jeffbass/imagezmq

Reading the last message in the queue with ImageHub

Opened this issue · 2 comments

gabrielesartor commented

Is there a way to read directly the last image in a queue with class ImageHub?
For example, I'm receiving continuously images on the TCP channel, but the connection goes down.
After 20 minutes the connection is restored and I want to receive the images from that moment and not the previous ones.

In other words, I'd like to have a behaviour similar to a receiver using the flag CONFLATE. An example below:

context = zmq.Context()
socket = context.socket(zmq.SUB)
socket.setsockopt(zmq.CONFLATE,1)
print("Collecting updates from weather server...")
socket.connect ("tcp://*:5555")

However, it seems that this flag cannot be used with images. Any suggestions?
Thank you in advance.

MichouTest commented

Hello, I use this code to get last frame https://github.com/jeffbass/imagezmq/blob/master/examples/pub_sub_receive.py
But if you restart the sub you may have some delay ...

jeffbass commented

The above suggestion from @MichouTest is a good one and may solve your problem.

There are 2 other things that might be affecting this:

  1. ZMQ options behave differently with multi-part messages. imageZMQ sends multi-part messages (the message part and the image part). There is a good explanation in this Stackoverflow question. If you read all the answers, there is also an explanation of ZMQ option setting. It may not be possible to use imageZMQ with the zmq.CONFLATE option because of this.
  2. Most ZMQ options require that the context be closed, then the option set, then the context and socket re-opened in order for the option to be set correctly. That is not easy to do in imageZMQ and would probably require modification of the context and socket setup in the imageZMQ classes. When I need to set ZMQ options, this is how I do it. It works for me for some ZMQ options, including zmq.RCVTIMEO and zmq.SNDTIMEO. You may want to try this way of setting the zmq.CONFLATE option. I have never used the zmq.CONFLATE option, so I don't have suggestions of the best way to use it or if it would help your problem.