From 417bb3123af9b53ad30442d17641460f239718a4 Mon Sep 17 00:00:00 2001
From: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
Date: Wed, 26 Jan 2022 14:42:44 +0100
Subject: [PATCH] Fonts: only pre-render glyphs for fonts in the current egui
 styles

---
 egui/src/context.rs     |  8 ++++++++
 egui/src/memory.rs      | 21 ++++++++++++++++++++-
 epaint/src/text/font.rs | 12 +++++++-----
 3 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/egui/src/context.rs b/egui/src/context.rs
index d7701ead..c997ea7d 100644
--- a/egui/src/context.rs
+++ b/egui/src/context.rs
@@ -92,6 +92,14 @@ impl ContextImpl {
         });
 
         fonts.begin_frame(pixels_per_point, max_texture_side);
+
+        if self.memory.options.preload_font_glyphs {
+            // Preload the most common characters for the most common fonts.
+            // This is not very important to do, but may a few GPU operations.
+            for font_id in self.memory.options.style.text_styles.values() {
+                fonts.lock().fonts.font(font_id).preload_common_characters();
+            }
+        }
     }
 }
 
diff --git a/egui/src/memory.rs b/egui/src/memory.rs
index 3e683a0b..4afdfeb8 100644
--- a/egui/src/memory.rs
+++ b/egui/src/memory.rs
@@ -93,7 +93,7 @@ pub struct Memory {
 // ----------------------------------------------------------------------------
 
 /// Some global options that you can read and write.
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "serde", serde(default))]
 pub struct Options {
@@ -108,6 +108,25 @@ pub struct Options {
     /// but is a signal to any backend that we want the [`crate::Output::events`] read out loud.
     /// Screen readers is an experimental feature of egui, and not supported on all platforms.
     pub screen_reader: bool,
+
+    /// If true, the most common glyphs (ASCII) are pre-rendered to the texture atlas.
+    ///
+    /// Only the fonts in [`Style::text_styles`] will be pre-cached.
+    ///
+    /// This can lead to fewer texture operations, but may use up the texture atlas quicker
+    /// if you are changing [`Style::text_styles`], of have a lot of text styles.
+    pub preload_font_glyphs: bool,
+}
+
+impl Default for Options {
+    fn default() -> Self {
+        Self {
+            style: Default::default(),
+            tessellation_options: Default::default(),
+            screen_reader: false,
+            preload_font_glyphs: true,
+        }
+    }
 }
 
 // ----------------------------------------------------------------------------
diff --git a/epaint/src/text/font.rs b/epaint/src/text/font.rs
index aab77be1..1dca6030 100644
--- a/epaint/src/text/font.rs
+++ b/epaint/src/text/font.rs
@@ -242,16 +242,18 @@ impl Font {
             });
         slf.replacement_glyph = replacement_glyph;
 
+        slf
+    }
+
+    pub fn preload_common_characters(&mut self) {
         // Preload the printable ASCII characters [32, 126] (which excludes control codes):
         const FIRST_ASCII: usize = 32; // 32 == space
         const LAST_ASCII: usize = 126;
         for c in (FIRST_ASCII..=LAST_ASCII).map(|c| c as u8 as char) {
-            slf.glyph_info(c);
+            self.glyph_info(c);
         }
-        slf.glyph_info('°');
-        slf.glyph_info(crate::text::PASSWORD_REPLACEMENT_CHAR);
-
-        slf
+        self.glyph_info('°');
+        self.glyph_info(crate::text::PASSWORD_REPLACEMENT_CHAR);
     }
 
     /// All supported characters