为什么ROS发布者不发布第一条消息？

Question

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

发布者从两个文件夹中读取图像，并在同一循环中发布它们。每个图像都有对应的深度，并且这两个图像以相同的名称映射。发布速度为1hz。

订户没有获得第一对图像。我试图转储订阅到文件夹的图像。除第一对图像外，所有发布的图像都将被转储。

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

这是订户的代码

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()

执行发布者时，将列出所有图像（来自print（fn）行）。但是订户没有得到第一副图像。

我尝试使用'rosrecord'记录消息。它也没有获得第一对。

可能是什么原因？