wireshark - USB mapping with python -

while reading ctf write-ups came across script

#!/usr/bin/env python import struct import image import dpkt  init_x, init_y = 100, 400  def print_map(pcap, device):     picture = image.new("rgb", (1200, 500), "white")     pixels = picture.load()       x, y = init_x, init_y      ts, buf in pcap:         device_id, = struct.unpack("b", buf[0x0b])          if device_id != device:             continue          data = struct.unpack("bbbb", buf[-4:])          status = data[0]         x = x + data[1]         y = y + data[2]          if (status == 1):             in range(-5, 5):                 j in range(-5, 5):                     pixels[x + , y + j] = (0, 0, 0, 0)         else:             pixels[x, y] = (255, 0, 0, 0)     picture.save("riverside-map.png", "png")  if __name__ == "__main__":      f = open("usb.pcap", "rb")     pcap = dpkt.pcap.reader(f)     print_map(pcap, 5)     f.close() 

and when run on usb.pcap error:

 traceback (most recent call last):       file "test.py", line 39, in <module>         print_map(pcap, n)       file "test.py", line 31, in print_map         pixels[x, y] = (255, 0, 0, 0)   indexerror: image index out of range 

why happening?

depending on dataset in usb.pcap file, may need adjust init_x , init_y variables. problem struct.unpack returns signed value, if data on 127 appears negative , exceeding array boundaries. if data positive, can test , force positive value. like:

data = [item + 256 if item < 0 else item item in data]