The beginning of this documentation showed a screenshot of a Mac computer displaying simultaneous video streams from 8 Raspberry Pi cameras:
This display was generated by running timing_send_jpg_buf.py on 8
Raspberry Pi computers with PiCamera modules and running
timing_receive_jpg_buf.py on a single Mac. Here is a more detailed
description of the test programs and how they work.
The Raspberry Pi computers send images and the Mac computer displays them. The imagezmq python classes transport the images between the computers. There can be a single Raspberry Pi sending images, or there can be 8 or more. There is always exactly one computer (a Mac in this example) receiving and displaying the images. In ZMQ parlance, the Raspberry Pi computers are acting as clients sending requests (REQ) and the Mac is acting as the server sending back replies (REP). Each "request" from the Raspberry Pi is a 2 part message consisting of an OpenCV image and the Raspberry Pi hostname. Each "reply" sent by the Mac message is an "OK" that tells the Raspberry Pi that it can send another image.
ZMQ is a powerful messaging library that allows many patterns for sending and receiving messages. The ZMQ pattern used by imagezmq is the REQ/REP pattern. That means that every time a Raspberry Pi sends an image, it waits for an "OK" from the Mac before sending another image. It also means that there can be multiple Raspberry Pi computers sending messages to the Mac at the same time, since the ZMQ REQ/REP pattern allows many clients to send REQ messages to a single REP server. With each image sent, the Raspberry Pi sends an identifier (the Raspberry Pi hostname, in these test programs), so that the Mac can display the images from each Raspberry Pi in a different window.
Let's look at the Python code in the Raspberry Pi sending program:
1 # run this program on each RPi to send a labelled image stream
2 import socket
3 import time
4 from imutils.video import VideoStream
5 import imagezmq
6
7 sender = imagezmq.ImageSender(connect_to='tcp://jeff-macbook:5555')
8
9 rpi_name = socket.gethostname() # send unique RPi hostname with each image
10 picam = VideoStream(usePiCamera=True).start()
11 time.sleep(2.0) # allow camera sensor to warm up
12 while True: # send images as stream until Ctrl-C
13 image = picam.read()
14 sender.send_image(rpi_name, image)Lines 2 to 5 import the Python packages we will be using. Line 7 instantiates an ImageSender class from imagezmq. Line 9 sets rpi_name to the hostname of the Raspberry Pi. This will keep each Raspberry Pi's image stream in a separate window on the Mac (provided that each Raspberry Pi has a unique hostname). Line 10 starts a VideoSteam from the PiCamera. Line 11 allows the PiCamera sensor to warm up (if we grab the first frame from the PiCamera without this warmup time, it will fail with an error). Line 12 begins a forever loop of 2 lines: Line 13 reads a frame from the PiCamera into the image variable. Line 14 uses imagezmq's send_image method to send the Raspberry Pi hostname and the image to the Mac. These "read and send" lines repeat until Ctrl-C is pressed to stop the program. This effectively sends a continuous stream of images (up to 32 images per second) to the Mac, with each image labeled with the hostname of the Raspberry Pi that is sending it. If there are multiple Raspberry Pi computers sending images at the same time, the Mac receiving the images is able to sort them to labelled windows because of the unique Raspberry Pi hostname sent with each image.
Now, lets look at the Python code on the Mac (or other display computer):
1 # run this program on the Mac to display image streams from multiple RPis
2 import cv2
3 import imagezmq
4
5 image_hub = imagezmq.ImageHub()
6 while True: # show streamed images until Ctrl-C
7 rpi_name, image = image_hub.recv_image()
8 cv2.imshow(rpi_name, image) # 1 window for each RPi
9 cv2.waitKey(1)
10 image_hub.send_reply(b'OK')Lines 2 and 3 import the Python packages we will be using: cv2 (OpenCV) and
imagezmq. Line 5 instantiates an ImageHub class from imagezmq.
Line 6 begins a forever loop: line 7 receives an rpi_name and an image
from imagezmq's recv_image method. Line 8 shows the image in a display
window with a window title of rpi_name. Line 9 waits for a millisecond,
then line 10 sends the required "reply" back to the Raspberry Pi per the ZMQ
REQ/REP pattern. Lines 9 and 10 repeatedly receive and display images as they
come in. The cv2.imshow() method displays each image received in a window
corresponding to the window name. If all the images come from a single
rpi_name, then all the image streams will appear in a single window. But if
the income stream has images from multiple rpi_name's, then cv2.imshow()
automatically sorts the images by rpi_name into unique windows. Thus, if
3 Raspberry Pi computers are sending images, the images will be displayed in
3 separate windows with each one labelled by its rpi_name. The ZMQ library
is fast enough to make these 3 streams of images appear as 3 continuous video
streams in separate windows. To create the picture at the top of this page, 8
Raspberry Pi computers were sending images to a single Mac. The picture is a
screenshot of the Mac's display with the 8 cv2.imshow() windows arranged
in 2 rows.