> ## Documentation Index > Fetch the complete documentation index at: https://docs.eucaengine.com/llms.txt > Use this file to discover all available pages before exploring further. # Game engine and world engine > Euca is both a game engine that runs worlds and a world engine that reads out their exact state — over one ECS substrate and one typed API. Euca is **one engine** — ECS-native, written in Rust, white-box — that exposes two interfaces over the same state: * **As a game engine, it *runs* worlds.** Real ECS simulation, physics, and a forward PBR renderer — a world you build, play, watch, and act in, through one typed [HTTP API](/api-reference/introduction). * **As a world engine, it *knows* worlds.** The same ECS state reads out exactly ([`observe`](/concepts/query-verbs)), its next-step odds read analytically (`peek`), it advances deterministically (`step`), and it branches counterfactually (`fork`). It hands you the **answer key** to the world it's running. ## One substrate, two interfaces A world engine needs a world to be the truth *of*. The game-engine interface produces the world; the world-engine interface exposes its exact state. Both operate on the same ECS data through one deterministic core, so a richer simulation directly yields richer state to query — the two are requirements on one artifact, not a trade-off. The one thing you must never do is let **appearance leak into truth** — and the architecture enforces exactly that (see [the CPU/GPU split](/concepts/cpu-core-gpu-edge)): authoritative state lives on the CPU and is hashed into a [`state_digest`](/concepts/determinism); the GPU is a read-only consumer. Two AI-generated visual skins of the same world produce a byte-identical digest. ## Which face matters, by use Resolve the "is it about graphics or about ground truth?" question by **use**, not by ranking — both faces are first-class. | Using the engine as… | The face that matters | What it optimizes for | | ---------------------------------------------------------------------- | --------------------- | ------------------------------------------------------------ | | **The grader** (benchmark, regret, [evaluation](/evaluation/overview)) | World engine | Exact ground truth, `peek == step`, determinism, forkability | | **The environment** models live in (experience, training, demos) | Game engine | Rendering, physics richness, performance, world breadth | Two render paths keep them from contaminating each other: an **observation path** (crisp, deterministic, frame↔tick↔entity-aligned — the data/eval signal) and a **presentation path** (the full feature set — demos, marketing, editor; never a data source). Both are kept genuinely good; only determinism-breaking effects (TAA accumulation, motion blur) are confined to presentation. ## Two ways to drive it: the agent API and the editor The world-engine API above is the primary surface — it is how a model or agent reads, steps, and forks the exact state, headless and at scale. But the game-engine face is something you can also drive *by hand*: the **`euca-studio` visual editor** (`cargo run -p euca-studio`) is one app that discovers every project, lets you pick one, edit its scene as data, and press **Play** to run it — the experience face, the way a person authors and watches a world. The same engine serves the same world both ways: `euca-studio --headless` runs a project with no window and exposes exactly the agent HTTP API on `:3917`, so what you author and play in the editor is the same world a model is graded against. The editor is a complement to the data API, never a replacement for it — appearance stays out of [truth](/concepts/determinism) on both paths. ## Why the answer key must be white-box Only a white-box world can report its *own* exact next-step distribution, so grading a model against the truth is only possible from the engine side, where the world is executable, inspectable code. Euca therefore acts as the grader: it scores every model architecture — learned, video, or latent — against the same exact truth, with no architecture given special treatment. See [Evaluation](/evaluation/overview). observe · peek · step · fork — the four verbs of the world-engine face.