vixy-vision/server/detector.py

#!/usr/bin/env python3
"""
Object Detection Module

Lightweight object detection using TensorFlow Lite with MobileNet V2 SSD.
Designed to run on Raspberry Pi 4/5 with minimal overhead.

The model is lazy-loaded on first detect() call to avoid startup delay.
"""

import cv2
import logging
import numpy as np
from dataclasses import dataclass
from pathlib import Path
from typing import Optional

logger = logging.getLogger(__name__)


@dataclass
class Detection:
    """A single detected object"""
    label: str
    confidence: float
    bbox: tuple  # (x_min, y_min, x_max, y_max) normalized 0-1
    class_id: int


class ObjectDetector:
    """
    Object detection using TFLite MobileNet V2 SSD.

    Lazy-loads the model on first detect() call. Designed to be called
    from the motion detection thread after motion is confirmed.
    """

    def __init__(
        self,
        model_path: str,
        labels_path: str,
        confidence_threshold: float = 0.5,
    ):
        self.model_path = Path(model_path)
        self.labels_path = Path(labels_path)
        self.confidence_threshold = confidence_threshold

        self._interpreter = None
        self._input_details = None
        self._output_details = None
        self._labels: list[str] = []
        self._input_height = 0
        self._input_width = 0

    def _load_model(self):
        """Load TFLite model and label map"""
        # Try ai-edge-litert (modern), then tflite-runtime (legacy)
        try:
            from ai_edge_litert import interpreter as tflite
        except ImportError:
            try:
                import tflite_runtime.interpreter as tflite
            except ImportError:
                raise ImportError(
                    "No TFLite runtime found. Install one of:\n"
                    "  pip install ai-edge-litert    (Python 3.12+)\n"
                    "  pip install tflite-runtime    (Python 3.9-3.11)"
                )

        if not self.model_path.exists():
            raise FileNotFoundError(
                f"Model file not found: {self.model_path}\n"
                f"Run download_model.sh to download the model."
            )

        # Load labels
        if self.labels_path.exists():
            self._labels = self.labels_path.read_text().strip().splitlines()
        else:
            logger.warning(f"Labels file not found: {self.labels_path}")
            self._labels = []

        # Try XNNPACK delegate for ARM acceleration
        delegates = []
        try:
            delegates = [tflite.load_delegate('libXNNPACK.so')]
            logger.info("XNNPACK delegate loaded")
        except (ValueError, OSError):
            logger.info("XNNPACK delegate not available, using default CPU")

        # Load model
        self._interpreter = tflite.Interpreter(
            model_path=str(self.model_path),
            experimental_delegates=delegates if delegates else None,
        )
        self._interpreter.allocate_tensors()

        self._input_details = self._interpreter.get_input_details()
        self._output_details = self._interpreter.get_output_details()

        # Get expected input size
        input_shape = self._input_details[0]['shape']
        self._input_height = input_shape[1]
        self._input_width = input_shape[2]

        logger.info(
            f"Object detection model loaded: {self.model_path.name} "
            f"(input: {self._input_width}x{self._input_height}, "
            f"{len(self._labels)} classes)"
        )

    def detect(self, frame: np.ndarray) -> list[Detection]:
        """
        Run object detection on a frame.

        Args:
            frame: BGR numpy array from OpenCV

        Returns:
            List of Detection objects above confidence threshold
        """
        # Lazy load
        if self._interpreter is None:
            self._load_model()

        # Preprocess: resize and convert BGR to RGB
        input_frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
        input_frame = cv2.resize(input_frame, (self._input_width, self._input_height))
        input_data = np.expand_dims(input_frame, axis=0)

        # Ensure correct dtype
        input_dtype = self._input_details[0]['dtype']
        if input_dtype == np.uint8:
            input_data = input_data.astype(np.uint8)
        elif input_dtype == np.float32:
            input_data = (input_data / 255.0).astype(np.float32)

        # Run inference
        self._interpreter.set_tensor(self._input_details[0]['index'], input_data)
        self._interpreter.invoke()

        # Parse outputs (SSD MobileNet post-processed format):
        # [0] bounding boxes: [1, N, 4] (y_min, x_min, y_max, x_max) normalized
        # [1] class IDs: [1, N]
        # [2] scores: [1, N]
        # [3] number of detections: [1]
        boxes = self._interpreter.get_tensor(self._output_details[0]['index'])[0]
        class_ids = self._interpreter.get_tensor(self._output_details[1]['index'])[0]
        scores = self._interpreter.get_tensor(self._output_details[2]['index'])[0]
        num_detections = int(self._interpreter.get_tensor(self._output_details[3]['index'])[0])

        # Filter by confidence
        detections = []
        for i in range(num_detections):
            score = float(scores[i])
            if score < self.confidence_threshold:
                continue

            class_id = int(class_ids[i])
            label = self._labels[class_id] if class_id < len(self._labels) else f"class_{class_id}"

            # Convert from (y_min, x_min, y_max, x_max) to (x_min, y_min, x_max, y_max)
            y_min, x_min, y_max, x_max = boxes[i]
            bbox = (
                float(np.clip(x_min, 0, 1)),
                float(np.clip(y_min, 0, 1)),
                float(np.clip(x_max, 0, 1)),
                float(np.clip(y_max, 0, 1)),
            )

            detections.append(Detection(
                label=label,
                confidence=score,
                bbox=bbox,
                class_id=class_id,
            ))

        return detections


def annotate_frame(frame: np.ndarray, detections: list[Detection]) -> np.ndarray:
    """
    Draw bounding boxes and labels on a frame.

    Args:
        frame: BGR numpy array (will be copied, not modified in place)
        detections: List of Detection objects

    Returns:
        Annotated copy of the frame
    """
    annotated = frame.copy()
    h, w = annotated.shape[:2]

    for det in detections:
        x1 = int(det.bbox[0] * w)
        y1 = int(det.bbox[1] * h)
        x2 = int(det.bbox[2] * w)
        y2 = int(det.bbox[3] * h)

        # Green box with label
        color = (0, 255, 0)
        cv2.rectangle(annotated, (x1, y1), (x2, y2), color, 2)

        label_text = f"{det.label} {det.confidence:.0%}"
        font_scale = 0.6
        thickness = 1
        (tw, th), _ = cv2.getTextSize(label_text, cv2.FONT_HERSHEY_SIMPLEX, font_scale, thickness)

        # Background for text
        cv2.rectangle(annotated, (x1, y1 - th - 8), (x1 + tw + 4, y1), color, -1)
        cv2.putText(annotated, label_text, (x1 + 2, y1 - 4),
                    cv2.FONT_HERSHEY_SIMPLEX, font_scale, (0, 0, 0), thickness)

    return annotated