How to update matplotlib's imshow() window interactively?

You don't need to call imshow all the time. It is much faster to use the object's set_data method:

myobj = imshow(first_image)
for pixel in pixels:
    addpixel(pixel)
    myobj.set_data(segmentedimg)
    draw()

The draw() should make sure that the backend updates the image.

UPDATE: your question was significantly modified. In such cases it is better to ask another question. Here is a way to deal with your second question:

Matplotlib's animation only deals with one increasing dimension (time), so your double loop won't do. You need to convert your indices to a single index. Here is an example:

import numpy as np
from matplotlib import pyplot as plt
from matplotlib import animation

nx = 150
ny = 50

fig = plt.figure()
data = np.zeros((nx, ny))
im = plt.imshow(data, cmap='gist_gray_r', vmin=0, vmax=1)

def init():
    im.set_data(np.zeros((nx, ny)))

def animate(i):
    xi = i // ny
    yi = i % ny
    data[xi, yi] = 1
    im.set_data(data)
    return im

anim = animation.FuncAnimation(fig, animate, init_func=init, frames=nx * ny,
                               interval=50)

If you are using Jupyter, maybe this answer interests you. I read in this site that the emmbebed function of clear_output can make the trick:

%matplotlib inline
from matplotlib import pyplot as plt
from IPython.display import clear_output

plt.figure()
for i in range(len(list_of_frames)):
    plt.imshow(list_of_frames[i])
    plt.title('Frame %d' % i)
    plt.show()
    clear_output(wait=True)

It is true that this method is quite slow, but it can be used for testing purposes.

I implemented a handy script that just suits your needs. Try it out here

An example that shows images in a custom directory is like this:

  import os
  import glob
  from scipy.misc import imread

  img_dir = 'YOUR-IMAGE-DIRECTORY'
  img_files = glob.glob(os.path.join(video_dir, '*.jpg'))

  def redraw_fn(f, axes):
    img_file = img_files[f]
    img = imread(img_file)
    if not redraw_fn.initialized:
      redraw_fn.im = axes.imshow(img, animated=True)
      redraw_fn.initialized = True
    else:
      redraw_fn.im.set_array(img)
  redraw_fn.initialized = False

  videofig(len(img_files), redraw_fn, play_fps=30)