QR Code

1. Experiment Objective

Use OpenCV and pyzbar to detect and decode QR codes in the camera image, with support for simultaneous recognition of multiple codes and dynamic parameter tuning.

2. Experiment Principle

1. Key Terms

2. Technical Architecture

Terminal 1: start agent + camera + joint models
Terminal 2: start QR code recognition

Data Flow: Image subscription → image conversion → orientation correction → resize → grayscale conversion → pyzbar / OpenCV detection → draw rectangle boxes + data content labels → real-time display.

3. Experiment Steps

1. Hardware Preparation

2. Start Agent + Camera + Joint Model (Terminal 1)

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ros2 launch microros_v2_bringup car_base.launch.py

3. Start QR Code Recognition (Terminal 2)

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ros2 launch microros_v2_opencv_visual_pkg detect_qr.launch.py

4. Operation Instructions

Dual-backend strategy:

• pyzbar (default): grayscale conversion → pyzbar.decode(), outputs payload, type, rectangle box, and polygon corners, with high stability
• OpenCV: tries detectAndDecodeMulti (multi-code) first, falls back to detectAndDecode (single-code)
• auto: tries pyzbar first, then OpenCV if no result

Dynamic parameter tuning:

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ros2 param list /detect_qr_node
ros2 param set /detect_qr_node qr_select_strategy nearest_center

Modify parameters such as detection backend, image orientation, and window size at runtime without restarting.

Interaction: Press q or ESC to exit.

5. How to Stop

Press Ctrl+C to stop the terminals one by one.

4. Experiment Observations

• Place a QR code in the camera's field of view; the code area is marked with a colored rectangle and the decoded content is displayed above it
• In multi-code scenarios, each code is annotated independently, and the primary code (largest area / closest to center) is highlighted

5. Program Analysis

Source code paths:

• Launch: ~/microros_ws/src/microros_v2_opencv_visual_pkg/launch/detect_qr.launch.py
• Node: ~/microros_ws/src/microros_v2_opencv_visual_pkg/microros_v2_opencv_visual_pkg/detect_qr.py

1. Core Node Analysis

DetectQRNode implements dual-backend QR code detection: pyzbar first, with OpenCV as the fallback.

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# Source file: ~/microros_ws/src/microros_v2_opencv_visual_pkg/microros_v2_opencv_visual_pkg/detect_qr.py
class DetectQRNode(Node):
    def __init__(self):
        super().__init__('detect_qr_node')
        self.declare_parameter('qr_detector_backend', 'pyzbar')
        self.declare_parameter('qr_select_strategy', 'largest')
        self.bridge = CvBridge()
​
    def detect_with_pyzbar(self, gray):
        from pyzbar.pyzbar import decode as pyzbar_decode
        results = pyzbar_decode(gray)
        codes = []
        for barcode in results:
            payload = barcode.data.decode('utf-8', errors='ignore')
            rect = barcode.rect
            polygon = barcode.polygon
            codes.append({'data': payload, 'rect': rect, 'polygon': polygon})
        return codes
​
    def image_callback(self, msg):
        frame = self.bridge.imgmsg_to_cv2(msg, 'bgr8')
        gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
        # Call different detection functions based on the selected backend
        backend = self.get_parameter('qr_detector_backend').value
        if backend == 'pyzbar':
            codes = self.detect_with_pyzbar(gray)
        elif backend == 'opencv':
            codes = self.detect_with_opencv(frame, gray)
        # Draw the detection results
        self.draw_qr_codes(frame, codes)
        cv.imshow('QR Detection', frame)

Key logic notes:

• The dual backends are switched at runtime through the qr_detector_backend parameter; pyzbar's decoding is more robust than OpenCV's QUIRC
• pyzbar provides barcode.polygon with precise positions of multiple corners, while OpenCV only returns a rectangle box
• In multi-code scenarios, the primary code is selected by qr_select_strategy: largest (largest area, default) / nearest_center / first

2. Key Parameters

Parameter definition files:

• Launch: ~/microros_ws/src/microros_v2_opencv_visual_pkg/launch/detect_qr.launch.py
• Node: ~/microros_ws/src/microros_v2_opencv_visual_pkg/microros_v2_opencv_visual_pkg/detect_qr.py

3. Node Description

6. Frequently Asked Questions