Fix text sizes being too small

Closes #2068

Before this PR, the default font, Ubuntu-Light, was ~11% smaller
than it should have been, and the default monospace font, Hack,
was ~14% smaller. This means that setting the font size `12` in egui
would yield smaller text than using that font size in any other app.
Ooops!

The change is that this PR now takes into account the ttf properties
`units_per_em` and `height_unscaled`.

If your egui application has specified you own font sizes or text styles
you will see the text in your application grow
larger, unless you go in and compensate by dividing all font sizes by
~1.21 for Ubuntu-Light/Proportional and ~1.16 for Hack/Monospace,
and with something else if you are using a custom font!
This effects any use of `FontId`, `RichText::size`, etc.

This PR changes the default `Style::text_styles` to compensate,
so the default egui style should look the same before and after this PR.

crates/egui/src/introspection.rs

@@ -14,7 +14,7 @@ pub fn font_family_ui(ui: &mut Ui, font_family: &mut FontFamily) {
 pub fn font_id_ui(ui: &mut Ui, font_id: &mut FontId) {
     let families = ui.fonts().families();
     ui.horizontal(|ui| {
-        ui.add(Slider::new(&mut font_id.size, 4.0..=40.0).max_decimals(0));
+        ui.add(Slider::new(&mut font_id.size, 4.0..=40.0).max_decimals(1));
         for alternative in families {
             let text = alternative.to_string();
             ui.radio_value(&mut font_id.family, alternative, text);

crates/egui/src/style.rs

@@ -602,25 +602,16 @@ pub struct DebugOptions {
 
 /// The default text styles of the default egui theme.
 pub fn default_text_styles() -> BTreeMap<TextStyle, FontId> {
-    let mut text_styles = BTreeMap::new();
-    text_styles.insert(
-        TextStyle::Small,
-        FontId::new(10.0, FontFamily::Proportional),
-    );
-    text_styles.insert(TextStyle::Body, FontId::new(14.0, FontFamily::Proportional));
-    text_styles.insert(
-        TextStyle::Button,
-        FontId::new(14.0, FontFamily::Proportional),
-    );
-    text_styles.insert(
-        TextStyle::Heading,
-        FontId::new(20.0, FontFamily::Proportional),
-    );
-    text_styles.insert(
-        TextStyle::Monospace,
-        FontId::new(14.0, FontFamily::Monospace),
-    );
-    text_styles
+    use FontFamily::{Monospace, Proportional};
+
+    [
+        (TextStyle::Small, FontId::new(9.0, Proportional)),
+        (TextStyle::Body, FontId::new(12.5, Proportional)),
+        (TextStyle::Button, FontId::new(12.5, Proportional)),
+        (TextStyle::Heading, FontId::new(18.0, Proportional)),
+        (TextStyle::Monospace, FontId::new(12.0, Monospace)),
+    ]
+    .into()
 }
 
 impl Default for Style {

crates/egui_demo_lib/src/demo/font_book.rs

@@ -10,7 +10,7 @@ impl Default for FontBook {
     fn default() -> Self {
         Self {
             filter: Default::default(),
-            font_id: egui::FontId::proportional(20.0),
+            font_id: egui::FontId::proportional(18.0),
             named_chars: Default::default(),
         }
     }

crates/epaint/src/text/fonts.rs

@@ -1,5 +1,4 @@
-use std::collections::BTreeMap;
-use std::sync::Arc;
+use std::{collections::BTreeMap, sync::Arc};
 
 use crate::{
     mutex::{Mutex, MutexGuard},
@@ -271,15 +270,21 @@ impl Default for FontDefinitions {
         // Some good looking emojis. Use as first priority:
         font_data.insert(
             "NotoEmoji-Regular".to_owned(),
-            FontData::from_static(include_bytes!("../../fonts/NotoEmoji-Regular.ttf")),
+            FontData::from_static(include_bytes!("../../fonts/NotoEmoji-Regular.ttf")).tweak(
+                FontTweak {
+                    scale: 0.8,            // make it smaller
+                    y_offset_factor: -0.2, // move it up
+                    y_offset: 0.0,
+                },
+            ),
         );
 
         // Bigger emojis, and more. <http://jslegers.github.io/emoji-icon-font/>:
         font_data.insert(
             "emoji-icon-font".to_owned(),
             FontData::from_static(include_bytes!("../../fonts/emoji-icon-font.ttf")).tweak(
                 FontTweak {
-                    scale: 0.8,            // make it smaller
+                    scale: 0.9,            // make it smaller
                     y_offset_factor: 0.07, // move it down slightly
                     y_offset: 0.0,
                 },
@@ -526,6 +531,21 @@ impl FontsAndCache {
 
 // ----------------------------------------------------------------------------
 
+#[derive(Clone, Copy, Debug, PartialEq)]
+struct HashableF32(f32);
+
+#[allow(clippy::derive_hash_xor_eq)]
+impl std::hash::Hash for HashableF32 {
+    #[inline(always)]
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        crate::f32_hash(state, self.0);
+    }
+}
+
+impl Eq for HashableF32 {}
+
+// ----------------------------------------------------------------------------
+
 /// The collection of fonts used by `epaint`.
 ///
 /// Required in order to paint text.
@@ -535,7 +555,7 @@ pub struct FontsImpl {
     definitions: FontDefinitions,
     atlas: Arc<Mutex<TextureAtlas>>,
     font_impl_cache: FontImplCache,
-    sized_family: ahash::HashMap<(u32, FontFamily), Font>,
+    sized_family: ahash::HashMap<(HashableF32, FontFamily), Font>,
 }
 
 impl FontsImpl {
@@ -584,10 +604,9 @@ impl FontsImpl {
     /// Get the right font implementation from size and [`FontFamily`].
     pub fn font(&mut self, font_id: &FontId) -> &mut Font {
         let FontId { size, family } = font_id;
-        let scale_in_pixels = self.font_impl_cache.scale_as_pixels(*size);
 
         self.sized_family
-            .entry((scale_in_pixels, family.clone()))
+            .entry((HashableF32(*size), family.clone()))
             .or_insert_with(|| {
                 let fonts = &self.definitions.families.get(family);
                 let fonts = fonts.unwrap_or_else(|| {
@@ -596,7 +615,7 @@ impl FontsImpl {
 
                 let fonts: Vec<Arc<FontImpl>> = fonts
                     .iter()
-                    .map(|font_name| self.font_impl_cache.font_impl(scale_in_pixels, font_name))
+                    .map(|font_name| self.font_impl_cache.font_impl(*size, font_name))
                     .collect();
 
                 Font::new(fonts)
@@ -699,23 +718,33 @@ impl FontImplCache {
         }
     }
 
-    #[inline]
-    pub fn scale_as_pixels(&self, scale_in_points: f32) -> u32 {
-        let scale_in_pixels = self.pixels_per_point * scale_in_points;
-
-        // Round to an even number of physical pixels to get even kerning.
-        // See https://github.com/emilk/egui/issues/382
-        scale_in_pixels.round() as u32
-    }
+    pub fn font_impl(&mut self, scale_in_points: f32, font_name: &str) -> Arc<FontImpl> {
+        use ab_glyph::Font as _;
 
-    pub fn font_impl(&mut self, scale_in_pixels: u32, font_name: &str) -> Arc<FontImpl> {
         let (tweak, ab_glyph_font) = self
             .ab_glyph_fonts
             .get(font_name)
             .unwrap_or_else(|| panic!("No font data found for {:?}", font_name))
             .clone();
 
-        let scale_in_pixels = (scale_in_pixels as f32 * tweak.scale).round() as u32;
+        let scale_in_pixels = self.pixels_per_point * scale_in_points;
+
+        // Scale the font properly (see https://github.com/emilk/egui/issues/2068).
+        let units_per_em = ab_glyph_font.units_per_em().unwrap_or_else(|| {
+            panic!(
+                "The font unit size of {:?} exceeds the expected range (16..=16384)",
+                font_name
+            )
+        });
+        let font_scaling = ab_glyph_font.height_unscaled() / units_per_em;
+        let scale_in_pixels = scale_in_pixels * font_scaling;
+
+        // Tweak the scale as the user desired:
+        let scale_in_pixels = scale_in_pixels * tweak.scale;
+
+        // Round to an even number of physical pixels to get even kerning.
+        // See https://github.com/emilk/egui/issues/382
+        let scale_in_pixels = scale_in_pixels.round() as u32;
 
         let y_offset_points = {
             let scale_in_points = scale_in_pixels as f32 / self.pixels_per_point;