Let's assume that your images are all .png files and they are all stored in the current working directory. The python code below will do what you want. As Ignacio suggests, using numpy along with PIL is the key here. You just need to be a little bit careful about switching between integer and float arrays when building your average pixel intensities.
import os, numpy, PIL
from PIL import Image
# Access all PNG files in directory
allfiles=os.listdir(os.getcwd())
imlist=[filename for filename in allfiles if filename[-4:] in [".png",".PNG"]]
# Assuming all images are the same size, get dimensions of first image
w,h=Image.open(imlist[0]).size
N=len(imlist)
# Create a numpy array of floats to store the average (assume RGB images)
arr=numpy.zeros((h,w,3),numpy.float)
# Build up average pixel intensities, casting each image as an array of floats
for im in imlist:
imarr=numpy.array(Image.open(im),dtype=numpy.float)
arr=arr+imarr/N
# Round values in array and cast as 8-bit integer
arr=numpy.array(numpy.round(arr),dtype=numpy.uint8)
# Generate, save and preview final image
out=Image.fromarray(arr,mode="RGB")
out.save("Average.png")
out.show()
The image below was generated from a sequence of HD video frames using the code above.
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