Render: presets PowerPoint mitres by default get sharp corners; miter limit 8

An outline naming no join was always rounded. PowerPoint rounds most
presets but mitres some by default: classified per preset by comparing
its export (8 pt outline, no join) with round- and miter-joined renders:
the plus, right/up/down block arrows (not the left one), chevrons, stars
from five points up, the equation shapes, curved ribbons and the action
buttons. Miter joins also take PowerPoint's default limit of 8 widths
(kurbo's default was 4), so sharp star points no longer bevel off.

Wild synthetic set: joins slide 0.43 -> 0.21, outline-on-pattern slide
3.95 -> 3.76, preset galleries slightly better.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/geom/src/preset.rs

@@ -339,6 +339,45 @@ pub fn info(name: &str) -> Option<&'static PresetInfo> {
     CATALOG.iter().find(|p| p.name == name)
 }
 
+/// Whether PowerPoint joins this preset's outline with sharp (miter) corners when the line names
+/// no join (it rounds them otherwise). Observed per preset on its export: the plus, the right,
+/// up and down block arrows (not the left one), chevrons, stars from five points up, the
+/// equation shapes, curved ribbons and the action buttons are mitred.
+pub fn mitred_by_default(name: &str) -> bool {
+    matches!(
+        name,
+        "plus"
+            | "rightArrow"
+            | "upArrow"
+            | "downArrow"
+            | "upDownArrow"
+            | "leftUpArrow"
+            | "bentUpArrow"
+            | "stripedRightArrow"
+            | "notchedRightArrow"
+            | "chevron"
+            | "leftArrowCallout"
+            | "leftRightArrowCallout"
+            | "upDownArrowCallout"
+            | "quadArrowCallout"
+            | "mathPlus"
+            | "mathMinus"
+            | "mathMultiply"
+            | "mathEqual"
+            | "mathNotEqual"
+            | "star5"
+            | "star6"
+            | "star8"
+            | "star10"
+            | "star12"
+            | "star16"
+            | "star24"
+            | "star32"
+            | "ellipseRibbon"
+            | "ellipseRibbon2"
+    ) || name.starts_with("actionButton")
+}
+
 pub fn is_line_like(name: &str) -> bool {
     matches!(name, "line" | "straightConnector1" | "bentConnector2" | "bentConnector3" | "curvedConnector3" | "arc")
         || name.starts_with("bentConnector")

crates/render/src/lib.rs

@@ -998,6 +998,13 @@ impl Renderer {
                 ctx.pop_layer();
             }
         }
+        // Some presets' outlines default to sharp corners (PowerPoint's own per-shape default).
+        let line = match (&s.geom, line.join) {
+            (Geom::Preset { name, .. }, None) if preset::mitred_by_default(name) => {
+                Line { join: Some(deckcraft_model::style::LineJoin::Miter), ..line }
+            }
+            _ => line,
+        };
         // A 3-D bevel: the face and a band inside the outline lit as surfaces.
         if let Some(bv) = effects.as_ref().and_then(|e| e.raw3d.as_deref()).and_then(bevel::from_raw) {
             let solid = match &fill {
@@ -1404,7 +1411,8 @@ pub(crate) fn stroke(
     // Hairlines stay visible at any zoom.
     let min_w = 0.5 / scale_of(m).max(1e-6);
     let sw = lw.max(min_w);
-    let mut sk = kurbo::Stroke::new(sw).with_caps(cap).with_join(join);
+    // PowerPoint's default miter limit is 8 widths (`a:miter lim="800000"`).
+    let mut sk = kurbo::Stroke::new(sw).with_caps(cap).with_join(join).with_miter_limit(8.0);
     // Square caps on a dashed line square off only the line's own two ends (where a dash is
     // drawn): each dash itself ends flat, as PowerPoint draws them.
     let mut square_ends = None;

crates/render/src/tests_wild.rs

@@ -662,3 +662,24 @@ fn a_gradient_not_rotating_with_its_shape_keeps_to_the_slide() {
         }
     }
 }
+
+#[test]
+fn stars_and_right_arrows_have_sharp_corners_by_default() {
+    let shape = |name: &str, join: Option<deckcraft_model::style::LineJoin>| Shape {
+        id: ShapeId(29),
+        xfrm: Some(Xfrm::new(100.0, 100.0, 100.0, 100.0)),
+        geom: Geom::preset(name),
+        fill: solid(WHITE),
+        line: Some(Line { join, ..Line::solid(ColorRef::rgb(RED), 10.0) }),
+        ..Default::default()
+    };
+    // A star5's top point mitred reaches about 16 pt above the box; rounded only 5.
+    let img = render(vec![shape("star5", None)]);
+    assert!(close(img.pixel(150, 90), RED, 40), "mitred point {:?}", img.pixel(150, 90));
+    let img = render(vec![shape("star5", Some(deckcraft_model::style::LineJoin::Round))]);
+    assert!(close(img.pixel(150, 90), WHITE, 10), "an explicit round join stays round");
+    // A rectangle keeps PowerPoint's round default: its corner is cut round.
+    let img = render(vec![shape("rect", None)]);
+    assert!(close(img.pixel(95, 95), WHITE, 30), "{:?}", img.pixel(95, 95));
+    assert!(deckcraft_geom::preset::mitred_by_default("rightArrow") && !deckcraft_geom::preset::mitred_by_default("leftArrow"));
+}