Kerning support (#588)

Some fonts look a bit weird if you don't do kerning.

@corwinkuiper can you check if I've done the correct thing for object
font rendering? I'm not entirely sure... Although it does render
correctly in my tests :D

- [x] Changelog updated / no changelog update needed

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Initial unicode support for font rendering.
+- Kerning support for font rendering.
 
 ### Fixed
 

agb-image-converter/src/font_loader.rs

@@ -3,6 +3,11 @@ use quote::quote;
 
 use proc_macro2::TokenStream;
 
+struct KerningData {
+    previous_character: char,
+    amount: f32,
+}
+
 struct LetterData {
     character: char,
     width: usize,
@@ -11,6 +16,7 @@ struct LetterData {
     ymin: i32,
     advance_width: f32,
     rendered: Vec<u8>,
+    kerning_data: Vec<KerningData>,
 }
 
 pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
@@ -31,8 +37,8 @@ pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
     let mut letters: Vec<_> = font
         .chars()
         .iter()
-        .map(|(&c, _)| (c, font.rasterize(c, pixels_per_em)))
-        .map(|(c, (metrics, bitmap))| {
+        .map(|(&c, &index)| (c, index, font.rasterize(c, pixels_per_em)))
+        .map(|(c, index, (metrics, bitmap))| {
             let width = metrics.width;
             let height = metrics.height;
 
@@ -50,6 +56,25 @@ pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
                 })
                 .collect();
 
+            let mut kerning_data: Vec<_> = font
+                .chars()
+                .iter()
+                .filter_map(|(&left_char, &left_index)| {
+                    let kerning = font.horizontal_kern_indexed(
+                        left_index.into(),
+                        index.into(),
+                        pixels_per_em,
+                    )?;
+
+                    Some(KerningData {
+                        previous_character: left_char,
+                        amount: kerning,
+                    })
+                })
+                .collect();
+
+            kerning_data.sort_unstable_by_key(|kd| kd.previous_character);
+
             LetterData {
                 character: c,
                 width,
@@ -58,6 +83,7 @@ pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
                 xmin: metrics.xmin,
                 ymin: metrics.ymin,
                 advance_width: metrics.advance_width,
+                kerning_data,
             }
         })
         .collect();
@@ -82,6 +108,13 @@ pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
         let xmin = letter_data.xmin as i8;
         let ymin = letter_data.ymin as i8;
         let advance_width = letter_data.advance_width.ceil() as u8;
+        let kerning_amounts = letter_data.kerning_data.iter().map(|kerning_data| {
+            let amount = kerning_data.amount as i8;
+            let c = kerning_data.previous_character;
+            quote! {
+                (#c, #amount)
+            }
+        });
 
         quote!(
             display::FontLetter::new(
@@ -92,6 +125,9 @@ pub fn load_font(font_data: &[u8], pixels_per_em: f32) -> TokenStream {
                 #xmin,
                 #ymin,
                 #advance_width,
+                &[
+                    #(#kerning_amounts),*
+                ]
             )
         )
     });

agb/src/display/font.rs

@@ -17,10 +17,12 @@ pub struct FontLetter {
     pub(crate) xmin: i8,
     pub(crate) ymin: i8,
     pub(crate) advance_width: u8,
+    kerning_amounts: &'static [(char, i8)],
 }
 
 impl FontLetter {
     #[must_use]
+    #[allow(clippy::too_many_arguments)] // only used in macro
     pub const fn new(
         character: char,
         width: u8,
@@ -29,6 +31,7 @@ impl FontLetter {
         xmin: i8,
         ymin: i8,
         advance_width: u8,
+        kerning_amounts: &'static [(char, i8)],
     ) -> Self {
         Self {
             character,
@@ -38,6 +41,7 @@ impl FontLetter {
             xmin,
             ymin,
             advance_width,
+            kerning_amounts,
         }
     }
 
@@ -47,6 +51,17 @@ impl FontLetter {
         let bit = position % 8;
         ((byte >> bit) & 1) != 0
     }
+
+    pub(crate) fn kerning_amount(&self, previous_char: char) -> i32 {
+        if let Ok(index) = self
+            .kerning_amounts
+            .binary_search_by_key(&previous_char, |kerning_data| kerning_data.0)
+        {
+            self.kerning_amounts[index].1 as i32
+        } else {
+            0
+        }
+    }
 }
 
 pub struct Font {
@@ -92,6 +107,7 @@ impl Font {
         TextRenderer {
             current_x_pos: 0,
             current_y_pos: 0,
+            previous_character: None,
             font: self,
             tile_pos: tile_pos.into(),
             tiles: Default::default(),
@@ -103,6 +119,7 @@ impl Font {
 pub struct TextRenderer<'a> {
     current_x_pos: i32,
     current_y_pos: i32,
+    previous_character: Option<char>,
     font: &'a Font,
     tile_pos: Vector2D<u16>,
     tiles: HashMap<(i32, i32), DynamicTile<'a>>,
@@ -258,6 +275,12 @@ impl<'a, 'b> TextRenderer<'b> {
             self.current_x_pos = 0;
         } else {
             let letter = self.font.letter(c);
+
+            if let Some(previous_character) = self.previous_character {
+                self.current_x_pos += letter.kerning_amount(previous_character);
+            }
+            self.previous_character = Some(c);
+
             self.render_letter(letter, vram_manager, foreground_colour, background_colour);
             self.current_x_pos += i32::from(letter.advance_width);
         }

agb/src/display/object/font/preprocess.rs

@@ -44,6 +44,7 @@ pub(crate) struct Preprocessed {
 
 #[derive(Debug, Default)]
 struct Preprocessor {
+    previous_character: Option<char>,
     width_in_sprite: i32,
 }
 
@@ -72,6 +73,10 @@ impl Preprocessor {
             }
             letter => {
                 let letter = font.letter(letter);
+                if let Some(previous_character) = self.previous_character {
+                    self.width_in_sprite += letter.kerning_amount(previous_character);
+                }
+
                 if self.width_in_sprite + letter.width as i32 > sprite_width {
                     widths.push_back(
                         PreprocessedElement::LetterGroup {
@@ -87,6 +92,8 @@ impl Preprocessor {
                 self.width_in_sprite += letter.advance_width as i32;
             }
         }
+
+        self.previous_character = Some(character);
     }
 }
 

agb/src/display/object/font/renderer.rs

@@ -43,6 +43,8 @@ pub(crate) struct WordRender {
     working: WorkingLetter,
     config: Configuration,
     colour: usize,
+
+    previous_character: Option<char>,
 }
 
 impl WordRender {
@@ -56,6 +58,7 @@ impl WordRender {
             working: WorkingLetter::new(config.sprite_size),
             config,
             colour: 1,
+            previous_character: None,
         }
     }
 
@@ -82,6 +85,11 @@ impl WordRender {
 
         let font_letter: &crate::display::FontLetter = font.letter(c);
 
+        if let Some(previous_character) = self.previous_character {
+            self.working.x_offset += font_letter.kerning_amount(previous_character);
+        }
+        self.previous_character = Some(c);
+
         // uses more than the sprite can hold
         let group = if self.working.x_offset + font_letter.width as i32
             > self.config.sprite_size.to_width_height().0 as i32