Configurable font edges ⚙

Adds top-edge and bottom-edge parameters to the font function. These define how
the box around a word is computed. The possible values are:
- ascender
- cap-height (default top edge)
- x-height
- baseline (default bottom edge)
- descender

The defaults are chosen so that it's easy to create good-looking designs with
vertical alignment. Since they are much tighter than what most other software
uses by default, the default leading had to be increased to 50% of the font size
and paragraph spacing to 100% of the font size.

The values cap-height and x-height fall back to ascender in case they are zero
because this value may occur in fonts that don't have glyphs with cap- or
x-height (like Twitter Color Emoji). Since cap-height is the default top edge,
doing no fallback would break things badly.

Removes softness in favor of a simple boolean for pages and a more finegread u8
for spacing. This is needed to make paragraph spacing consume line spacing
created by hard line breaks.

4 files changed, 37 insertions, 9 deletions

diff --git a/src/library/font.rs b/src/library/font.rs
index 460cfba1..ecc15d96 100644
--- a/src/library/font.rs
+++ b/src/library/font.rs
@@ -1,6 +1,7 @@
 use fontdock::{FontStretch, FontStyle, FontWeight};
 
 use super::*;
+use crate::shaping::VerticalFontMetric;
 
 /// `font`: Configure the font.
 ///
@@ -13,6 +14,8 @@ use super::*;
 /// - Font Style: `style`, of type `font-style`.
 /// - Font Weight: `weight`, of type `font-weight`.
 /// - Font Stretch: `stretch`, of type `relative`, between 0.5 and 2.0.
+/// - Top edge of the font: `top-edge`, of type `vertical-font-metric`.
+/// - Bottom edge of the font: `bottom-edge`, of type `vertical-font-metric`.
 /// - Serif family definition: `serif`, of type `font-familiy-list`.
 /// - Sans-serif family definition: `sans-serif`, of type `font-familiy-list`.
 /// - Monospace family definition: `monospace`, of type `font-familiy-list`.
@@ -22,15 +25,15 @@ use super::*;
 /// if present.
 ///
 /// # Relevant types and constants
-/// - Type `font-family-list`
-///   - coerces from `string`
-///   - coerces from `array`
-///   - coerces from `font-family`
 /// - Type `font-family`
 ///   - `serif`
 ///   - `sans-serif`
 ///   - `monospace`
 ///   - coerces from `string`
+/// - Type `font-family-list`
+///   - coerces from `string`
+///   - coerces from `array`
+///   - coerces from `font-family`
 /// - Type `font-style`
 ///   - `normal`
 ///   - `italic`
@@ -46,12 +49,20 @@ use super::*;
 ///   - `extrabold` (800)
 ///   - `black` (900)
 ///   - coerces from `integer`
+/// - Type `vertical-font-metric`
+///   - `ascender`
+///   - `cap-height`
+///   - `x-height`
+///   - `baseline`
+///   - `descender`
 pub fn font(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
     let size = args.find::<Linear>(ctx);
     let list: Vec<_> = args.filter::<FontFamily>(ctx).map(|f| f.to_string()).collect();
     let style = args.get(ctx, "style");
     let weight = args.get(ctx, "weight");
     let stretch = args.get(ctx, "stretch");
+    let top_edge = args.get(ctx, "top-edge");
+    let bottom_edge = args.get(ctx, "bottom-edge");
     let serif = args.get(ctx, "serif");
     let sans_serif = args.get(ctx, "sans-serif");
     let monospace = args.get(ctx, "monospace");
@@ -87,6 +98,14 @@ pub fn font(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
             ctx.state.font.variant.stretch = stretch;
         }
 
+        if let Some(top_edge) = top_edge {
+            ctx.state.font.top_edge = top_edge;
+        }
+
+        if let Some(bottom_edge) = bottom_edge {
+            ctx.state.font.bottom_edge = bottom_edge;
+        }
+
         for (variant, arg) in &[
             (FontFamily::Serif, &serif),
             (FontFamily::SansSerif, &sans_serif),
@@ -185,3 +204,7 @@ typify! {
         };
     },
 }
+
+typify! {
+    VerticalFontMetric: "vertical font metric",
+}
diff --git a/src/library/mod.rs b/src/library/mod.rs
index 09fda20f..453eab26 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -26,9 +26,9 @@ use std::fmt::{self, Display, Formatter};
 use fontdock::{FontStyle, FontWeight};
 
 use crate::eval::{Scope, ValueAny, ValueFunc};
-use crate::exec::Softness;
 use crate::layout::*;
 use crate::prelude::*;
+use crate::shaping::VerticalFontMetric;
 
 /// Construct a scope containing all standard library definitions.
 pub fn new() -> Scope {
@@ -81,6 +81,11 @@ pub fn new() -> Scope {
     set!(any: "bold", FontWeight::BOLD);
     set!(any: "extrabold", FontWeight::EXTRABOLD);
     set!(any: "black", FontWeight::BLACK);
+    set!(any: "ascender", VerticalFontMetric::Ascender);
+    set!(any: "cap-height", VerticalFontMetric::CapHeight);
+    set!(any: "x-height", VerticalFontMetric::XHeight);
+    set!(any: "baseline", VerticalFontMetric::Baseline);
+    set!(any: "descender", VerticalFontMetric::Descender);
 
     std
 }
diff --git a/src/library/page.rs b/src/library/page.rs
index 17021607..f7d76eaf 100644
--- a/src/library/page.rs
+++ b/src/library/page.rs
@@ -95,13 +95,13 @@ pub fn page(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
         }
 
         ctx.set_dirs(Gen::new(main, cross));
-        ctx.finish_page(false, Softness::Hard, span);
+        ctx.finish_page(false, true, span);
 
         if let Some(body) = &body {
             // TODO: Restrict body to a single page?
             body.exec(ctx);
             ctx.state = snapshot;
-            ctx.finish_page(true, Softness::Soft, span);
+            ctx.finish_page(true, false, span);
         }
     })
 }
@@ -113,6 +113,6 @@ pub fn page(ctx: &mut EvalContext, args: &mut ValueArgs) -> Value {
 pub fn pagebreak(_: &mut EvalContext, args: &mut ValueArgs) -> Value {
     let span = args.span;
     Value::template("pagebreak", move |ctx| {
-        ctx.finish_page(true, Softness::Hard, span);
+        ctx.finish_page(true, true, span);
     })
 }
diff --git a/src/library/spacing.rs b/src/library/spacing.rs
index d8f18f41..4965a220 100644
--- a/src/library/spacing.rs
+++ b/src/library/spacing.rs
@@ -28,7 +28,7 @@ fn spacing(ctx: &mut EvalContext, args: &mut ValueArgs, axis: SpecAxis) -> Value
     Value::template("spacing", move |ctx| {
         if let Some(linear) = spacing {
             let amount = linear.resolve(ctx.state.font.font_size());
-            let spacing = NodeSpacing { amount, softness: Softness::Hard };
+            let spacing = NodeSpacing { amount, softness: 0 };
             if axis == ctx.state.dirs.main.axis() {
                 ctx.push_into_stack(spacing);
             } else {