Text rendering#

YouTube titles, channel names and comments arrive in every script, often mixed in one string (an Arabic title with Latin words and digits, a Korean title with emoji). Dear ImGui draws one glyph per code point, left to right, so SlopTube adds font fallback, Arabic shaping and bidirectional reordering on top of it. Glyphs are rasterised by FreeType (imgui_freetype). The code is in src/app/ui/Fonts.* and src/app/ui/text/.

Fonts (ui/Fonts.cpp)#

loadFonts() builds one face per Weight (Regular, Medium, SemiBold, Bold: Roboto's 400, 500, 600 and 700, the weights the official app uses), each with the same chain of fallbacks merged in:

1. Roboto (the system UI font), then colour emoji (below), then the NotoSansSymbols files;
2. Arabic (NotoNaskhArabicUI), Hebrew, CJK, Korean;
3. one font for every other script found in /system/fonts (Devanagari, Thai, …).

• Files are memory-mapped, not read. Large CJK collections cost no start-up time, and only the pages touched use memory.
• Glyphs are rasterised on demand by ImGui 1.92's dynamic atlas, so no glyph ranges are pre-baked.
• Missing files are skipped, so the same build works on other phones.
• CJK: the variant (Japanese, Simplified or Traditional Chinese, Korean) follows the phone's language.
• Weights: since Android 12, Roboto is one variable font. inspectPrimary reads its named instances with FreeType and picks, for each weight, the instance at that wght with every other axis at its default (upright, normal width); FreeType loads it through the face index (instance << 16). Older phones have a static file per weight (Roboto-Medium.ttf, …). A weight found in neither is the regular face emboldened by FreeType.
• Sizes are Android text sizes. A size in sp is the em square, as Android's textSize, not ImGui's ascender-to-descender height: FreeType is asked for the nominal size (see kLoaderFlags). Lines are spaced by Roboto's own line height (hhea ascender to descender, 1.17 em), Android's default, and every fallback font is sized the same way, as on the phone.
• Rendering as on Android: no hinting, and glyph advances rounded to whole pixels (PixelSnapH), so a line is as wide as the official app's (about 1 % wider than with fractional advances).
• Variable CJK: FreeType reads CFF2, so the variable Noto Sans CJK is used when present.

Colour emoji#

ImGui's rasteriser cannot read colour fonts (CBDT/sbix), so emoji come from Android itself. A custom ImFontLoader (emojiLoader in Fonts.cpp), merged right after Roboto, claims the emoji blocks (U+2300–23FF, U+2600–27BF, U+2B00–2BFF, U+1F000–1FAFF) when the system can draw them (Paint.hasGlyph). Each glyph is drawn on demand by Platform::drawGlyph (JNI: Paint, Canvas, Bitmap) at the text's size, as everywhere else on the phone, then copied into ImGui's RGBA atlas as a coloured glyph (ImFontGlyph::Colored, so text colour does not tint it). Glyphs are baked once per size, like any other.

Coming before the symbol fonts, it also draws in colour the pictographs they carry (hearts, stars), as everywhere else on the phone. Sequence controls (zero-width joiner, variation selectors, skin-tone modifiers, tags) take no space.

Cost on the Galaxy A14:

Typography#

Sizes and weights were measured on screenshots of the official app on the same phone: the ink box and the inked area of each text were compared with Roboto rendered at every candidate size and weight. They live in Theme.hpp:

The same measurements on SlopTube's screens give the same sizes and weights.

Pipeline (ui/text/)#

Strings without right-to-left characters take a fast path that skips everything above. Once cached, a string costs nothing; its first appearance costs tens to a few hundred microseconds, mostly ImGui rasterising new glyphs.

Canvas API#

• measure, text, paragraph and line take UTF-8 in logical order, exactly as received.
• paragraph and line take an optional TextAlign:
• Natural (default) aligns left-to-right text left and right-to-left text right, like the official app;
• Center centres the text.
• A truncated line gets its ellipsis at the logical end, which is the left side for Arabic.
• Canvas::with(weight) returns a canvas drawing in that weight. Screens use the roles of Theme.hpp (kHeadingWeight, kTitleWeight, kLabelWeight) rather than naming weights.
• Empty text, including a default-constructed std::string_view, is a no-op.

Known limitations#

• Emoji sequences are drawn one code point at a time. A family (joined with U+200D) shows as its members side by side, a skin tone falls back to the default yellow, and a flag shows as its two regional-indicator letters.
• No Indic or Thai shaping. Glyphs show, but Devanagari conjuncts and Thai stacked marks may be imperfect.
• Arabic vowel marks are not positioned from font data.
• Bidi subset: explicit embedding and isolate controls are ignored, and a multi-line string is one paragraph.
• The search field shows every script but does not shape or reorder while typing.

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