From a9cf934b5f86c49b86647a7ae7be8dc317165dc9 Mon Sep 17 00:00:00 2001
From: Michael Ragazzon <michael.ragazzon@gmail.com>
Date: Sat, 6 Jul 2024 22:01:50 +0200
Subject: [PATCH] Font optimization: Store whether font size has been set
 during kerning queries

Helps speed-up layout with non-ascii text especially.
---
 .../FontFaceHandleDefault.cpp                 | 22 ++++++++++++-------
 .../FontEngineDefault/FontFaceHandleDefault.h |  2 +-
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/Source/Core/FontEngineDefault/FontFaceHandleDefault.cpp b/Source/Core/FontEngineDefault/FontFaceHandleDefault.cpp
index 4e08723f0..b9867e67e 100644
--- a/Source/Core/FontEngineDefault/FontFaceHandleDefault.cpp
+++ b/Source/Core/FontEngineDefault/FontFaceHandleDefault.cpp
@@ -87,6 +87,7 @@ int FontFaceHandleDefault::GetStringWidth(const String& string, float letter_spa
 {
 	RMLUI_ZoneScoped;
 
+	bool has_set_size = false;
 	int width = 0;
 	for (auto it_string = StringIteratorU8(string); it_string; ++it_string)
 	{
@@ -97,7 +98,7 @@ int FontFaceHandleDefault::GetStringWidth(const String& string, float letter_spa
 			continue;
 
 		// Adjust the cursor for the kerning between this character and the previous one.
-		width += GetKerning(prior_character, character);
+		width += GetKerning(prior_character, character, has_set_size);
 
 		// Adjust the cursor for this character's advance.
 		width += glyph->advance;
@@ -195,12 +196,13 @@ bool FontFaceHandleDefault::GenerateLayerTexture(Vector<byte>& texture_data, Vec
 int FontFaceHandleDefault::GenerateString(RenderManager& render_manager, TexturedMeshList& mesh_list, const String& string, const Vector2f position,
 	const ColourbPremultiplied colour, const float opacity, const float letter_spacing, const int layer_configuration_index)
 {
-	int geometry_index = 0;
-	int line_width = 0;
-
 	RMLUI_ASSERT(layer_configuration_index >= 0);
 	RMLUI_ASSERT(layer_configuration_index < (int)layer_configurations.size());
 
+	int geometry_index = 0;
+	int line_width = 0;
+	bool has_set_size = false;
+
 	UpdateLayersOnDirty();
 
 	// Fetch the requested configuration and generate the geometry for each one.
@@ -247,7 +249,7 @@ int FontFaceHandleDefault::GenerateString(RenderManager& render_manager, Texture
 				continue;
 
 			// Adjust the cursor for the kerning between this character and the previous one.
-			line_width += GetKerning(prior_character, character);
+			line_width += GetKerning(prior_character, character, has_set_size);
 
 			ColourbPremultiplied glyph_color = layer_colour;
 			// Use white vertex colors on RGB glyphs.
@@ -323,11 +325,11 @@ void FontFaceHandleDefault::FillKerningPairCache()
 	}
 }
 
-int FontFaceHandleDefault::GetKerning(Character lhs, Character rhs) const
+int FontFaceHandleDefault::GetKerning(Character lhs, Character rhs, bool& has_set_size) const
 {
 	static_assert(' ' == 32, "Only ASCII/UTF8 character set supported.");
 
-	// Check if we have no kerning, or if we are have a control character.
+	// Check if we have no kerning, or if we have a control character.
 	if (!has_kerning || char32_t(lhs) < ' ' || char32_t(rhs) < ' ')
 		return 0;
 
@@ -348,7 +350,11 @@ int FontFaceHandleDefault::GetKerning(Character lhs, Character rhs) const
 	}
 
 	// Fetch it from the font face instead.
-	const int result = FreeType::GetKerning(ft_face, metrics.size, lhs, rhs);
+	const int result = FreeType::GetKerning(ft_face, has_set_size ? 0 : metrics.size, lhs, rhs);
+
+	// This is purely an optimization to avoid repeatedly setting the font size in FreeType, which can be a measurable performance hit.
+	has_set_size = true;
+
 	return result;
 }
 
diff --git a/Source/Core/FontEngineDefault/FontFaceHandleDefault.h b/Source/Core/FontEngineDefault/FontFaceHandleDefault.h
index 33b797e42..a5972fc54 100644
--- a/Source/Core/FontEngineDefault/FontFaceHandleDefault.h
+++ b/Source/Core/FontEngineDefault/FontFaceHandleDefault.h
@@ -100,7 +100,7 @@ class FontFaceHandleDefault final : public NonCopyMoveable {
 	void FillKerningPairCache();
 
 	// Return the kerning for a character pair.
-	int GetKerning(Character lhs, Character rhs) const;
+	int GetKerning(Character lhs, Character rhs, bool& has_set_size) const;
 
 	/// Retrieve a glyph from the given code point, building and appending a new glyph if not already built.
 	/// @param[in-out] character  The character, can be changed e.g. to the replacement character if no glyph is found.