Datums: drawn at the model's extent, never framed, the selected one shown

A datum plane is drawn as the model's extent within it (where it cuts the
model's box, in the plane's axes) plus a 5 % margin, a datum axis along the
model's extent plus the margin, instead of squares 1.2 diagonals wide.
Fit, Home, the standard views and headless pictures frame the model
without the datums (they stay inside the clipping radius). The datum
selected in the browser is drawn highlighted and on top, even when hidden.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/view.rs

@@ -164,6 +164,8 @@ struct Picture {
     shown: Shown,
     glyphs: Glyphs,
     colors: GlyphColors,
+    /// The box around the datums drawn: inside the clipping radius, never framed (#295).
+    datums: Aabb3,
 }
 
 impl Picture {
@@ -185,15 +187,10 @@ fn picture(s: &Session, p: &Palette) -> Picture {
     let plot = s.results_view.as_ref().and_then(|v| s.latest_result(&v.step).map(|(r, _)| (v, r)));
     let Some((view, result)) = plot else {
         let mut scene = model_scene(&base, p);
-        let mut bounds = base.bounds;
-        bounds.union(fieldcraft_render::glyphs::datums(
-            &s.model,
-            &base.bounds,
-            &|n| s.shows(fieldcraft_io::ItemKind::Datum, n),
-            &p.glyphs,
-            &mut scene,
-        ));
-        return Picture { scene, drawn: base, bounds, shown, glyphs, colors: p.glyphs };
+        let bounds = base.bounds;
+        let shows = |n: &str| s.shows(fieldcraft_io::ItemKind::Datum, n);
+        let datums = fieldcraft_render::glyphs::datums(&s.model, &base.bounds, &shows, None, &p.glyphs, &mut scene);
+        return Picture { scene, drawn: base, bounds, shown, glyphs, colors: p.glyphs, datums };
     };
     shown.step = Some(view.step.clone());
     let pick = view.pick();
@@ -237,7 +234,7 @@ fn picture(s: &Session, p: &Palette) -> Picture {
     }
     let mut bounds = base.bounds;
     bounds.union(drawn.bounds);
-    Picture { scene, drawn, bounds, shown, glyphs, colors: p.glyphs.over_results(p.background.1) }
+    Picture { scene, drawn, bounds, shown, glyphs, colors: p.glyphs.over_results(p.background.1), datums: Aabb3::EMPTY }
 }
 
 /// The scene of the session (the model, or the results view's plot) with the setup's symbols
@@ -269,7 +266,8 @@ pub fn orientation(p: &Value) -> Result<Orientation> {
     Orientation::checked(yaw, pitch, distance).ok_or_else(|| bad("`yaw` and `pitch` must be finite and `distance` finite and above 0"))
 }
 
-/// A PNG of the session seen from `at`, `w` × `h` pixels: framing everything drawn, or from
+/// A PNG of the session seen from `at`, `w` × `h` pixels: framing everything drawn but the
+/// datums (they are drawn at the model's extent, and framing them would only add margin), or from
 /// `at`'s distance when it has one. The symbols are drawn at their default size on screen
 /// ([`glyphs::DEFAULT_SIZE`] pixels) for that camera.
 pub fn render_png(s: &Session, at: &Orientation, w: usize, h: usize, p: &Palette) -> Result<(Vec<u8>, Shown)> {
@@ -289,6 +287,7 @@ pub fn render_png(s: &Session, at: &Orientation, w: usize, h: usize, p: &Palette
     symbols = Scene::default();
     let mut bounds = pic.bounds;
     bounds.union(pic.symbols(unit(&cam), &mut symbols));
+    bounds.union(pic.datums);
     pic.scene.radius = if bounds.is_empty() { 1.0 } else { (cam.target.dist(bounds.center()) + bounds.diagonal() * 0.5).max(1e-3) };
     let png = fieldcraft_render::render_layers_png(&pic.scene, &[&symbols], &cam, w, h).map_err(|e| EngineError::Params(format!("render: {e}")))?;
     Ok((png, pic.shown))