Hera

Overview

Hera is a compositor and render server for Embedded Linux, written in Rust. The server draws everything: clients send scene graph diffs, glyph runs, and shared buffers, while the compositor renders, composites, and presents via DRM/KMS with a Vulkan backend.

What Hera Is

Hera is not a UI toolkit by itself. It is the server-side engine that receives high-level UI changes and turns them into frames. Client toolkits (Hera Kit, React Native via rn_host, or other custom apps) focus on building a scene graph and emitting diffs; the server owns timing, composition, and GPU work.

Why A Server-Rendered Compositor

  • Hera carries semantic scene data down to the compositor and renderer, unlocking advanced rendering and animation that can span multiple client surfaces.
  • The server owns frame timing and animation so updates stay aligned with the display clock even if clients stall.
  • Multiple client processes can contribute surfaces without each client managing KMS/DRM devices.
  • Rendering is centralized, enabling consistent effects and a single GPU cache for glyphs, images, and shared buffers.

Core Building Blocks

  • hera::server handles IPC, input, lifecycle, and frame timing.
  • hera::compositor schedules frames, applies animation, and computes damage.
  • hera::graph holds the retained scene graph and per-surface roots.
  • hera::renderer records Vulkan work, manages buffers/atlases, and presents.
  • hera::protocol defines the wire types and transport helpers.

Data Flow At A Glance

  1. A client builds a UI tree and diffs it into protocol DiffOp messages.
  2. Diffs, glyph runs, and shared buffers flow over Unix sockets to the server.
  3. The compositor merges updates into the retained scene graph and runs animation and hit-testing on the server clock.
  4. The renderer builds command buffers, imports DMA-BUFs, and page-flips.

Key Concepts

  • Surface: a per-client root with its own scene graph tree.
  • Node: a render primitive (view, text run, image, embedded surface).
  • DiffOp: an incremental update to the retained graph.
  • Commit: a batch of diffs applied atomically and acknowledged by the server.
  • Frame Scheduling: server-driven timing that aligns client updates with presentation.

Client Entry Points

  • Hera Kit: a Rust-native UI runtime that emits diffs directly.
  • React Native host: a Bridgeless + Fabric host that translates RN mount transactions into Hera diff ops.

Where To Go Next

Start with the architecture page for the full system flow, then dive into Hera Kit or the React Native host depending on how you plan to build your UI.