Blob Brush: the tip preview draws 0% Roundness as thin as the brush

The Tool Options tip preview floored the short axis at 1% of the long one, while the brush draws 0% Roundness at 0.1%. The preview now uses the same 0.1% minimum, with a test that compares it with the tip the brush draws.

crates/ui-egui/src/dialogs/freehand.rs

@@ -276,7 +276,7 @@ fn blob_tip_preview(ui: &mut egui::Ui, d: &mut Dialog) {
     let u = egui::vec2(a.cos() as f32, -a.sin() as f32);
     let v = egui::vec2(-u.y, u.x);
     let long = 18.0;
-    let short = long * (d.f64("roundness", 100.0).clamp(0.0, 100.0) as f32 / 100.0).max(0.01);
+    let short = long * blob_tip_short_ratio(d.f64("roundness", 100.0));
     let dab = |center: egui::Pos2, color: egui::Color32| {
         let points = (0..32)
             .map(|i| {
@@ -295,6 +295,12 @@ fn blob_tip_preview(ui: &mut egui::Ui, d: &mut Dialog) {
     }
 }
 
+/// The preview tip's short axis as a fraction of its long axis: Roundness in percent, with 0
+/// percent drawn as 0.1 percent, the minimum the brush draws with.
+fn blob_tip_short_ratio(roundness: f64) -> f32 {
+    (roundness.clamp(0.0, 100.0) as f32 / 100.0).max(0.001)
+}
+
 /// OK: set the tool's options.
 fn confirm(app: &mut VectorcraftApp, d: &Dialog) -> Result<Value, String> {
     let values: Map<String, Value> = KEYS.iter().filter_map(|k| Some((k.to_string(), d.fields.get(*k)?.clone()))).collect();

crates/ui-egui/src/dialogs/freehand/tests.rs

@@ -28,6 +28,14 @@ fn blob_tip_fields_confirm_and_cancel() {
     assert_eq!(app.session.tool_options_of("blobBrush")["roundness"], json!(0.0));
 }
 
+#[test]
+fn the_tip_preview_is_as_flat_as_the_tip_the_brush_draws() {
+    for roundness in [-5.0, 0.0, 0.05, 0.1, 1.0, 50.0, 100.0, 120.0] {
+        let drawn = vectorcraft_tools::draw2::blob_tip(0.0, roundness).as_coeffs()[3] as f32;
+        assert!((blob_tip_short_ratio(roundness) - drawn).abs() < 1e-6, "roundness {roundness}: drawn {drawn}");
+    }
+}
+
 /// The labels the open dialog paints.
 fn painted(app: &mut VectorcraftApp) -> String {
     crate::tests_labels::painted_text(app, |app, ui| super::super::show(app, ui.ctx()))