Fast capture and 2D image stacking as 3D numpy array with Python and Raspberry Pi

[Agustin Cruz]

I'm working on a Python - Raspberry Pi project in which I need to take about 30 
images per second (no movie) and stack each 2D image to a 3D array using numpy 
array, without saving each 2D capture as a file (because is slow).

I found this Python code to take images as fast as possible, but i don't know 
how to stack all images fast to a 3D stack of images.

import io
import time
import picamera
#from PIL import Image

def outputs():
    stream = io.BytesIO()
    for i in range(40):
        # This returns the stream for the camera to capture to
        yield stream
        # Once the capture is complete, the loop continues here
        # (read up on generator functions in Python to understand
        # the yield statement). Here you could do some processing
        # on the image...
        #stream.seek(0)
        #img = Image.open(stream)
        # Finally, reset the stream for the next capture
        stream.seek(0)
        stream.truncate()

with picamera.PiCamera() as camera:
    camera.resolution = (640, 480)
    camera.framerate = 80
    time.sleep(2)
    start = time.time()
    camera.capture_sequence(outputs(), 'jpeg', use_video_port=True)
    finish = time.time()
    print('Captured 40 images at %.2ffps' % (40 / (finish - start)))

Does anyone of you know how to stack the 2D images taken in this code to a 3D 
numpy array using Python and the Raspberry Pi camera module? Without saving 
each 2D capture as a file

Best regards, Agustín
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