cpp

MapLibre Native Slint - C++ Implementation

This directory hosts the C++ version of the MapLibre Native + Slint example. It mirrors the Rust implementation under rust/ but uses the C++ MapLibre API and the Slint C++ bindings.

Build

From the repository root:

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

Run

./build/maplibre-slint-example

Files

• main.cpp — application entry point and UI wiring
• map_window.slint — Slint UI definition that generates map_window.h
• src/slint_maplibre_headless.* — MapLibre headless integration and rendering

Zero-copy OpenGL example (maplibre-slint-gl)

maplibre-slint-example (above) renders the map with mbgl::HeadlessFrontend and copies every frame back to the CPU (readStillImage) before handing it to Slint. maplibre-slint-gl instead renders MapLibre Native directly into an OpenGL texture inside Slint's own GL context and hands that texture to Slint as a borrowed texture (slint::Image::create_from_borrowed_gl_2d_rgba_texture). There is no GPU->CPU readback and no second GL context, which suits GPU-constrained devices such as the Raspberry Pi.

How it works:

• A custom mbgl::gl::RendererBackend + mbgl::gfx::Renderable (src/slint_gl_backend.*, modelled on the upstream GLFW backend) renders into an FBO whose colour texture lives in Slint's GL context. GL entry points are loaded via eglGetProcAddress; activate()/deactivate() are no-ops because Slint's context is already current inside the rendering-notifier callback.
• main_gl.cpp creates the FBO/texture (+ depth/stencil) during RenderingState::RenderingSetup, renders the map into it during BeforeRendering, and publishes the texture with create_from_borrowed_gl_2d_rgba_texture(..., BottomLeft).
• gl_map_window.slint is a small-panel / touch layout (large buttons, a right-edge vertical zoom slider + zoom buttons, no pitch/bearing sliders).

Build

Requires the OpenGL backend (not WebGPU) and Slint's FemtoVG GL renderer. On a Raspberry Pi running on the console over DRM/KMS, also use the libseat linuxkms backend:

cmake -B build -DCMAKE_BUILD_TYPE=Release \
  -DMLN_WITH_OPENGL=ON -DMLN_WITH_WEBGPU=OFF -DMLN_WITH_GLFW=OFF \
  -DSLINT_FEATURE_RENDERER_FEMTOVG=ON \
  -DSLINT_FEATURE_BACKEND_LINUXKMS=ON
cmake --build build --target maplibre-slint-gl

The target is compiled with -fno-rtti to match mbgl-core (MLN_WITH_RTTI=OFF).

Run

SLINT_BACKEND=linuxkms ./build/cpp/maplibre-slint-gl

Variable	Effect
MAPLIBRE_STYLE_URL	Initial style URL
MAPLIBRE_WIDTH / MAPLIBRE_HEIGHT	Render size (default: the display resolution)
MAPLIBRE_FLY_MS	flyTo duration in ms for the city buttons (default 2500)

Raspberry Pi notes

Verified on a Raspberry Pi 4 (Debian trixie, aarch64, V3D / vc4-kms-v3d) with a 480x320 HDMI touch panel (Quimat MPI3508) on the console over DRM/KMS:

• DRM master: install/enable seatd, add the user to the video group, and build Slint with SLINT_FEATURE_BACKEND_LINUXKMS=ON (libseat). seatd grants DRM master without an X11/Wayland session or an active VT.
• Display mode: the MPI3508 only advertises standard HDMI modes (it scales internally to 480x320); a raw 480x320 CVT signal is rejected. For less downscaling, force e.g. video=HDMI-A-1:720x576@50 in cmdline.txt.
• Touch: the XPT2046/ADS7846 resistive controller is single-touch (no pinch). Enable dtparam=spi=on and dtoverlay=ads7846,cs=1,penirq=25,..., then set a libinput calibration matrix via udev, e.g. ENV{LIBINPUT_CALIBRATION_MATRIX}="0 -1 1 -1 0 1" for this panel/orientation. Resistive panels have a dead zone at the very edges, so keep interactive controls inset with a margin (the Pi layout does this). Double-tap zooms in (detected in handle_mouse_press); zoom out with the on-screen zoom buttons/slider.
• maplibre-native renders on the Pi's V3D GPU, composited zero-copy by Slint; the flyTo camera animation stays smooth.

A few integration details matter when extending the zero-copy GL example on V3D:

• Re-render every frame. V3D is a tiled GPU and treats the FBO colour attachment as transient: skipping frontend->render() on idle frames discards the borrowed texture and the map turns white or black. Drive a repaint every frame rather than only on map invalidation.
• Save and restore GL state around the render. Slint's FemtoVG renderer shares the GL context, so the BeforeRendering callback snapshots and restores the framebuffer binding, viewport, current program, array/element buffer bindings, active texture, and the BLEND/DEPTH_TEST/SCISSOR_TEST/CULL_FACE enables around frontend->render(). Without that, Slint's own drawing is corrupted.
• Borrowed-texture size. A borrowed GL texture is composited at its native size (it is not scaled to the element via image-fit), so create the FBO at the display resolution.