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_docs/02_document/modules/onnx_engine.md

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+# Module: onnx_engine
+
+## Purpose
+
+ONNX Runtime-based inference engine — CPU/CUDA fallback when TensorRT is unavailable.
+
+## Public Interface
+
+### Class: OnnxEngine (extends InferenceEngine)
+
+| Method | Signature | Description |
+|--------|-----------|-------------|
+| `__init__` | `(bytes model_bytes, int batch_size=1, **kwargs)` | Loads ONNX model from bytes, creates InferenceSession with CUDA > CPU provider priority. Reads input shape and batch size from model metadata. |
+| `get_input_shape` | `() -> tuple` | Returns `(height, width)` from input tensor shape |
+| `get_batch_size` | `() -> int` | Returns batch size (from model if not dynamic, else from constructor) |
+| `run` | `(input_data) -> list` | Runs session inference, returns output tensors |
+
+## Internal Logic
+
+- Provider order: `["CUDAExecutionProvider", "CPUExecutionProvider"]` — ONNX Runtime selects the best available.
+- If the model's batch dimension is dynamic (-1), uses the constructor's `batch_size` parameter.
+- Logs model input metadata and custom metadata map at init.
+
+## Dependencies
+
+- **External**: `onnxruntime`
+- **Internal**: `inference_engine` (base class), `constants_inf` (logging)
+
+## Consumers
+
+- `inference` — instantiated when no compatible NVIDIA GPU is found
+
+## Data Models
+
+None (wraps onnxruntime.InferenceSession).
+
+## Configuration
+
+None.
+
+## External Integrations
+
+None directly — model bytes are provided by caller (loaded via `loader_http_client`).
+
+## Security
+
+None.
+
+## Tests
+
+None found.