Variable-radius fillets through middle points: radii at positions along the edge, straight or smooth between

kernel::fillet_variable_points(body, edge, start, [(position, radius)...], smooth): the radius law is a B-spline of the edge parameter (straight pieces, or C1 cubic pieces with Catmull-Rom slopes), and since every section's arc is affine in its radius the blend is exactly a rational B-spline (degree 2 across, the law's degree along) with B-spline contact curves on the faces. The largest radius along the edge decides whether the blend fits. Fillet type variable takes mid: [[position, radius expr], ...] and smooth; middle radii are parameters of the feature (Mid radius) and follow user parameters. Tests: straight pieces against (1 - pi/4) L (a^2 + ab + b^2) / 3 per piece, smooth against the integrated law, either end as start, refusals; engine test with a parameter-driven middle radius.

Refs #9

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/doc/src/document.rs

@@ -773,8 +773,16 @@ pub enum FilletStyle {
     Constant,
     /// `radius` is the chord: the width across the blend.
     Chord,
-    /// `radius` at the end of each edge nearest `start`, `radius2` at the other, linear between.
-    Variable { radius2: String, start: Vec3 },
+    /// `radius` at the end of each edge nearest `start`, `radius2` at the other; `mid` radii at
+    /// positions between (0 to 1 from the start end); straight between them, or `smooth`.
+    Variable {
+        radius2: String,
+        start: Vec3,
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        mid: Vec<(f64, String)>,
+        #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+        smooth: bool,
+    },
 }
 
 impl FilletStyle {
@@ -1148,8 +1156,9 @@ impl FeatureKind {
             }
             FeatureKind::Fillet { radius, style, .. } => {
                 v.push(radius);
-                if let FilletStyle::Variable { radius2, .. } = style {
+                if let FilletStyle::Variable { radius2, mid, .. } = style {
                     v.push(radius2);
+                    v.extend(mid.iter().map(|(_, r)| r.as_str()));
                 }
             }
             FeatureKind::Chamfer { distance, distance2, angle, .. } => {

crates/doc/src/eval.rs

@@ -2620,11 +2620,15 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             };
             let nb = match &f.kind {
                 FeatureKind::Fillet { style: crate::FilletStyle::Chord, .. } => kernel::fillet_chord(&mb.body, &edges, r)?,
-                FeatureKind::Fillet { style: crate::FilletStyle::Variable { radius2, start }, .. } => {
+                FeatureKind::Fillet { style: crate::FilletStyle::Variable { radius2, start, mid, smooth }, .. } => {
                     let r2 = val(vals, radius2, Kind::Length)?;
+                    let mut pts = vec![(0.0, r), (1.0, r2)];
+                    for (t, e) in mid {
+                        pts.push((*t, val(vals, e, Kind::Length)?));
+                    }
                     let mut b = mb.body.clone();
                     for p in &edges {
-                        b = kernel::fillet_variable(&b, *p, *start, r, r2)?;
+                        b = kernel::fillet_variable_points(&b, *p, *start, &pts, *smooth)?;
                     }
                     b
                 }

crates/doc/src/params.rs

@@ -75,9 +75,10 @@ impl FeatureKind {
             FeatureKind::Fillet { radius, style, .. } => match style {
                 crate::FilletStyle::Constant => v.push(("Radius", radius, L)),
                 crate::FilletStyle::Chord => v.push(("Chord length", radius, L)),
-                crate::FilletStyle::Variable { radius2, .. } => {
+                crate::FilletStyle::Variable { radius2, mid, .. } => {
                     v.push(("Start radius", radius, L));
                     v.push(("End radius", radius2, L));
+                    v.extend(mid.iter_mut().map(|(_, r)| ("Mid radius", r, L)));
                 }
             },
             FeatureKind::Chamfer { distance, distance2, angle, .. } => {