValue::new(&mut scale).speed(plot.scale.abs().max(1e-6) * 0.01).range(1e-9..=1e12).prefix("× ");
+            let drag = egui::DragValue::new(&mut scale).speed(plot.scale.abs().max(1e-6) * 0.01).range(0.0..=1e12).prefix("× ");
             if ui.add_enabled(plot.view.deformed, drag).changed() {
                 run.push(("results.deformed", json!({"scale": scale})));
             }