feat: implement optimized text rendering by caching font characters into special atlas

src/texture/mod.rs

@@ -2,3 +2,4 @@ pub mod animated;
 pub mod blinking;
 pub mod sprite;
 pub mod text;
+pub mod ttf;

src/texture/ttf.rs

@@ -0,0 +1,272 @@
+//! TTF font rendering using pre-rendered character atlas.
+//!
+//! This module provides efficient TTF font rendering by pre-rendering all needed
+//! characters into a texture atlas at startup, avoiding expensive SDL2 font
+//! surface-to-texture conversions every frame.
+
+use glam::{UVec2, Vec2};
+use sdl2::pixels::Color;
+use sdl2::rect::Rect;
+use sdl2::render::{Canvas, RenderTarget, Texture, TextureCreator};
+
+use sdl2::ttf::Font;
+use sdl2::video::WindowContext;
+use std::collections::HashMap;
+
+use crate::error::{GameError, TextureError};
+
+/// Character atlas tile representing a single rendered character
+#[derive(Clone, Copy, Debug)]
+pub struct TtfCharTile {
+    pub pos: UVec2,
+    pub size: UVec2,
+    pub advance: u32, // Character advance width for proportional fonts
+}
+
+/// TTF text atlas containing pre-rendered characters for efficient rendering
+pub struct TtfAtlas {
+    /// The texture containing all rendered characters
+    texture: Texture,
+    /// Mapping from character to its position and size in the atlas
+    char_tiles: HashMap<char, TtfCharTile>,
+    /// Cached color modulation state to avoid redundant SDL2 calls
+    last_modulation: Option<Color>,
+}
+
+const TTF_CHARS: &str = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz.,:-/()ms μµ%± ";
+
+impl TtfAtlas {
+    /// Creates a new TTF text atlas by pre-rendering all needed characters.
+    ///
+    /// This should be called once at startup. It renders all characters that might
+    /// be used in text rendering into a single texture atlas for efficient GPU rendering.
+    pub fn new(texture_creator: &TextureCreator<WindowContext>, font: &Font) -> Result<Self, GameError> {
+        // Calculate character dimensions and advance widths for proportional fonts
+        let mut char_tiles = HashMap::new();
+        let mut max_height = 0u32;
+        let mut total_width = 0u32;
+        let mut char_metrics = Vec::new();
+
+        // First pass: measure all characters
+        for c in TTF_CHARS.chars() {
+            if c == ' ' {
+                // Handle space character specially - measure a non-space character for height
+                let space_height = font.size_of("0").map_err(|e| GameError::Sdl(e.to_string()))?.1;
+                let space_advance = font.size_of(" ").map_err(|e| GameError::Sdl(e.to_string()))?.0;
+                char_tiles.insert(
+                    c,
+                    TtfCharTile {
+                        pos: UVec2::ZERO,                  // Will be set during population
+                        size: UVec2::new(0, space_height), // Space has no visual content
+                        advance: space_advance,
+                    },
+                );
+                max_height = max_height.max(space_height);
+                char_metrics.push((c, 0, space_height, space_advance));
+            } else {
+                let (advance, height) = font.size_of(&c.to_string()).map_err(|e| GameError::Sdl(e.to_string()))?;
+                char_tiles.insert(
+                    c,
+                    TtfCharTile {
+                        pos: UVec2::ZERO, // Will be set during population
+                        size: UVec2::new(advance, height),
+                        advance,
+                    },
+                );
+                max_height = max_height.max(height);
+                total_width += advance;
+                char_metrics.push((c, advance, height, advance));
+            }
+        }
+
+        // Calculate atlas dimensions (pack characters horizontally for better space utilization)
+        let atlas_size = UVec2::new(total_width, max_height);
+
+        // Create atlas texture as a render target
+        let mut atlas_texture = texture_creator
+            .create_texture_target(None, atlas_size.x, atlas_size.y)
+            .map_err(|e| GameError::Sdl(e.to_string()))?;
+        atlas_texture.set_blend_mode(sdl2::render::BlendMode::Blend);
+
+        // Second pass: calculate positions
+        let mut current_x = 0u32;
+        for (c, width, _height, _advance) in char_metrics {
+            if let Some(tile) = char_tiles.get_mut(&c) {
+                tile.pos = UVec2::new(current_x, 0);
+                current_x += width;
+            }
+        }
+
+        Ok(Self {
+            texture: atlas_texture,
+            char_tiles,
+            last_modulation: None,
+        })
+    }
+
+    /// Renders all characters to the atlas texture using a canvas.
+    /// This must be called after creation to populate the atlas.
+    pub fn populate_atlas<C: RenderTarget>(
+        &mut self,
+        canvas: &mut Canvas<C>,
+        texture_creator: &TextureCreator<WindowContext>,
+        font: &Font,
+    ) -> Result<(), GameError> {
+        let mut render_error: Option<GameError> = None;
+
+        let result = canvas.with_texture_canvas(&mut self.texture, |atlas_canvas| {
+            // Clear with transparent background
+            atlas_canvas.set_draw_color(Color::RGBA(0, 0, 0, 0));
+            atlas_canvas.clear();
+
+            for c in TTF_CHARS.chars() {
+                if c == ' ' {
+                    // Skip rendering space character - it has no visual content
+                    continue;
+                }
+
+                // Render character to surface
+                let surface = match font.render(&c.to_string()).blended(Color::WHITE) {
+                    Ok(s) => s,
+                    Err(e) => {
+                        render_error = Some(GameError::Sdl(e.to_string()));
+                        return;
+                    }
+                };
+
+                // Create texture from surface
+                let char_texture = match texture_creator.create_texture_from_surface(&surface) {
+                    Ok(t) => t,
+                    Err(e) => {
+                        render_error = Some(GameError::Sdl(e.to_string()));
+                        return;
+                    }
+                };
+
+                // Get character tile info
+                let tile = match self.char_tiles.get(&c) {
+                    Some(t) => t,
+                    None => {
+                        render_error = Some(GameError::Sdl(format!("Character '{}' not found in atlas tiles", c)));
+                        return;
+                    }
+                };
+
+                // Copy character to atlas
+                let dest = Rect::new(tile.pos.x as i32, tile.pos.y as i32, tile.size.x, tile.size.y);
+                if let Err(e) = atlas_canvas.copy(&char_texture, None, dest) {
+                    render_error = Some(GameError::Sdl(e.to_string()));
+                    return;
+                }
+            }
+        });
+
+        // Check the result of with_texture_canvas and any render error
+        if let Err(e) = result {
+            return Err(GameError::Sdl(e.to_string()));
+        }
+
+        if let Some(error) = render_error {
+            return Err(error);
+        }
+
+        Ok(())
+    }
+
+    /// Gets a character tile from the atlas
+    pub fn get_char_tile(&self, c: char) -> Option<&TtfCharTile> {
+        self.char_tiles.get(&c)
+    }
+}
+
+/// TTF text renderer that uses the pre-rendered character atlas
+pub struct TtfRenderer {
+    scale: f32,
+}
+
+impl TtfRenderer {
+    pub fn new(scale: f32) -> Self {
+        Self { scale }
+    }
+
+    /// Renders a string of text at the given position with the specified color
+    pub fn render_text<C: RenderTarget>(
+        &self,
+        canvas: &mut Canvas<C>,
+        atlas: &mut TtfAtlas,
+        text: &str,
+        position: Vec2,
+        color: Color,
+    ) -> Result<(), TextureError> {
+        let mut x_offset = 0.0;
+
+        // Apply color modulation once at the beginning if needed
+        if atlas.last_modulation != Some(color) {
+            atlas.texture.set_color_mod(color.r, color.g, color.b);
+            atlas.texture.set_alpha_mod(color.a);
+            atlas.last_modulation = Some(color);
+        }
+
+        for c in text.chars() {
+            // Get character tile info first to avoid borrowing conflicts
+            let char_tile = atlas.get_char_tile(c);
+
+            if let Some(char_tile) = char_tile {
+                if char_tile.size.x > 0 && char_tile.size.y > 0 {
+                    // Only render non-space characters
+                    let dest = Rect::new(
+                        (position.x + x_offset) as i32,
+                        position.y as i32,
+                        (char_tile.size.x as f32 * self.scale) as u32,
+                        (char_tile.size.y as f32 * self.scale) as u32,
+                    );
+
+                    // Render the character directly
+                    let src = Rect::new(
+                        char_tile.pos.x as i32,
+                        char_tile.pos.y as i32,
+                        char_tile.size.x,
+                        char_tile.size.y,
+                    );
+                    canvas.copy(&atlas.texture, src, dest).map_err(TextureError::RenderFailed)?;
+                }
+
+                // Advance by character advance width (proportional spacing)
+                x_offset += char_tile.advance as f32 * self.scale;
+            } else {
+                // Fallback for unsupported characters - use a reasonable default
+                x_offset += 8.0 * self.scale;
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Calculate the width of a text string in pixels
+    pub fn text_width(&self, atlas: &TtfAtlas, text: &str) -> u32 {
+        let mut total_width = 0u32;
+
+        for c in text.chars() {
+            if let Some(char_tile) = atlas.get_char_tile(c) {
+                total_width += (char_tile.advance as f32 * self.scale) as u32;
+            } else {
+                // Fallback for unsupported characters
+                total_width += (8.0 * self.scale) as u32;
+            }
+        }
+
+        total_width
+    }
+
+    /// Calculate the height of text in pixels
+    pub fn text_height(&self, atlas: &TtfAtlas) -> u32 {
+        // Find the maximum height among all characters
+        atlas
+            .char_tiles
+            .values()
+            .map(|tile| tile.size.y)
+            .max()
+            .unwrap_or(0)
+            .saturating_mul(self.scale as u32)
+    }
+}