Fix bottom accent positioning in math

crates/typst-layout/src/math/accent.rs

@@ -1,7 +1,7 @@
 use typst_library::diag::SourceResult;
 use typst_library::foundations::{Packed, StyleChain};
 use typst_library::layout::{Em, Frame, Point, Size};
-use typst_library::math::{Accent, AccentElem};
+use typst_library::math::AccentElem;
 
 use super::{style_cramped, FrameFragment, GlyphFragment, MathContext, MathFragment};
 
@@ -18,8 +18,11 @@ pub fn layout_accent(
     let cramped = style_cramped();
     let mut base = ctx.layout_into_fragment(&elem.base, styles.chain(&cramped))?;
 
-    // Try to replace a glyph with its dotless variant.
-    if elem.dotless(styles) {
+    let accent = elem.accent;
+    let top_accent = !accent.is_bottom();
+
+    // Try to replace base glyph with its dotless variant.
+    if top_accent && elem.dotless(styles) {
         if let MathFragment::Glyph(glyph) = &mut base {
             glyph.make_dotless_form(ctx);
         }
@@ -29,41 +32,54 @@ pub fn layout_accent(
     let base_class = base.class();
     let base_attach = base.accent_attach();
 
-    let width = elem.size(styles).relative_to(base.width());
+    let mut glyph = GlyphFragment::new(ctx, styles, accent.0, elem.span());
 
-    let Accent(c) = elem.accent;
-    let mut glyph = GlyphFragment::new(ctx, styles, c, elem.span());
-
-    // Try to replace accent glyph with flattened variant.
-    let flattened_base_height = scaled!(ctx, styles, flattened_accent_base_height);
-    if base.ascent() > flattened_base_height {
-        glyph.make_flattened_accent_form(ctx);
+    // Try to replace accent glyph with its flattened variant.
+    if top_accent {
+        let flattened_base_height = scaled!(ctx, styles, flattened_accent_base_height);
+        if base.ascent() > flattened_base_height {
+            glyph.make_flattened_accent_form(ctx);
+        }
     }
 
     // Forcing the accent to be at least as large as the base makes it too
     // wide in many case.
+    let width = elem.size(styles).relative_to(base.width());
     let short_fall = ACCENT_SHORT_FALL.at(glyph.font_size);
     let variant = glyph.stretch_horizontal(ctx, width, short_fall);
     let accent = variant.frame;
-    let accent_attach = variant.accent_attach;
+    let accent_attach = variant.accent_attach.0;
+
+    let (gap, accent_pos, base_pos) = if top_accent {
+        // Descent is negative because the accent's ink bottom is above the
+        // baseline. Therefore, the default gap is the accent's negated descent
+        // minus the accent base height. Only if the base is very small, we
+        // need a larger gap so that the accent doesn't move too low.
+        let accent_base_height = scaled!(ctx, styles, accent_base_height);
+        let gap = -accent.descent() - base.ascent().min(accent_base_height);
+        let accent_pos = Point::with_x(base_attach.0 - accent_attach);
+        let base_pos = Point::with_y(accent.height() + gap);
+        (gap, accent_pos, base_pos)
+    } else {
+        let gap = -accent.ascent();
+        let accent_pos = Point::new(base_attach.1 - accent_attach, base.height() + gap);
+        let base_pos = Point::zero();
+        (gap, accent_pos, base_pos)
+    };
 
-    // Descent is negative because the accent's ink bottom is above the
-    // baseline. Therefore, the default gap is the accent's negated descent
-    // minus the accent base height. Only if the base is very small, we need
-    // a larger gap so that the accent doesn't move too low.
-    let accent_base_height = scaled!(ctx, styles, accent_base_height);
-    let gap = -accent.descent() - base.ascent().min(accent_base_height);
     let size = Size::new(base.width(), accent.height() + gap + base.height());
-    let accent_pos = Point::with_x(base_attach - accent_attach);
-    let base_pos = Point::with_y(accent.height() + gap);
     let baseline = base_pos.y + base.ascent();
+
     let base_italics_correction = base.italics_correction();
     let base_text_like = base.is_text_like();
-
     let base_ascent = match &base {
         MathFragment::Frame(frame) => frame.base_ascent,
         _ => base.ascent(),
     };
+    let base_descent = match &base {
+        MathFragment::Frame(frame) => frame.base_descent,
+        _ => base.descent(),
+    };
 
     let mut frame = Frame::soft(size);
     frame.set_baseline(baseline);
@@ -73,6 +89,7 @@ pub fn layout_accent(
         FrameFragment::new(styles, frame)
             .with_class(base_class)
             .with_base_ascent(base_ascent)
+            .with_base_descent(base_descent)
             .with_italics_correction(base_italics_correction)
             .with_accent_attach(base_attach)
             .with_text_like(base_text_like),

crates/typst-layout/src/math/attach.rs

@@ -434,9 +434,13 @@ fn compute_script_shifts(
     }
 
     if bl.is_some() || br.is_some() {
+        let descent = match &base {
+            MathFragment::Frame(frame) => frame.base_descent,
+            _ => base.descent(),
+        };
         shift_down = shift_down
             .max(sub_shift_down)
-            .max(if is_text_like { Abs::zero() } else { base.descent() + sub_drop_min })
+            .max(if is_text_like { Abs::zero() } else { descent + sub_drop_min })
             .max(measure!(bl, ascent) - sub_top_max)
             .max(measure!(br, ascent) - sub_top_max);
     }

crates/typst-layout/src/math/fragment.rs

@@ -164,12 +164,12 @@ impl MathFragment {
         }
     }
 
-    pub fn accent_attach(&self) -> Abs {
+    pub fn accent_attach(&self) -> (Abs, Abs) {
         match self {
             Self::Glyph(glyph) => glyph.accent_attach,
             Self::Variant(variant) => variant.accent_attach,
             Self::Frame(fragment) => fragment.accent_attach,
-            _ => self.width() / 2.0,
+            _ => (self.width() / 2.0, self.width() / 2.0),
         }
     }
 
@@ -240,7 +240,7 @@ pub struct GlyphFragment {
     pub ascent: Abs,
     pub descent: Abs,
     pub italics_correction: Abs,
-    pub accent_attach: Abs,
+    pub accent_attach: (Abs, Abs),
     pub font_size: Abs,
     pub class: MathClass,
     pub math_size: MathSize,
@@ -294,7 +294,7 @@ impl GlyphFragment {
             descent: Abs::zero(),
             limits: Limits::for_char(c),
             italics_correction: Abs::zero(),
-            accent_attach: Abs::zero(),
+            accent_attach: (Abs::zero(), Abs::zero()),
             class,
             span,
             modifiers: FrameModifiers::get_in(styles),
@@ -326,8 +326,14 @@ impl GlyphFragment {
         });
 
         let mut width = advance.scaled(ctx, self.font_size);
-        let accent_attach =
+
+        // The fallback for accents is half the width plus or minus the italics
+        // correction. This is similar to how top and bottom attachments are
+        // shifted. For bottom accents we do not use the accent attach of the
+        // base as it is meant for top acccents.
+        let top_accent_attach =
             accent_attach(ctx, id, self.font_size).unwrap_or((width + italics) / 2.0);
+        let bottom_accent_attach = (width - italics) / 2.0;
 
         let extended_shape = is_extended_shape(ctx, id);
         if !extended_shape {
@@ -339,7 +345,7 @@ impl GlyphFragment {
         self.ascent = bbox.y_max.scaled(ctx, self.font_size);
         self.descent = -bbox.y_min.scaled(ctx, self.font_size);
         self.italics_correction = italics;
-        self.accent_attach = accent_attach;
+        self.accent_attach = (top_accent_attach, bottom_accent_attach);
         self.extended_shape = extended_shape;
     }
 
@@ -457,7 +463,7 @@ impl Debug for GlyphFragment {
 pub struct VariantFragment {
     pub c: char,
     pub italics_correction: Abs,
-    pub accent_attach: Abs,
+    pub accent_attach: (Abs, Abs),
     pub frame: Frame,
     pub font_size: Abs,
     pub class: MathClass,
@@ -499,8 +505,9 @@ pub struct FrameFragment {
     pub limits: Limits,
     pub spaced: bool,
     pub base_ascent: Abs,
+    pub base_descent: Abs,
     pub italics_correction: Abs,
-    pub accent_attach: Abs,
+    pub accent_attach: (Abs, Abs),
     pub text_like: bool,
     pub ignorant: bool,
 }
@@ -508,6 +515,7 @@ pub struct FrameFragment {
 impl FrameFragment {
     pub fn new(styles: StyleChain, frame: Frame) -> Self {
         let base_ascent = frame.ascent();
+        let base_descent = frame.descent();
         let accent_attach = frame.width() / 2.0;
         Self {
             frame: frame.modified(&FrameModifiers::get_in(styles)),
@@ -517,8 +525,9 @@ impl FrameFragment {
             limits: Limits::Never,
             spaced: false,
             base_ascent,
+            base_descent,
             italics_correction: Abs::zero(),
-            accent_attach,
+            accent_attach: (accent_attach, accent_attach),
             text_like: false,
             ignorant: false,
         }
@@ -540,11 +549,15 @@ impl FrameFragment {
         Self { base_ascent, ..self }
     }
 
+    pub fn with_base_descent(self, base_descent: Abs) -> Self {
+        Self { base_descent, ..self }
+    }
+
     pub fn with_italics_correction(self, italics_correction: Abs) -> Self {
         Self { italics_correction, ..self }
     }
 
-    pub fn with_accent_attach(self, accent_attach: Abs) -> Self {
+    pub fn with_accent_attach(self, accent_attach: (Abs, Abs)) -> Self {
         Self { accent_attach, ..self }
     }
 

crates/typst-layout/src/math/stretch.rs

@@ -278,7 +278,7 @@ fn assemble(
     }
 
     let accent_attach = match axis {
-        Axis::X => frame.width() / 2.0,
+        Axis::X => (frame.width() / 2.0, frame.width() / 2.0),
         Axis::Y => base.accent_attach,
     };
 

crates/typst-library/src/math/accent.rs

@@ -80,6 +80,19 @@ impl Accent {
     pub fn new(c: char) -> Self {
         Self(Self::combine(c).unwrap_or(c))
     }
+
+    /// List of bottom accents. Currently just a list of ones included in the
+    /// Unicode math class document.
+    const BOTTOM: &[char] = &[
+        '\u{0323}', '\u{032C}', '\u{032D}', '\u{032E}', '\u{032F}', '\u{0330}',
+        '\u{0331}', '\u{0332}', '\u{0333}', '\u{033A}', '\u{20E8}', '\u{20EC}',
+        '\u{20ED}', '\u{20EE}', '\u{20EF}',
+    ];
+
+    /// Whether this accent is a bottom accent or not.
+    pub fn is_bottom(&self) -> bool {
+        Self::BOTTOM.contains(&self.0)
+    }
 }
 
 /// This macro generates accent-related functions.

tests/suite/math/accent.typ

@@ -58,3 +58,31 @@ $hat(a) hat(A)$
 $tilde(w) tilde(W)$
 $grave(i) grave(j)$
 $grave(I) grave(J)$
+
+--- math-accent-bottom ---
+// Test bottom accents.
+$accent(a, \u{20EE}), accent(T, \u{0323}), accent(xi, \u{0332}),
+  accent(f, \u{20ED}), accent(F, \u{20E8}), accent(y, \u{032E}),
+  accent(!, \u{032F}), accent(J, \u{0333}), accent(p, \u{0331})$
+
+--- math-accent-bottom-wide-base ---
+// Test wide base with bottom accents.
+$accent(x + y, \u{20EF}), accent(sum, \u{032D})$
+
+--- math-accent-bottom-subscript ---
+// Test effect of bottom accent on subscript.
+$q_x != accent(q, \u{032C})_x != accent(accent(q, \u{032C}), \u{032C})_x$
+
+--- math-accent-bottom-high-base ---
+// Test high base with bottom accents.
+$ accent(integral, \u{20EC}), accent(integral, \u{20EC})_a^b, accent(integral_a^b, \u{20EC}) $
+
+--- math-accent-bottom-sized ---
+// Test bottom accent size.
+$accent(sum, \u{0330}), accent(sum, \u{0330}, size: #50%), accent(H, \u{032D}, size: #200%)$
+
+--- math-accent-nested ---
+// Test nested top and bottom accents.
+$hat(accent(L, \u{0330})), accent(circle(p), \u{0323}),
+  macron(accent(caron(accent(A, \u{20ED})), \u{0333})) \
+  breve(accent(eta, \u{032E})) = accent(breve(eta), \u{032E})$