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ADR-0010 — CP437 display pipeline

Status: accepted Source: spec 015/016 (display text); FR-094, FR-100; components/cdc_views/src/RenderHelpers.cpp

Context

Section titled “Context”

The text pipeline is canonical CP437: T9 input, plugin to_display, and the i18n loader all produce CP437. The CalEPD submodule overrides Epd::print(const std::string&) with a hardcoded UTF-8 assumption (Epd::_unicodeEasy): it skips bytes 0xC3/0xC2 and adds 64 to every byte in 0x84..0xBE. Applied to CP437 this corrupts text (ae 0x84 -> 0xC4 renders as a line glyph, oe 0x94 -> 0xD4), while ue 0x81 and ASCII pass through. gfx->print(char) (single char) routes to Epd::print(const char) and is not transformed.

Decision

Section titled “Decision”

On-screen CP437 text is drawn through the render helpers, never through gfx->print(const char*) / gfx->print(std::string) for user or i18n text.

  • Built-in 6x8 font: cdc::ui::render::printText(gfx, text) (writes each byte via gfx->write(), bypassing the transform).
  • Font-aware drawing: cdc::ui::render::drawText(gfx, text, font).
  • Shared chrome helpers (printTruncated, drawHeaderLeft/Centered, drawFooterBar) already do this internally.
  • gfx->print(...) is acceptable only for pure-ASCII literals (e.g. "[Y]", digit buffers).

i18n strings are CP437 at draw time: lang_de.json carries real UTF-8 umlauts and the loader converts UTF-8 → CP437 (cdc::core::cp437::fromUtf8); in-code English fallbacks stay ASCII.

Consequences

Section titled “Consequences”
  • Enables: correct rendering of extended glyphs (umlauts, box-drawing) across i18n, T9 input, and plugin output on the CalEPD display.
  • Must hold: new views and helpers draw user/i18n text only via printText/drawText; in-code string literals drawn directly stay ASCII (no UTF-8 umlauts in code that bypasses the loader).
  • Cost: developers cannot use the convenient gfx->print overload for non-ASCII text; the helper path is mandatory.