improv: extract decoration metrics from the font

src/attrs.rs

@@ -255,6 +255,12 @@ impl TextDecoration {
     }
 }
 
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+pub struct DecorationMetrics {
+    pub offset: f32,
+    pub thickness: f32,
+}
+
 /// Text attributes
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
 pub struct Attrs<'a> {

src/layout.rs

@@ -2,7 +2,7 @@
 
 use core::fmt::Display;
 
-use crate::{math, CacheKey, CacheKeyFlags, Color, TextDecoration};
+use crate::{math, CacheKey, CacheKeyFlags, Color, DecorationMetrics, TextDecoration};
 #[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
@@ -60,6 +60,10 @@ pub struct LayoutGlyph {
     pub cache_key_flags: CacheKeyFlags,
     /// Text decoration (underline, strikethough, overline)
     pub text_decoration: TextDecoration,
+    /// Underline offset and thickness extracted from the font
+    pub underline_metrics: DecorationMetrics,
+    /// Strikethrough offset and thickness extracted from the font
+    pub strikethrough_metrics: DecorationMetrics,
 }
 
 #[derive(Clone, Debug)]

src/render.rs

@@ -67,8 +67,13 @@ fn draw_decoration_group<R: Renderer>(
     let font_size = first.font_size;
     let x_start = first.x;
     let x_end = last.x + last.w;
-    let width = (x_end - x_start) as u32;
-    if width <= 0 {
+    let width = x_end - x_start;
+    if width <= 0.0 {
+        // first check to see if it's below 0.0
+        return;
+    }
+    let width = width as u32;
+    if width == 0 {
         return;
     }
     // Underline
@@ -79,8 +84,8 @@ fn draw_decoration_group<R: Renderer>(
                 .underline_color_opt
                 .or(first.color_opt)
                 .unwrap_or(default_color);
-            let thickness = (font_size * 14.0).max(1.0);
-            let y = run.line_y + font_size * 0.125;
+            let thickness = (first.underline_metrics.thickness * font_size).max(1.0);
+            let y = run.line_y - first.underline_metrics.offset * font_size;
             renderer.rectangle(x_start as i32, y as i32, width, thickness as u32, color);
         }
         UnderlineStyle::Double => {
@@ -88,9 +93,9 @@ fn draw_decoration_group<R: Renderer>(
                 .underline_color_opt
                 .or(first.color_opt)
                 .unwrap_or(default_color);
-            let thickness = (font_size * 14.0).max(1.0);
+            let thickness = (first.underline_metrics.thickness * font_size).max(1.0);
             let gap = thickness;
-            let y = run.line_y + font_size * 0.125;
+            let y = run.line_y - first.underline_metrics.offset * font_size;
             renderer.rectangle(x_start as i32, y as i32, width, thickness as u32, color);
             renderer.rectangle(
                 x_start as i32,
@@ -108,8 +113,8 @@ fn draw_decoration_group<R: Renderer>(
             .strikethrough_color_opt
             .or(first.color_opt)
             .unwrap_or(default_color);
-        let thickness = (font_size / 14.0).max(1.0);
-        let y = run.line_y - font_size * 0.3;
+        let thickness = (first.strikethrough_metrics.thickness * font_size).max(1.0);
+        let y = run.line_y - first.strikethrough_metrics.offset * font_size;
         renderer.rectangle(x_start as i32, y as i32, width, thickness as u32, color);
     }
 
@@ -119,8 +124,11 @@ fn draw_decoration_group<R: Renderer>(
             .overline_color_opt
             .or(first.color_opt)
             .unwrap_or(default_color);
-        let thickness = (font_size / 14.0).max(1.0);
-        let y = run.line_top;
+        // we're reusing underline thickness for overline
+        let thickness = (first.underline_metrics.thickness * font_size).max(1.0);
+        let y = run.line_top; //TODO: this should be run.line_y - ascent
+                              // but we don't have ascent in GlyphLayout
+                              // using line_top as an approximation for now, which should be good enough for most fonts
         renderer.rectangle(x_start as i32, y as i32, width, thickness as u32, color);
     }
 }

src/shape.rs

@@ -4,8 +4,9 @@
 
 use crate::fallback::FontFallbackIter;
 use crate::{
-    math, Align, Attrs, AttrsList, CacheKeyFlags, Color, Ellipsize, EllipsizeHeightLimit, Font,
-    FontSystem, Hinting, LayoutGlyph, LayoutLine, Metrics, TextDecoration, Wrap,
+    math, Align, Attrs, AttrsList, CacheKeyFlags, Color, DecorationMetrics, Ellipsize,
+    EllipsizeHeightLimit, Font, FontSystem, Hinting, LayoutGlyph, LayoutLine, Metrics,
+    TextDecoration, Wrap,
 };
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
@@ -15,6 +16,7 @@ use core::cmp::{max, min};
 use core::fmt;
 use core::mem;
 use core::ops::Range;
+use skrifa::metrics::Decoration;
 
 #[cfg(not(feature = "std"))]
 use core_maths::CoreFloat;
@@ -130,6 +132,7 @@ fn shape_fallback(
     let font_scale = font.metrics().units_per_em as f32;
     let ascent = font.metrics().ascent / font_scale;
     let descent = -font.metrics().descent / font_scale;
+    let (underline_metrics, strikethrough_metrics) = decoration_metrics(font);
 
     let mut buffer = scratch.harfrust_buffer.take().unwrap_or_default();
     buffer.set_direction(if span_rtl {
@@ -237,6 +240,8 @@ fn shape_fallback(
             cache_key_flags: override_fake_italic(attrs.cache_key_flags, font, &attrs),
             metrics_opt: attrs.metrics_opt.map(Into::into),
             text_decoration: attrs.text_decoration,
+            underline_metrics,
+            strikethrough_metrics,
         });
     }
 
@@ -504,6 +509,8 @@ fn shape_skip(
     let metrics = swash_font.metrics(&[]);
     let glyph_metrics = swash_font.glyph_metrics(&[]).scale(1.0);
 
+    let (underline_metrics, strikethrough_metrics) = decoration_metrics(font.as_ref());
+
     let ascent = metrics.ascent / f32::from(metrics.units_per_em);
     let descent = metrics.descent / f32::from(metrics.units_per_em);
 
@@ -538,6 +545,8 @@ fn shape_skip(
                     ),
                     metrics_opt: attrs.metrics_opt.map(Into::into),
                     text_decoration: attrs.text_decoration,
+                    underline_metrics,
+                    strikethrough_metrics,
                 }
             }),
     );
@@ -575,6 +584,8 @@ pub struct ShapeGlyph {
     pub cache_key_flags: CacheKeyFlags,
     pub metrics_opt: Option<Metrics>,
     pub text_decoration: TextDecoration,
+    pub underline_metrics: DecorationMetrics,
+    pub strikethrough_metrics: DecorationMetrics,
 }
 
 impl ShapeGlyph {
@@ -605,6 +616,8 @@ impl ShapeGlyph {
             metadata: self.metadata,
             cache_key_flags: self.cache_key_flags,
             text_decoration: self.text_decoration,
+            underline_metrics: self.underline_metrics,
+            strikethrough_metrics: self.strikethrough_metrics,
         }
     }
 
@@ -615,6 +628,24 @@ impl ShapeGlyph {
     }
 }
 
+fn decoration_metrics(font: &Font) -> (DecorationMetrics, DecorationMetrics) {
+    let metrics = font.metrics();
+    let upem = metrics.units_per_em as f32;
+    if upem == 0.0 {
+        return (DecorationMetrics::default(), DecorationMetrics::default());
+    }
+    (
+        DecorationMetrics {
+            offset: metrics.underline.map_or(-0.125, |d| d.offset / upem),
+            thickness: metrics.underline.map_or(1.0 / 14.0, |d| d.thickness / upem),
+        },
+        DecorationMetrics {
+            offset: metrics.strikeout.map_or(0.3, |d| d.offset / upem),
+            thickness: metrics.strikeout.map_or(1.0 / 14.0, |d| d.thickness / upem),
+        },
+    )
+}
+
 /// span index used in VlRange to indicate this range is the ellipsis.
 const ELLIPSIS_SPAN: usize = usize::MAX;