Sample code for ROS ImageMarker tutorial: https://foxglove.dev/blog/annotate-your-robots-camera-images-with-image-markers

step1.py

# Step 1: a basic ROS node that receives camera images.

import rospy
from sensor_msgs.msg import Image


def image_callback(msg: Image):
    rospy.loginfo("Got an image!")


def main():
    rospy.init_node("my_node")
    rospy.Subscriber("/usb_cam/image_raw", Image, image_callback, queue_size=1)
    rospy.spin()


if __name__ == "__main__":
    main()

step2.py

# Step 2: run the dlib face detector and log the number of faces found.

import cv2
import rospy
import dlib
from cv_bridge import CvBridge
from sensor_msgs.msg import Image


cv_bridge = CvBridge()
face_detector = dlib.get_frontal_face_detector()


def image_callback(msg: Image):
    # Convert the ROS Image to a grayscale OpenCV image
    cv_img = cv_bridge.imgmsg_to_cv2(msg)
    grayscale_img = cv2.cvtColor(cv_img, cv2.COLOR_BGR2GRAY)

    # Run the face detector on the grayscale image
    rects = face_detector(grayscale_img, 0)
    rospy.loginfo(f"Detected {len(rects)} faces")


def main():
    rospy.init_node("my_node")
    rospy.Subscriber("/usb_cam/image_raw", Image, image_callback, queue_size=1)
    rospy.spin()


if __name__ == "__main__":
    main()

step3.py

# Step 3: publish an ImageMarkerArray topic to draw a box around each face.

import cv2
import rospy
import dlib
from cv_bridge import CvBridge

from std_msgs.msg import ColorRGBA
from sensor_msgs.msg import Image
from foxglove_msgs.msg import ImageMarkerArray
from visualization_msgs.msg import ImageMarker
from geometry_msgs.msg import Point

cv_bridge = CvBridge()
face_detector = dlib.get_frontal_face_detector()


class Node:
    pub_markers: rospy.Publisher

    def __init__(self):
        rospy.init_node("my_node")
        rospy.Subscriber("/usb_cam/image_raw", Image, self.image_callback, queue_size=1)
        self.pub_markers = rospy.Publisher(
            "/usb_cam/face_markers", ImageMarkerArray, queue_size=1
        )
        rospy.spin()

    def image_callback(self, msg: Image):
        # Convert the ROS Image to a grayscale OpenCV image
        cv_img = cv_bridge.imgmsg_to_cv2(msg)
        grayscale_img = cv2.cvtColor(cv_img, cv2.COLOR_BGR2GRAY)

        # Run the face detector on the grayscale image
        rects = face_detector(grayscale_img, 0)
        rospy.loginfo(f"Detected {len(rects)} faces")

        # Display each face as a cyan rectangle
        markers = ImageMarkerArray()
        for rect in rects:
            markers.markers.append(
                ImageMarker(
                    header=msg.header,
                    type=ImageMarker.LINE_STRIP,
                    outline_color=ColorRGBA(0, 255, 255, 1),
                    points=[
                        Point(rect.left(), rect.top(), 0),
                        Point(rect.right(), rect.top(), 0),
                        Point(rect.right(), rect.bottom(), 0),
                        Point(rect.left(), rect.bottom(), 0),
                        Point(rect.left(), rect.top(), 0),
                    ],
                )
            )
        self.pub_markers.publish(markers)


def main():
    Node()


if __name__ == "__main__":
    main()

step4.py

# Step 4: run the facial landmark predictor and draw a polygon for each facial feature.

import cv2
import rospy
import dlib
from imutils.face_utils import FACIAL_LANDMARKS_68_IDXS
from cv_bridge import CvBridge

from std_msgs.msg import ColorRGBA
from sensor_msgs.msg import Image
from foxglove_msgs.msg import ImageMarkerArray
from visualization_msgs.msg import ImageMarker
from geometry_msgs.msg import Point

cv_bridge = CvBridge()
face_detector = dlib.get_frontal_face_detector()
predictor = dlib.shape_predictor("shape_predictor_68_face_landmarks.dat")

COLORS = [
    ColorRGBA(0, 0, 0, 1),
    ColorRGBA(0, 0, 255, 1),
    ColorRGBA(0, 255, 0, 1),
    ColorRGBA(0, 255, 255, 1),
    ColorRGBA(255, 0, 0, 1),
    ColorRGBA(255, 0, 255, 1),
    ColorRGBA(255, 255, 0, 1),
    ColorRGBA(255, 255, 255, 1),
]


class Node:
    pub_markers: rospy.Publisher

    def __init__(self):
        rospy.init_node("my_node")
        rospy.Subscriber("/usb_cam/image_raw", Image, self.image_callback, queue_size=1)
        self.pub_markers = rospy.Publisher(
            "/usb_cam/face_markers", ImageMarkerArray, queue_size=1
        )
        rospy.spin()

    def image_callback(self, msg: Image):
        # Convert the ROS Image to a grayscale OpenCV image
        cv_img = cv_bridge.imgmsg_to_cv2(msg)
        grayscale_img = cv2.cvtColor(cv_img, cv2.COLOR_BGR2GRAY)

        # Run the face detector on the grayscale image
        rects = face_detector(grayscale_img, 0)
        rospy.loginfo(f"Detected {len(rects)} faces")

        markers = ImageMarkerArray()
        for rect in rects:
            # Run the predictor, which returns a list of 68 facial landmarks as (x,y) points
            points = predictor(grayscale_img, rect).parts()

            # Draw a line around each face region
            for region_idx, (name, (start_idx, end_idx)) in enumerate(
                FACIAL_LANDMARKS_68_IDXS.items()
            ):
                region_points = points[start_idx:end_idx]
                # Connect the points for each region in a loop, except for the jaw
                if name != "jaw":
                    region_points.append(region_points[0])

                markers.markers.append(
                    ImageMarker(
                        header=msg.header,
                        type=ImageMarker.LINE_STRIP,
                        points=[Point(p.x, p.y, 0) for p in region_points],
                        outline_color=COLORS[region_idx % len(COLORS)],
                    )
                )
        self.pub_markers.publish(markers)


def main():
    Node()


if __name__ == "__main__":
    main()

Updated at https://gist.github.com/foxymiles/09efce7cb134b3c39fada02fff983621 to account for changes in Foxglove Studio since the post was originally written.