Choosing a backend

Which of the two backends you want depends on one question: does something already own your panel?

No — I have a DrawTarget

Take xpui-embedded-graphics. It draws every pixel itself through any embedded-graphics DrawTarget — a driver crate for an e-paper panel, an OLED over SPI, a colour TFT, the desktop simulator's window. It supplies Canvas and TextMetrics from the u8g2 faces it ships, takes Chrome from xpui-chrome, and takes InputSource and Clock from whatever you feed it. Nothing else has to exist: a bare-metal firmware with a panel driver and a button poll is a complete host.

It is monochrome by design and generic over PixelColor anyway: a Palette of two colours maps ink and background onto whatever the panel takes.

Yes — a C++ firmware draws through FreeInkUI

Take xpui-fui. A firmware that already owns the screen and paints its own screens through FreeInkUI has a component library that knows what a list row, a dialog and a slider look like. The backend calls that library over a C ABI, so a Rust screen and a native one come out as the same pixels, in the user's theme, with no second drawing path. The firmware supplies input and the clock through one trait, Platform, and adds one C++ file to its build.

A firmware with its own themed renderer can implement the same ABI itself rather than binding to FreeInkUI; that is a supported path, not a fork.

Writing a third

xpui's docs/writing-a-backend.md is the path through the five traits, and the obligations the compiler cannot check sit on the methods that carry them: draw_text takes a top-left origin, not a baseline, and a font id of 0 means none, so a backend whose own handles start at 0 offsets them. Both are wrong in a way that compiles. xpui-screenshot is how a new backend proves what it painted, and xpui-abi-check is how one that crosses a C boundary proves its signatures agree.