Multiprocessing not working while plotting

Question

Ok I am trying to traverse two paths (path1 and path2) simultaneously using multiprocessing. But the paths are not plotted together in the graph, they are plotted one after another. Please let me know how can I traverse the two paths together at the same time in the graph? Here is my code:

import os
import sys
import matplotlib.pyplot as plt
import numpy as np
from scipy import ndimage
import threading
from multiprocessing import Process
do_animation = True

def visualize_path(grid_map, start, goal, path):  # pragma: no cover
    oy, ox = start
    gy, gx = goal
    px, py = np.transpose(np.flipud(np.fliplr(path)))
    if not do_animation:
        plt.imshow(grid_map, cmap='Greys')
        plt.plot(ox, oy, "-xy")
        plt.plot(px, py, "-r")
        plt.plot(gx, gy, "-pg")
        plt.show()
    else:
        for ipx, ipy in zip(px, py):
            plt.cla()
            # for stopping simulation with the esc key.
            plt.gcf().canvas.mpl_connect(
                'key_release_event',
                lambda event: [exit(0) if event.key == 'escape' else None])
            plt.imshow(grid_map, cmap='Greys')
            plt.plot(ox, oy, "-xb")
            plt.plot(px, py, "-r")
            plt.plot(gx, gy, "-pg")
            plt.plot(ipx, ipy, "or")
            plt.axis("equal")
            plt.grid(True)
            plt.pause(0.5)

def main():
    start = [0,0]
    goal = [20,20]
    path1 = [[0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 5]]
    path2 = [[7, 15], [8, 16], [9, 17], [10, 18], [11, 19], [12, 20]]
    grid = [[0.0 for i in range(20)] for j in range(20)]
    print(grid)
    print(path1)
    print(path2)
    Process(target=visualize_path(grid, start, goal, path1)).start()
    Process(target=visualize_path(grid, start, goal, path2)).start()

if __name__ == "__main__":
    main()

user_2823719 · Answer

You are not doing multiprocessing at all. You are calling visualize_path(grid, start, goal, path1) from the main process and passing to the Process constructor its return value, which is None and only then are you executing the next Process constructor and doing likewise. What you want to do is:

    Process(target=visualize_path, args=(grid, start, goal, path1)).start()
    Process(target=visualize_path, args=(grid, start, goal, path2)).start()

Or:

    p1 = Process(target=visualize_path, args=(grid, start, goal, path1))
    p2 = Process(target=visualize_path, args=(grid, start, goal, path2))
    p1.start()
    p2.start()
    # Explicitly wait for the processes to complete:
    p1.join()
    p2.join()

But you are creating separate processes each running in their own address space and each will be creating its own separate plot -- albeit this now this will occur in parallel. But it doesn't appear that that you can have two separate processes plotting on the same plot if that is your intention.

Multiprocessing not working while plotting

1 Answers