Camera-Based Perception

Visual sensing and basic image processing techniques

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Tutorial Overview

🎯 Learning Objectives

By the end of this tutorial, you will:

• ✅ Access and view camera streams

• ✅ Understand image topics and formats

• ✅ Perform basic image processing (filters, thresholding, edge detection)

• ✅ Detect colors and shapes

• ✅ Record and process video data

• ✅ Understand camera calibration

• ✅ Explore potential perception applications

• ✅ Integrate vision with robot control

⏱️ Time Required

• Reading & Setup: 20 minutes

• Basic Image Access: 25 minutes

• Image Processing: 40 minutes

• Color/Shape Detection: 35 minutes

• Applications: 30 minutes

• Total: ~150 minutes

📚 Prerequisites

• ✅ Completed Sensor Data Visualization

• ✅ Completed ROS2 Communication & Tools

• ✅ Basic Python knowledge

• ✅ Understanding of image concepts (pixels, RGB)

• ✅ OpenCV basics helpful but not required

• ✅ Can record and play rosbags

🛠️ What You'll Need

• ✅ Beetlebot (powered, camera working)

• ✅ Laptop with ROS2 Jazzy

• ✅ Wireless controller

• ✅ Test objects with distinct colors

• ✅ Good lighting (avoid direct sunlight on camera)

• ✅ Printed patterns for detection tests (optional)

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Part 1: Camera System Overview

Raspberry Pi Camera V1.3 Specifications

Your robot's camera:

Hardware:

• Sensor: OmniVision OV5647

• Resolution: 5MP (2592×1944 max)

• Typical streaming: 1080p (1920×1080) or 720p (1280×720)

• Frame rate: 30 fps (1080p), 60 fps (720p)

• FOV: ~54° horizontal, ~41° vertical

• Focus: Fixed (best 30cm - 3m)

• Interface: CSI (Camera Serial Interface) to Pi

Mounting:

• Height: 8.8cm from ground

• Position: 15cm forward from center

• Orientation: Horizontal, forward-facing

• Topic: /pi_camera/image_raw

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Camera Topics

Available topics:

ros2 topic list | grep camera

# Output:
# /pi_camera/camera_info        ← Calibration data
# /pi_camera/image_raw           ← Uncompressed images
# /pi_camera/image_raw/compressed ← JPEG compressed

Topic details:

/pi_camera/image_raw (sensor_msgs/Image)

• Uncompressed RGB8 or BGR8 format

• ~30 MB/s bandwidth (1080p @ 30fps)

• Best quality

• Use for local processing

/pi_camera/image_raw/compressed (sensor_msgs/CompressedImage)

• JPEG compression

• ~3-5 MB/s bandwidth

• Slight quality loss

• Better for remote viewing

/pi_camera/camera_info (sensor_msgs/CameraInfo)

• Intrinsic parameters (focal length, principal point)

• Distortion coefficients

• Rectification matrix

• Projection matrix

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Understanding Image Messages

Image message structure:

ros2 interface show sensor_msgs/msg/Image

# Fields:
# std_msgs/Header header
#   builtin_interfaces/Time stamp
#   string frame_id
# uint32 height           # rows
# uint32 width            # columns
# string encoding         # rgb8, bgr8, mono8, etc.
# uint8 is_bigendian
# uint32 step             # bytes per row
# uint8[] data            # actual pixel data

Common encodings:

• rgb8: 8-bit RGB (Red-Green-Blue)

• bgr8: 8-bit BGR (OpenCV default)

• mono8: 8-bit grayscale

• rgba8: 8-bit RGBA (with alpha channel)

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Part 2: Accessing Camera Stream

Quick View with rqt_image_view

Simplest method:

# On laptop
ros2 run rqt_image_view rqt_image_view

# Window opens
# Dropdown: Select "/pi_camera/image_raw/compressed"
# Image appears!

Controls:

• Zoom: Mouse wheel

• Pan: Right-click + drag

• Refresh: Click dropdown again

• Rotate: Image → Transform menu

[PLACEHOLDER: Screenshot of rqt_image_view showing camera feed]

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Exercise 9.1: Camera Field of View Test

Task: Measure camera's actual field of view

Materials needed:

• Measuring tape

• Flat wall

• Markers or tape

Procedure:

1. Place robot 1 meter from wall (measure precisely)
2. Robot facing wall directly (perpendicular)
3. View camera in rqt_image_view
4. Mark left edge of camera view on wall (left_edge)
5. Mark right edge of camera view on wall (right_edge)
6. Measure distance between marks (width)

Calculation:
  FOV = 2 × atan(width / (2 × distance))
  FOV = 2 × atan(width / 2.0)
  
Example:
  width = 1.1 meters
  FOV = 2 × atan(1.1 / 2.0) = 2 × atan(0.55)
      = 2 × 28.8° = 57.6°

Expected: ~54° (spec says 54°, you should get close!)

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Viewing in RViz

Integrated visualization:

# Launch RViz
rviz2

# Add Camera display:
Add → Camera
  Image Topic: /pi_camera/image_raw
  Transport Hint: compressed (for better performance)

# New window shows camera feed with RViz overlays

Benefits of RViz camera view:

• See camera + LiDAR + map simultaneously

• Overlay detection results

• 3D context for camera view

• Can add measurement tools

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Part 3: Basic Image Processing with OpenCV

Installing Dependencies

# Install OpenCV for Python
sudo apt install python3-opencv python3-numpy --break-system-packages

# Or pip (if needed)
pip3 install opencv-python numpy --break-system-packages

# Verify installation
python3 -c "import cv2; print(cv2.__version__)"
# Should show: 4.x.x

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Creating Image Processing Node

[!WARNING] TODO: Exercise Script Not Included in Core Repository The camera_processor.py script below is an exercise for the user to practice processing camera frames with OpenCV. It does not come pre-installed.

Basic template for image processing:

nano ~/camera_processor.py

Script:

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
import numpy as np

class CameraProcessor(Node):
    def __init__(self):
        super().__init__('camera_processor')
        
        # Bridge converts ROS Image ↔ OpenCV image
        self.bridge = CvBridge()
        
        # Subscribe to camera
        self.subscription = self.create_subscription(
            Image,
            '/pi_camera/image_raw',
            self.image_callback,
            10)
        
        # Publisher for processed images (optional)
        self.publisher = self.create_publisher(
            Image,
            '/camera/processed',
            10)
        
        self.get_logger().info('Camera Processor started!')
    
    def image_callback(self, msg):
        try:
            # Convert ROS Image to OpenCV format
            cv_image = self.bridge.imgmsg_to_cv2(msg, desired_encoding='bgr8')
            
            # **PROCESSING HAPPENS HERE**
            # Example: Convert to grayscale
            gray = cv2.cvtColor(cv_image, cv2.COLOR_BGR2GRAY)
            
            # Display (local window)
            cv2.imshow('Original', cv_image)
            cv2.imshow('Grayscale', gray)
            cv2.waitKey(1)
            
            # Publish processed image (optional)
            processed_msg = self.bridge.cv2_to_imgmsg(gray, encoding='mono8')
            self.publisher.publish(processed_msg)
            
        except Exception as e:
            self.get_logger().error(f'Error processing image: {e}')

def main(args=None):
    rclpy.init(args=args)
    node = CameraProcessor()
    
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    finally:
        cv2.destroyAllWindows()
        node.destroy_node()
        rclpy.shutdown()

if __name__ == '__main__':
    main()

Run:

chmod +x ~/camera_processor.py
python3 ~/camera_processor.py

Two windows appear:

• Original: Color camera feed

• Grayscale: Converted to grayscale

Press Ctrl+C to stop

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Image Filtering

Add filters to processing node:

# In image_callback, replace processing section:

# 1. Gaussian Blur (smoothing, noise reduction)
blurred = cv2.GaussianBlur(cv_image, (15, 15), 0)

# 2. Edge Detection (Canny)
edges = cv2.Canny(cv_image, 50, 150)

# 3. Median Blur (removes salt-and-pepper noise)
median = cv2.medianBlur(cv_image, 5)

# 4. Sharpening
kernel = np.array([[-1,-1,-1],
                   [-1, 9,-1],
                   [-1,-1,-1]])
sharpened = cv2.filter2D(cv_image, -1, kernel)

# Display all
cv2.imshow('Original', cv_image)
cv2.imshow('Blurred', blurred)
cv2.imshow('Edges', edges)
cv2.imshow('Median', median)
cv2.imshow('Sharpened', sharpened)

[PLACEHOLDER: Screenshot showing original vs filtered images]

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Exercise 9.2: Edge Detection for Obstacle Boundaries

Task: Detect obstacles using edge detection

Modify processing node:

def image_callback(self, msg):
    cv_image = self.bridge.imgmsg_to_cv2(msg, 'bgr8')
    
    # Convert to grayscale
    gray = cv2.cvtColor(cv_image, cv2.COLOR_BGR2GRAY)
    
    # Blur to reduce noise
    blurred = cv2.GaussianBlur(gray, (5, 5), 0)
    
    # Edge detection
    edges = cv2.Canny(blurred, 50, 150)
    
    # Find contours (boundaries)
    contours, _ = cv2.findContours(edges, cv2.RETR_EXTERNAL, 
                                     cv2.CHAIN_APPROX_SIMPLE)
    
    # Draw contours on original image
    output = cv_image.copy()
    cv2.drawContours(output, contours, -1, (0, 255, 0), 2)
    
    # Display
    cv2.imshow('Edges', edges)
    cv2.imshow('Contours', output)
    cv2.waitKey(1)

Test:

• Place various objects in front of robot

• Observe edge detection outlining objects

• Try different Canny thresholds (50, 150) to tune sensitivity

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Part 4: Color Detection

HSV Color Space

Why HSV instead of RGB?

RGB (Red-Green-Blue):

• How cameras capture

• Lighting affects all channels

• Hard to isolate specific colors

HSV (Hue-Saturation-Value):

• Hue = Color (0-180° in OpenCV)

• Saturation = Color intensity (0-255)

• Value = Brightness (0-255)

• Lighting mainly affects Value

• Easier to detect specific colors

Color ranges in HSV (OpenCV):

Red:    Hue 0-10, 170-180 (wraps around)
Orange: Hue 10-25
Yellow: Hue 25-35
Green:  Hue 35-85
Blue:   Hue 85-125
Purple: Hue 125-155

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Color Detection Node

[!WARNING] TODO: Exercise Script Not Included in Core Repository The color_detector.py script below is an exercise for color tracking. It is not pre-installed. You are encouraged to create it yourself!

Create color detector:

nano ~/color_detector.py

Script:

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
import numpy as np

class ColorDetector(Node):
    def __init__(self):
        super().__init__('color_detector')
        self.bridge = CvBridge()
        
        self.subscription = self.create_subscription(
            Image, '/pi_camera/image_raw', self.image_callback, 10)
        
        # Define color ranges (HSV)
        # Red (example: red ball, red tape)
        self.lower_red1 = np.array([0, 100, 100])
        self.upper_red1 = np.array([10, 255, 255])
        self.lower_red2 = np.array([170, 100, 100])
        self.upper_red2 = np.array([180, 255, 255])
        
        # Green (example: green marker)
        self.lower_green = np.array([35, 50, 50])
        self.upper_green = np.array([85, 255, 255])
        
        # Blue (example: blue object)
        self.lower_blue = np.array([85, 50, 50])
        self.upper_blue = np.array([125, 255, 255])
        
        self.get_logger().info('Color Detector started!')
    
    def image_callback(self, msg):
        # Convert to OpenCV
        cv_image = self.bridge.imgmsg_to_cv2(msg, 'bgr8')
        
        # Convert to HSV
        hsv = cv2.cvtColor(cv_image, cv2.COLOR_BGR2HSV)
        
        # Create masks for each color
        # Red (two ranges because it wraps around)
        mask_red1 = cv2.inRange(hsv, self.lower_red1, self.upper_red1)
        mask_red2 = cv2.inRange(hsv, self.lower_red2, self.upper_red2)
        mask_red = cv2.bitwise_or(mask_red1, mask_red2)
        
        mask_green = cv2.inRange(hsv, self.lower_green, self.upper_green)
        mask_blue = cv2.inRange(hsv, self.lower_blue, self.upper_blue)
        
        # Find contours for each color
        contours_red, _ = cv2.findContours(mask_red, cv2.RETR_EXTERNAL, 
                                            cv2.CHAIN_APPROX_SIMPLE)
        contours_green, _ = cv2.findContours(mask_green, cv2.RETR_EXTERNAL, 
                                              cv2.CHAIN_APPROX_SIMPLE)
        contours_blue, _ = cv2.findContours(mask_blue, cv2.RETR_EXTERNAL, 
                                             cv2.CHAIN_APPROX_SIMPLE)
        
        # Draw on original image
        output = cv_image.copy()
        
        # Red objects
        for contour in contours_red:
            if cv2.contourArea(contour) > 500:  # Filter small noise
                x, y, w, h = cv2.boundingRect(contour)
                cv2.rectangle(output, (x, y), (x+w, y+h), (0, 0, 255), 2)
                cv2.putText(output, 'RED', (x, y-10), 
                           cv2.FONT_HERSHEY_SIMPLEX, 0.6, (0, 0, 255), 2)
        
        # Green objects
        for contour in contours_green:
            if cv2.contourArea(contour) > 500:
                x, y, w, h = cv2.boundingRect(contour)
                cv2.rectangle(output, (x, y), (x+w, y+h), (0, 255, 0), 2)
                cv2.putText(output, 'GREEN', (x, y-10), 
                           cv2.FONT_HERSHEY_SIMPLEX, 0.6, (0, 255, 0), 2)
        
        # Blue objects
        for contour in contours_blue:
            if cv2.contourArea(contour) > 500:
                x, y, w, h = cv2.boundingRect(contour)
                cv2.rectangle(output, (x, y), (x+w, y+h), (255, 0, 0), 2)
                cv2.putText(output, 'BLUE', (x, y-10), 
                           cv2.FONT_HERSHEY_SIMPLEX, 0.6, (255, 0, 0), 2)
        
        # Display
        cv2.imshow('Original', cv_image)
        cv2.imshow('Detected Colors', output)
        cv2.imshow('Red Mask', mask_red)
        cv2.imshow('Green Mask', mask_green)
        cv2.imshow('Blue Mask', mask_blue)
        cv2.waitKey(1)

def main(args=None):
    rclpy.init(args=args)
    node = ColorDetector()
    rclpy.spin(node)
    cv2.destroyAllWindows()
    node.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Run:

chmod +x ~/color_detector.py
python3 ~/color_detector.py

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Exercise 9.3: Color-Based Object Tracking

Task: Track colored object and display centroid

Modification to color_detector.py:

# After finding contours for red objects:
for contour in contours_red:
    if cv2.contourArea(contour) > 500:
        # Calculate centroid
        M = cv2.moments(contour)
        if M["m00"] != 0:
            cx = int(M["m10"] / M["m00"])
            cy = int(M["m01"] / M["m00"])
            
            # Draw centroid
            cv2.circle(output, (cx, cy), 5, (255, 255, 255), -1)
            
            # Draw bounding box
            x, y, w, h = cv2.boundingRect(contour)
            cv2.rectangle(output, (x, y), (x+w, y+h), (0, 0, 255), 2)
            
            # Display position info
            cv2.putText(output, f'RED ({cx}, {cy})', (x, y-10), 
                       cv2.FONT_HERSHEY_SIMPLEX, 0.6, (0, 0, 255), 2)
            
            # Log position
            self.get_logger().info(f'Red object at pixel ({cx}, {cy})')

Test:

• Hold red object in front of camera

• Move it around

• Observe centroid tracking

• Check console for position logs

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Part 5: Shape Detection

Detecting Simple Shapes

Add shape detection:

def detect_shape(self, contour):
    """Identify shape based on contour properties"""
    
    # Approximate contour to polygon
    peri = cv2.arcLength(contour, True)
    approx = cv2.approxPolyDP(contour, 0.04 * peri, True)
    
    # Number of vertices determines shape
    vertices = len(approx)
    
    if vertices == 3:
        return "Triangle"
    elif vertices == 4:
        # Check if square or rectangle
        x, y, w, h = cv2.boundingRect(approx)
        aspect_ratio = float(w) / h
        if 0.95 <= aspect_ratio <= 1.05:
            return "Square"
        else:
            return "Rectangle"
    elif vertices == 5:
        return "Pentagon"
    elif vertices > 5:
        # Check circularity
        area = cv2.contourArea(contour)
        perimeter = cv2.arcLength(contour, True)
        circularity = 4 * np.pi * area / (perimeter * perimeter)
        
        if circularity > 0.8:
            return "Circle"
        else:
            return "Polygon"
    else:
        return "Unknown"

# In image_callback, after finding contours:
for contour in contours_red:
    if cv2.contourArea(contour) > 500:
        shape = self.detect_shape(contour)
        x, y, w, h = cv2.boundingRect(contour)
        cv2.putText(output, shape, (x, y-10), 
                   cv2.FONT_HERSHEY_SIMPLEX, 0.6, (0, 0, 255), 2)

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Exercise 9.4: Shape and Color Recognition

Task: Create multi-property detector

Test scenario:

Objects to detect:
- Red circle (ball)
- Blue rectangle (book)
- Green triangle (folded paper)

Output should show:
- "RED CIRCLE at (320, 240)"
- "BLUE RECTANGLE at (150, 300)"
- "GREEN TRIANGLE at (500, 180)"

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Part 6: Camera Calibration

Why Calibrate?

Camera distortion:

• Lens distortion (barrel, pincushion)

• Affects measurements and 3D perception

• Need calibration to correct

What calibration provides:

• Intrinsic matrix (focal length, principal point)

• Distortion coefficients

• Allows undistortion of images

• Enables accurate 3D measurements

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Viewing Current Calibration

# Check camera_info topic
ros2 topic echo /pi_camera/camera_info --once

# Output shows:
# K: [fx, 0, cx, 0, fy, cy, 0, 0, 1]  ← Intrinsic matrix
#   fx, fy = focal length (pixels)
#   cx, cy = principal point (image center)
# 
# D: [k1, k2, t1, t2, k3]  ← Distortion coefficients
#   k1, k2, k3 = radial distortion
#   t1, t2 = tangential distortion

---

Using Calibration Data

Undistort image:

import cv2
import numpy as np

# Get calibration from camera_info (run once)
# camera_matrix = np.array([[fx, 0, cx],
#                           [0, fy, cy],
#                           [0,  0,  1]])
# dist_coeffs = np.array([k1, k2, t1, t2, k3])

# Example values (yours may differ):
camera_matrix = np.array([[640, 0, 640],
                          [0, 640, 360],
                          [0, 0, 1]], dtype=float)
dist_coeffs = np.array([0.1, -0.05, 0, 0, 0], dtype=float)

# In image_callback:
def image_callback(self, msg):
    cv_image = self.bridge.imgmsg_to_cv2(msg, 'bgr8')
    
    # Undistort
    h, w = cv_image.shape[:2]
    new_camera_matrix, roi = cv2.getOptimalNewCameraMatrix(
        camera_matrix, dist_coeffs, (w, h), 1, (w, h))
    
    undistorted = cv2.undistort(cv_image, camera_matrix, 
                                 dist_coeffs, None, new_camera_matrix)
    
    # Display both
    cv2.imshow('Original (Distorted)', cv_image)
    cv2.imshow('Undistorted', undistorted)
    cv2.waitKey(1)

---

Part 7: Practical Applications

Application 1: Line Following

Detect line on floor, steer toward it:

def detect_line(self, cv_image):
    """Detect line in lower portion of image"""
    
    # Region of interest (bottom half of image)
    height, width = cv_image.shape[:2]
    roi = cv_image[int(height/2):height, 0:width]
    
    # Convert to grayscale
    gray = cv2.cvtColor(roi, cv2.COLOR_BGR2GRAY)
    
    # Threshold (assuming dark line on light floor)
    _, thresh = cv2.threshold(gray, 80, 255, cv2.THRESH_BINARY_INV)
    
    # Find contours
    contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, 
                                     cv2.CHAIN_APPROX_SIMPLE)
    
    if contours:
        # Find largest contour (assumed to be line)
        largest = max(contours, key=cv2.contourArea)
        
        # Calculate centroid
        M = cv2.moments(largest)
        if M["m00"] != 0:
            cx = int(M["m10"] / M["m00"])
            
            # Error from center
            center_x = width // 2
            error = cx - center_x
            
            return error  # Positive = line to right, negative = line to left
    
    return None

Use error for steering:

# In control loop:
error = self.detect_line(cv_image)
if error is not None:
    # Simple proportional control
    angular_vel = -0.005 * error  # Scale factor
    
    cmd = Twist()
    cmd.linear.x = 0.3  # Constant forward speed
    cmd.angular.z = angular_vel
    self.cmd_vel_pub.publish(cmd)

---

Application 2: Obstacle Color Classification

Classify obstacles by color, react differently:

def classify_obstacle(self, cv_image):
    """Determine obstacle type by color"""
    
    hsv = cv2.cvtColor(cv_image, cv2.COLOR_BGR2HSV)
    
    # Red = STOP
    mask_red = cv2.inRange(hsv, self.lower_red, self.upper_red)
    if cv2.countNonZero(mask_red) > 1000:  # Significant red detected
        return "STOP"
    
    # Green = GO
    mask_green = cv2.inRange(hsv, self.lower_green, self.upper_green)
    if cv2.countNonZero(mask_green) > 1000:
        return "GO"
    
    # Yellow = SLOW
    lower_yellow = np.array([20, 100, 100])
    upper_yellow = np.array([30, 255, 255])
    mask_yellow = cv2.inRange(hsv, lower_yellow, upper_yellow)
    if cv2.countNonZero(mask_yellow) > 1000:
        return "SLOW"
    
    return "UNKNOWN"

# In control loop:
obstacle_type = self.classify_obstacle(cv_image)
if obstacle_type == "STOP":
    # Stop robot
    cmd.linear.x = 0.0
elif obstacle_type == "SLOW":
    # Reduce speed
    cmd.linear.x = 0.2
elif obstacle_type == "GO":
    # Full speed
    cmd.linear.x = 0.8

---

Application 3: AprilTag Detection

AprilTags = Fiducial markers (like QR codes for robots)

Install apriltag library:

sudo apt install ros-jazzy-apriltag-ros

Launch apriltag detector:

ros2 launch apriltag_ros tag_realsense.launch.py

Use cases:

• Precise localization (know exact position from tag)

• Object identification (each tag has unique ID)

• Docking stations (navigate to specific tag)

• AR applications (overlay virtual objects)

---

Part 8: Recording and Processing Video

Recording Camera Data

# Record camera feed
ros2 bag record -o camera_test /pi_camera/image_raw/compressed

# Drive around, collect data
# Ctrl+C to stop

# Play back
ros2 bag play camera_test

---

Offline Processing

[!WARNING] TODO: Exercise Script Not Included in Core Repository The offline_processor.py script below is an exercise to demonstrate processing rosbags programmatically. It is not pre-installed.

Python script: offline_processor.py

#!/usr/bin/env python3

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2

class OfflineProcessor(Node):
    def __init__(self):
        super().__init__('offline_processor')
        self.bridge = CvBridge()
        
        # Output video writer
        fourcc = cv2.VideoWriter_fourcc(*'XVID')
        self.out = cv2.VideoWriter('processed_output.avi', fourcc, 
                                    30.0, (1280, 720))
        
        self.subscription = self.create_subscription(
            Image, '/pi_camera/image_raw', self.process, 10)
        
        self.frame_count = 0
    
    def process(self, msg):
        cv_image = self.bridge.imgmsg_to_cv2(msg, 'bgr8')
        
        # **YOUR PROCESSING HERE**
        # Example: Add timestamp overlay
        timestamp = msg.header.stamp.sec + msg.header.stamp.nanosec / 1e9
        cv2.putText(cv_image, f'Time: {timestamp:.2f}s', (10, 30),
                   cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)
        
        # Write to output video
        self.out.write(cv_image)
        
        self.frame_count += 1
        if self.frame_count % 30 == 0:
            self.get_logger().info(f'Processed {self.frame_count} frames')

def main():
    rclpy.init()
    node = OfflineProcessor()
    
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    finally:
        node.out.release()
        node.get_logger().info('Video saved as processed_output.avi')
        node.destroy_node()
        rclpy.shutdown()

if __name__ == '__main__':
    main()

Run:

# Terminal 1: Play bag
ros2 bag play camera_test

# Terminal 2: Process
python3 offline_processor.py

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Part 9: Limitations and Considerations

What Camera Perception Can't Do (Without ML)

⚠️ Important: Basic image processing has limitations

Without machine learning:

❌ Cannot do:

• Object recognition (identify specific objects like "person", "chair", "dog")

• Face recognition

• Text reading (OCR - though basic, possible with libraries)

• Complex scene understanding

• Semantic segmentation

✅ Can do:

• Color detection

• Shape detection (simple geometries)

• Edge/contour detection

• Motion detection

• Line following

• Marker detection (AprilTags, QR codes)

• Brightness/contrast analysis

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When to Use Machine Learning

For advanced perception, need ML models:

Options:

• Pre-trained models (YOLO, MobileNet, etc.)

  • Object detection

  • Instance segmentation

  • Pose estimation

• Custom training (TensorFlow, PyTorch)

  • Train on specific objects

  • Requires dataset collection

  • Computationally intensive

Note: ML perception is beyond this tutorial but worth exploring!

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Lighting Considerations

Camera performance varies with lighting:

Good lighting:

• Indirect, even illumination

• No harsh shadows

• Consistent throughout environment

Poor lighting:

• Direct sunlight (overexposes)

• Backlit scenes (subject too dark)

• Rapid changes (drives in/out of shadows)

• Very low light (noisy images)

Tip: Test perception algorithms in actual target lighting conditions!

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Part 10: Knowledge Check

Concept Quiz

1. Why use HSV instead of RGB for color detection?

2. What does camera calibration provide?

3. What's the difference between /image_raw and /image_raw/compressed?

4. Can basic OpenCV detect "what" an object is (e.g., identify as "cup")?

5. Why is the camera mounted forward-facing at 8.8cm height?

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Hands-On Challenge

Task: Color-based navigation system

Requirements:

1. Robot follows green markers on floor

2. Stops at red markers

3. Ignores other colors

4. Publishes cmd_vel based on vision

5. Displays annotated video showing detections

6. Records demo video

Deliverable:

• Python script with vision processing

• Video recording of robot navigating course

• Documentation of color thresholds used

• Discussion of challenges encountered

Bonus:

• Add shape detection (only respond to green circles, not rectangles)

• Implement smooth steering (PID control based on centroid error)

• Handle cases where no marker visible

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Part 11: What You've Learned

✅ Congratulations!

You now understand:

Camera Fundamentals:

• ✅ Camera specifications and mounting

• ✅ Image topics and message formats

• ✅ Viewing camera streams (rqt, RViz)

• ✅ Recording and playing back video

Image Processing:

• ✅ OpenCV basics (CvBridge, cv2 functions)

• ✅ Image filtering (blur, edge detection)

• ✅ Color detection (HSV color space)

• ✅ Shape detection (contours, polygons)

Practical Applications:

• ✅ Line following

• ✅ Obstacle classification by color

• ✅ Object tracking (centroids)

• ✅ Marker detection (AprilTags)

Advanced Topics:

• ✅ Camera calibration concepts

• ✅ Undistorting images

• ✅ Offline video processing

• ✅ Limitations of basic vision

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Next Steps

🎯 You're Now Ready For:

Immediate Next: → Localization Techniques - Combine vision with other sensors

Autonomous Navigation: → Autonomous Navigation - Vision-aided obstacle avoidance

Advanced Vision:

• Machine learning object detection (YOLO, MobileNet)

• Visual odometry (estimate motion from camera)

• SLAM with visual features (ORB-SLAM)

• Depth estimation (if adding depth camera)

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Quick Reference

Essential Camera Commands

# --- View Camera ---
ros2 run rqt_image_view rqt_image_view
# Select: /pi_camera/image_raw/compressed

# --- Check Camera Status ---
ros2 topic hz /pi_camera/image_raw    # Should be ~30 Hz
ros2 topic echo /pi_camera/camera_info --once

# --- Record Video ---
ros2 bag record /pi_camera/image_raw/compressed

# --- Launch Processing Node ---
python3 camera_processor.py

# --- View Processed Output ---
ros2 run rqt_image_view rqt_image_view
# Select: /camera/processed

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OpenCV Quick Reference

# Convert ROS → OpenCV
cv_image = bridge.imgmsg_to_cv2(msg, 'bgr8')

# Convert OpenCV → ROS
ros_image = bridge.cv2_to_imgmsg(cv_image, 'bgr8')

# Color space conversion
hsv = cv2.cvtColor(cv_image, cv2.COLOR_BGR2HSV)
gray = cv2.cvtColor(cv_image, cv2.COLOR_BGR2GRAY)

# Filtering
blurred = cv2.GaussianBlur(cv_image, (5, 5), 0)
edges = cv2.Canny(gray, 50, 150)

# Color detection
mask = cv2.inRange(hsv, lower_color, upper_color)

# Contours
contours, _ = cv2.findContours(mask, cv2.RETR_EXTERNAL, 
                                 cv2.CHAIN_APPROX_SIMPLE)

# Drawing
cv2.rectangle(image, (x, y), (x+w, y+h), (0, 255, 0), 2)
cv2.circle(image, (cx, cy), 5, (255, 0, 0), -1)
cv2.putText(image, 'Text', (x, y), cv2.FONT_HERSHEY_SIMPLEX, 
           0.6, (0, 0, 255), 2)

# Display
cv2.imshow('Window', image)
cv2.waitKey(1)

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HSV Color Ranges (OpenCV)

Color	Hue Range	Typical Sat	Typical Val
Red	0-10, 170-180	100-255	100-255
Orange	10-25	100-255	100-255
Yellow	25-35	100-255	100-255
Green	35-85	50-255	50-255
Blue	85-125	50-255	50-255
Purple	125-155	50-255	50-255

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Completed Camera-Based Perception! 🎉

→ Continue to Localization Techniques → Or return to Tutorial Index

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Last Updated: January 2026 Tutorial 9 of 11 - Advanced Level Estimated completion time: 150 minutes