Dissolve (filmmaking)

Example Challenge

VolgaCTF 2020 - Dissolved (https://ctftime.org/writeup/19157)

from BitVector import BitVector
from PIL import Image
import numpy as np

img  = Image.open("stego.png")
data = np.array(img.getdata()) # get data as np array
mask = data[:,3]!=255          # only pixels where alpha isn't max     
data = data[mask]              # apply mask
data = data[:,2]&1             # only get green LSB
BitVector(bitlist=data).get_text_from_bitvector()