Prevent unwanted font face changes

[Gliese852]

Before PR -> After PR

When a character appears in the text, that is not in the default font face, it is looked up in the spare faces of the font. Moreover, the default range also began to be baked from the spare face. I made that only characters that are not in the default face are baked from the spare face.

To visually see what is this all about, start a new game and click on Toliman in the sector map. In it's description there is an "α" (alpha) symbol, this leads to a complete change of the font face. This PR fixes this issue.

Fixes #4807

[Gliese852]

I think it's worth explaining in more detail the logic of what is happening there.

The font contains several faces:

pioneer/src/pigui/PiGui.cpp

Lines 131 to 135 in 97f0641

	PiFont guifont("pionillium", {
	PiFace("DejaVuSans.ttf", 13.0 / 14.0),
	PiFace("wqy-microhei.ttc", 1.0),
	PiFace("PionilliumText22L-Medium.ttf", 1.0)
	});

it can be seen that the main face is at the end of the vector. How does it become main? A range is added to it (when a glyph is added, a range is created for it if it does not fall into an existing one, and in the beginning we don't have any ranges at all)

pioneer/src/pigui/PiGui.cpp

Line 202 in 97f0641

pifont.faces().back().addGlyph(0x20); // only add space

In the next function, the font texture is created, apparently

pioneer/src/pigui/PiGui.cpp

Lines 347 to 386 in 97f0641

	void Instance::BakeFont(PiFont &font)
	{
	PROFILE_SCOPED()
	ImGuiIO &io = ImGui::GetIO();
	ImFont *imfont = nullptr;
	for (PiFace &face : font.faces()) {
	ImFontConfig config;
	config.MergeMode = true;
	float size = font.pixelsize() * face.sizefactor();
	const std::string path = FileSystem::JoinPath(FileSystem::JoinPath(FileSystem::GetDataDir(), "fonts"), face.ttfname());
	// Output("- baking face %s at size %f\n", path.c_str(), size);
	face.sortUsedRanges();
	if (face.used_ranges().size() > 0) {
	face.m_imgui_ranges.clear();
	ImFontAtlas::GlyphRangesBuilder gb;
	// Always include the default range
	gb.AddRanges(io.Fonts->GetGlyphRangesDefault());
	ImWchar gr[3] = { 0, 0, 0 };
	for (auto &range : face.used_ranges()) {
	// Output("Used range: %x - %x", range.first, range.second);
	gr[0] = range.first;
	gr[1] = range.second;
	gb.AddRanges(gr);
	}
	gb.BuildRanges(&face.m_imgui_ranges);
	ImFont *f = io.Fonts->AddFontFromFileTTF(path.c_str(), size, imfont == nullptr ? nullptr : &config, face.m_imgui_ranges.Data);
	assert(f);
	if (imfont != nullptr)
	assert(f == imfont);
	imfont = f;
	}
	}
	m_im_fonts[imfont] = std::make_pair(font.name(), font.pixelsize());
	// Output("setting %s %i to %p\n", font.name(), font.pixelsize(), imfont);
	m_fonts[std::make_pair(font.name(), font.pixelsize())] = imfont;
	if (!imfont->MissingGlyphs.empty()) {
	Output("WARNING: glyphs missing in shiny new font\n");
	}
	imfont->MissingGlyphs.clear();
	}

It iterates over all faces of the font, in turn (note that the main face comes last)

pioneer/src/pigui/PiGui.cpp

Line 352 in 97f0641

for (PiFace &face : font.faces()) {

but it ignores faces without ranges, and in the beginning we have a range only for the main face.

pioneer/src/pigui/PiGui.cpp

Line 359 in 97f0641

if (face.used_ranges().size() > 0) {

А default range is added to each iterated face

pioneer/src/pigui/PiGui.cpp

Line 363 in 97f0641

gb.AddRanges(io.Fonts->GetGlyphRangesDefault());

that's what this range is

pioneer/contrib/imgui/imgui_draw.cpp

Lines 2225 to 2234 in 97f0641

	// Retrieve list of range (2 int per range, values are inclusive)
	const ImWchar* ImFontAtlas::GetGlyphRangesDefault()
	{
	static const ImWchar ranges[] =
	{
	0x0020, 0x00FF, // Basic Latin + Latin Supplement
	0,
	};
	return &ranges[0];
	}

If after rendering, the dear ImGui lacks a glyph, this function is called (I omitted the details, there are a lot of calls, as I understand it)

pioneer/src/pigui/PiGui.cpp

Lines 161 to 184 in 97f0641

	void Instance::AddGlyph(ImFont *font, unsigned short glyph)
	{
	PROFILE_SCOPED()
	// range glyph..glyph
	auto iter = m_im_fonts.find(font);
	if (iter == m_im_fonts.end()) {
	Error("Cannot find font instance for ImFont %p\n", font);
	assert(false);
	}
	auto pifont_iter = m_pi_fonts.find(iter->second);
	if (pifont_iter == m_pi_fonts.end()) {
	Error("No registered PiFont for name %s size %i\n", iter->second.first.c_str(), iter->second.second);
	assert(false);
	}
	PiFont &pifont = pifont_iter->second;
	for (PiFace &face : pifont.faces()) {
	if (face.isValidGlyph(glyph)) {
	face.addGlyph(glyph);
	m_should_bake_fonts = true;
	return;
	}
	}
	Error("No face in font %s handles glyph %i\n", pifont.name().c_str(), glyph);
	}

It checks all faces of the font for the presence of a given glyph, and also sets the flag that fonts need to be rebuilt

pioneer/src/pigui/PiGui.cpp

Lines 176 to 181 in 97f0641

	for (PiFace &face : pifont.faces()) {
	if (face.isValidGlyph(glyph)) {
	face.addGlyph(glyph);
	m_should_bake_fonts = true;
	return;
	}

Аnd now we have a range not only in the main face, but also in some other. When baking, it will go before the main face, and now the default range will be added to it. When it comes to the main face, the range will already be occupied.

Therefore, I simply rearranged the main face to the top of the list, and the default range will always be taken from it, and everything that cannot be found in it will be taken from other faces.

[sturnclaw]

@Gliese852 can I ask you to please transcribe that excellent write-up to function comments? The overall architecture is something I've had to reverse-engineer myself when doing imgui upgrades, and I'd really really appreciate having this overview as function comments so future maintainers (and future me!) don't have to recreate it.

[Gliese852]

@Web-eWorks Done, tried to adapt this information, because in this topic it is written how it works now, and in the comments in the code, I wrote how it will work after the patch.