为什么 ROS 发布者不发布第一条消息？

我有一个发布者发布了两个主题为“图像”和“深度”的图像，一个订阅者正在收听这两个主题。

发布者从两个文件夹中读取图像并在同一个循环中发布这两个文件夹。每个图像都有一个对应的深度，并且这两个映射到相同的名称。发布率为1hz。

订阅者没有得到第一对图像。我试图转储订阅文件夹的图像。除了第一对图像之外，所有发布的图像都被转储了。

这是发布者的代码

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


def talker():

    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(1) # 1hz
    bridge = CvBridge()
    hello_str = "hello world %s" % rospy.get_time()
    rospy.loginfo(hello_str)
    path = "/home/test_img/"
    imglist = os.listdir(path)
    path_depth = "/home/out_depth/"


    for i in range(0,len(imglist)):
        topic = 'image'
        print(topic)
        pub = rospy.Publisher(topic, Image, queue_size=10)
        fn = path+imglist[i]
        print(fn)
        img = cv2.imread(fn)
        imgMsg = bridge.cv2_to_imgmsg(img, "bgr8")
        pub.publish(imgMsg)




        topic = 'depth'
        print(topic)
        pub = rospy.Publisher(topic, Image, queue_size=10)
        fn = path_depth+imglist[i]
        print(fn)
        img = cv2.imread(fn)
        imgMsg = bridge.cv2_to_imgmsg(img, "bgr8")
        pub.publish(imgMsg)
        rate.sleep()




if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

这是订阅者的代码

import rospy
import numpy as np
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
import cv2
from datetime import datetime


def callback(data):


    bridge = CvBridge()
    # Convert your ROS Image message to OpenCV2
    cv2_img = bridge.imgmsg_to_cv2(data, "bgr8")
    cv2.imwrite('out_rgbd/'+datetime.now().strftime("%I:%M%S%f")+".jpg", cv2_img)

def listener():

    # In ROS, nodes are uniquely named. If two nodes with the same
    # name are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('listener', anonymous=True)



    topic = 'image'
    print(topic)
    rospy.Subscriber(topic, Image, callback)

    topic = 'depth'
    print(topic)
    rospy.Subscriber(topic, Image, callback)

    # spin() simply keeps python from exiting until this node is stopped
    rospy.spin()

if __name__ == '__main__':
    listener()

在执行发布者时，会列出所有图像（来自 print(fn) 行）。但是订阅者没有得到第一对图像。

我试过使用“rosrecord”记录消息。它也没有获得第一对。

原因可能是什么？