Try to keep font atlas texture below 1024x1024 to keep support for Wi…

…ndows GDI OpenGL renderer

src/openboardview/main_opengl.cpp

@@ -311,6 +311,18 @@ int main(int argc, char **argv) {
 
 	if (!customFont.empty()) fontList.push_front(customFont);
 
+	// Try to keep atlas size below 1024x1024 pixels (max texture size on e.g. Windows GDI)
+	io.Fonts->TexDesiredWidth = 1024;
+	// Max size is 76px for a single font with current glyph range (192 glyphs) and default oversampling (h*3, v*1), could break in future ImGui updates
+	// Different fonts can also overflow, e.g. Arial and Helvetica work but Algerian does not
+	// Expected max g.font_size is around 50, limit set in BoardView.cpp for Preferences panel, user can manually set higher value in config file but could break
+	// Rough estimate with some margin (72 instead of 76), assuming small font is 2 times smaller, max size for larger font is
+	double maxLargeFontSizeSquared = 72.0 * 72.0 - g.font_size * g.font_size - (g.font_size / 2.0) * (g.font_size / 2.0);
+	if (maxLargeFontSizeSquared < 1.0) {
+		maxLargeFontSizeSquared = 1.0;
+	}
+	double largeFontScaleFactor = std::min(8.0, std::sqrt(maxLargeFontSizeSquared) / g.font_size); // Find max scale factor for large font, use at most 8 times larger font
+
 	for (const auto &name : fontList) {
 		app.obvconfig.WriteStr("fontName", name.c_str());
 #ifdef _WIN32
@@ -321,17 +333,18 @@ int main(int argc, char **argv) {
 			io.Fonts->AddFontFromMemoryTTF(
 			    const_cast<void *>(reinterpret_cast<const void *>(ttf.data())), ttf.size(), g.font_size, &font_cfg);
 			io.Fonts->AddFontFromMemoryTTF(
-			    const_cast<void *>(reinterpret_cast<const void *>(ttf.data())), ttf.size(), g.font_size * 8, &font_cfg); // Larger font for resizeable (zoomed) text
+			    const_cast<void *>(reinterpret_cast<const void *>(ttf.data())), ttf.size(), g.font_size * largeFontScaleFactor, &font_cfg); // Larger font for resizeable (zoomed) text
 			io.Fonts->AddFontFromMemoryTTF(
 			    const_cast<void *>(reinterpret_cast<const void *>(ttf.data())), ttf.size(), g.font_size / 2, &font_cfg); // Smaller font for resizeable (zoomed) text
 			break;
 		}
+
 #else
 		const std::string fontpath = get_font_path(name);
 		if (fontpath.empty()) continue;        // Font not found
 		if (check_fileext(fontpath, ".ttf")) { // ImGui handles only TrueType fonts so exclude anything which has a different ext
 			io.Fonts->AddFontFromFileTTF(fontpath.c_str(), g.font_size);
-			io.Fonts->AddFontFromFileTTF(fontpath.c_str(), g.font_size * 8); // Larger font for resizeable (zoomed) text
+			io.Fonts->AddFontFromFileTTF(fontpath.c_str(), g.font_size * largeFontScaleFactor); // Larger font for resizeable (zoomed) text
 			io.Fonts->AddFontFromFileTTF(fontpath.c_str(), g.font_size / 2); // Smaller font for resizeable (zoomed) text
 			break;
 		}