AI System — MeMLP (Modular embedded Multi-layer Perceptron Model)

NV2 Engine embeds a small neural network — MeMLP (Modular embedded Multi-layer Perceptron Model) — that decides where vegetation belongs, classifies biomes from climate features and selects procedural texture styles. It trains in the background while you play with zero FPS impact (<1% CPU overhead). Everything lives in-process: pure CPU, one JSON checkpoint, no cloud, no GPU.

Implementation: world/memplp.rs (MeMLP core) + world/ai_generator.rs (engine-facing system), using ndarray.

Architecture overview

┌─────────────────────────────────────────────────────────────┐
│                    Main Game Thread                          │
│  World Generation                                           │
│   ├─ Chunk generation (BiomeGenerator)                      │
│   ├─ Tree placement (VegetationGenerator)                   │
│   └─ AI Vegetation (place_ai_vegetation) ◄──────┐           │
│                                                  │          │
└──────────────────────────────────────────────────┬──────────┘
                                                   │
            Queries AI for predictions (non-blocking
            via Arc)
                                                   │
┌──────────────────────────────────────────────────▼──────────┐
│              Background AI Thread (continuous)              │
│  TerrainAI → MeMLP modular model                            │
│   ├─ Vegetation head: 8 → 24 → 16 → 4 (deep MLP)           │
│   ├─ Biome head:      8 → 12 → 9                            │
│   ├─ Texture head:    8 → 12 → 6                            │
│   ├─ Player feedback  (world/ai_feedback.rs)                │
│   └─ Online datasets  (world/online_trainer.rs, offline-ok) │
│                                                             │
│  Training Loop (200+ samples/epoch):                        │
│   1. Drain player actions (highest priority)                │
│   2. Merge online climate data (every ~60 s, offline-safe)  │
│   3. Synthetic samples for all three heads                  │
│   4. Backpropagation (cross-entropy)                        │
│   5. Checkpoint every 20 epochs                             │
└─────────────────────────────────────────────────────────────┘

Model

MeMLP is modular: one checkpoint file contains several specialist multi-layer perceptrons, each solving one task.

• Total parameters: 1095 (fresh) / 623 (migrated legacy checkpoint)
• Checkpoint: single JSON file, ~1–4 KB (Core/checkpoints/ai_model.json)
• Inference: ~0.3 µs per head forward pass (release build)
• Training: ~0.5–1.2 M samples/s per head (release build)

Old pre-MeMLP checkpoints (single hidden layer 8→16→4) are detected and migrated automatically — trained weights are preserved, new heads start fresh. The shipped checkpoint is already v1 MeMLP.

Input features (8)

Outputs (vegetation head, 4 classes)

Mathematics

Forward pass (per head)

Hidden:  h1 = ReLU(input @ w1 + b1)          where ReLU(x) = max(0, x)
         h2 = ReLU(h1 @ w2 + b2)             (deep heads)
Logits:  logits = h_last @ wN + bN
Softmax: p_i = exp(logits_i - max(logits)) / sum_j(exp(logits_j - max(logits)))

Backward pass (cross-entropy, backprop through all layers)

Loss:         Loss = -sum_i(target_i * log(p_i))
Output grad:  dL/dz = p - target             (elementwise)
Weight grad:  dL/dw[k][i,j] = dL/dz[j] * h[k-1][i]
Hidden grad:  dL/dh = dL/dz @ w.T · ReLU'(z) (chain rule through ReLU)

Gradient descent

w := w - learning_rate * dL/dw
b := b - learning_rate * dL/db

Training loop

A background thread runs continuously, combining three signal sources:

1. Player feedback (highest priority) — world/ai_feedback.rs records every vegetation block the player places or breaks; the AI literally learns from what you do (buffer bounded at 4096 samples).
2. Online climate data — world/online_trainer.rs fetches real temperature/humidity from Open-Meteo (keyless, 8 cities spanning desert, rainforest, tundra…) every ~60 s, with a synthetic offline fallback.
3. Synthetic samples — heuristic targets keep all three heads sharp: 200 vegetation samples + 40 biome/texture samples per epoch.

Checkpoint saved every 20 epochs to Core/checkpoints/ai_model.json.

Hyperparameters

Public API (ai_generator.rs)

pub struct TerrainAI {  // wraps the MeMLP, engine-facing API is stable
    model: MeMLP,       // vegetation (8→24→16→4) + biome (8→12→9) + texture (8→12→6)
    learning_rate: f32,
    training_samples: usize,
    // ...
}

Integration points

1. World initialization — World::new_with_settings() spawns AISystem (background thread) and stores it with the message receiver.
2. Feature extraction — place_ai_vegetation() in vegetation.rs extracts the 8 terrain features per 3×3 cell.
3. Prediction & placement — predict_vegetation(&features) returns the block + confidence; blocks are placed only when confidence > 0.40.
4. Biome-aware decorations — DecorationAI uses predict_biome() to choose decoration style (fern in swamp/taiga, flowers in forests…).
5. Player learning — interaction.rs calls ai_feedback::record_place / record_break on every vegetation interaction.

Phase-2 features (2026-08-13)

Community model sharing

Models are portable. /ai_export <path> [author] writes an nv2-model-bundle — the full checkpoint wrapped with author / description / biome-hint metadata — and /ai_import <path> loads any shared bundle, sanitises it and persists it to the runtime checkpoint. API: AISystem::export_model / import_model. This is the local half of the cloud-sharing roadmap: files can already be exchanged between players and servers.

Training-dataset import

JSON datasets (samples: 8 terrain features, targets: 4-class vegetation distributions) are validated and trained on directly — /ai_dataset <path> [epochs] or AISystem::train_on_dataset. Empty or mismatched files are rejected; non-finite rows are skipped.

Player-preference learning

TerrainAI keeps per-class preference counters (flower / fern / stick / pebble) inside the checkpoint (#[serde(default)] — old checkpoints stay compatible). Placing a vegetation block increments its counter; the background loop blends heuristic targets with the learned distribution (30% weight), so the model leans toward what the player likes. /ai_stats shows the live counters.

Performance

Measured with cargo test --release qa_benchmark_report -- --ignored --nocapture — full numbers in TEST_REPORT.md.

Testing

32 AI/ML tests across world::ai_generator (16), world::memplp (10), world::online_trainer (2), world::vegetation (3) and world::biomes (1):

• Forward pass produces a valid probability distribution
• Training decreases loss and fits simple patterns
• Checkpoint JSON round-trips exactly
• Legacy checkpoints migrate (both synthetic and the shipped file)
• Procedural textures are deterministic per seed
• Player-feedback buffer stays bounded
• Heuristic targets stay in range
• Training survives extreme inputs — gradient clipping + bounded updates mean weights can never explode into NaN
• NaN inputs are rejected without touching the weights
• Poisoned checkpoints still load — null (NaN) weights read back as 0.0 instead of failing the whole load
• Model bundles round-trip — export → import preserves metadata and parameters; non-bundle files are rejected
• Datasets import and train — validation rejects empty/mismatched files
• Preferences shift targets — the blend leans toward the player's favourite class and survives checkpoint round-trips

Robustness: Mlp::train clips gradients (±5) and bounds per-parameter updates (±1), save_checkpoint sanitises NaN/Inf before writing, and load_checkpoint tolerates NaN weights serialised as JSON null.

Roadmap (Phase 2)

See the Roadmap page for: real-time AI texture generation, player-preference learning, cloud model sharing, seasonal vegetation, multi-biome coordination, and GPU-accelerated training.