How to get consecutive images with 3 seconds delay

edit

Each callback is getting called with every message published on image_topic after the callback is created, so cam_callback gets all the images and overwrites img1.jpep repeatedly, then a second later cam_callback2 becomes active and starts overwriting img2.jpeg repeatedly, then both callbacks are overwriting the two different files with the same image repeatedly until the node exits.

Try putting loginfos in the callbacks to see this more clearly:

import rospy
from std_msgs.msg import Float32


def callback1(msg):
    rospy.loginfo(f"callback1: {msg}")


def callback2(msg):
    rospy.loginfo(f"callback2: {msg}")


rospy.init_node("dual_callbacks")
rospy.Subscriber("test", Float32, callback1)
rospy.sleep(3.0)
rospy.Subscriber("test", Float32, callback2)
rospy.sleep(1.0)

run that node, then run this one:

import rospy
from std_msgs.msg import Float32


rospy.init_node("increment")
pub = rospy.Publisher("test", Float32, queue_size=2)
rate = rospy.Rate(3.0)
value = 0.0
while not rospy.is_shutdown():
    pub.publish(Float32(value))
    value += 1.0
    rate.sleep()

and you'll see output like this from the callback node:

[I] [1683550340.629]: callback1: data: 11.0
[I] [1683550340.962]: callback1: data: 12.0
[I] [1683550341.295]: callback1: data: 13.0
[I] [1683550341.628]: callback1: data: 14.0
[I] [1683550341.629]: callback2: data: 14.0
[I] [1683550341.961]: callback1: data: 15.0
[I] [1683550341.962]: callback2: data: 15.0
[I] [1683550342.294]: callback1: data: 16.0
[I] [1683550342.295]: callback2: data: 16.0

If the callbacks were unregistered or ignored later messages after receiving a message the first time you would get the desired effect:

import rospy
from std_msgs.msg import Float32


def callback1(msg):
    rospy.loginfo(f"callback1: {msg}")
    sub1.unregister()


def callback2(msg):
    rospy.loginfo(f"callback2: {msg}")
    sub2.unregister()


rospy.init_node("dual_callbacks")
sub1 = rospy.Subscriber("test", Float32, callback1)
rospy.sleep(3.0)
sub2 = rospy.Subscriber("test", Float32, callback2)
rospy.sleep(1.0)