basic decoding for 3ds game note files

gamenote.py

import numpy as np
from PIL import Image
from sys import argv

# format structure:

# header
#  4 bytes seem to be a checksum of some kind? they're definitely not a timestamp
#  8 bytes are some kind of magic/ident? (10RC1000 for me)
#  4 bytes padding ?

#data
#  34560 bytes for note image data
#  1428 bytes for note thumbnail image

# note images are 320 x 216 px
# note thumbnails are 68 x 42 px

# both images use the same format: 
# bitmap; 1 nibble (4 bits) per pixel; each pixel has a palette index value

# palette index values
palette = [
  # 0 - white
  0xFFFFFFFF,
  # 1 - black
  0x313131FF,
  # 2 - red
  0xF63D39FF,
  # 3 - blue
  0x0085FEFF,
  # 4 - grey 
  0x8A8A8AFF,
  # 5 - light red
  0xFB9795FF,
  # 6 - light blue
  0x7DC1FFFF
]

class gameMemo:
  def __init__(self, filename):
    with open(filename, 'rb') as f:
      self.data = f.read()
  
  def _readImageData(self, data):
    imageData = np.fromstring(data, dtype=np.uint8)
  
    pixelData_real = np.bitwise_and(imageData, 0x0f)
    pixelData_imag = np.bitwise_and(imageData >> 4, 0x0f)
  
    pixelData = np.array([palette[j] for i in zip(pixelData_imag, pixelData_real) for j in i], dtype=">u4")
  
    return pixelData
    
  def getImage(self):
    image = self._readImageData(self.data[16:-1428])
    image = image.tobytes("C")
    
    return Image.frombytes("RGBA", (320, 216), image)
    
  def getThumb(self):
    image = self._readImageData(self.data[-1428::])
    image = image.tobytes("C")
    
    return Image.frombytes("RGBA", (68, 42), image)
    
memo = gameMemo(argv[1])
memo.getThumb().save(argv[1] + '_thumb.png', 'png')
memo.getImage().save(argv[1] + '_image.png', 'png')