def extractor(line,position):
bit_str = ''
for pixel in line:
for intensity in pixel:
bit_str += bits.get_bit(intensity,position)
line_str = bits.bits_to_message(bit_str)
line_n = int(line_str)
return line_n
def decode(image):
"""
Takes an image as its argument and returns a string of ASCII characters
as its result.
Decoding process:
- Iterate over each intensity value in each pixel and do the following:
- Convert intensity value of image to binary
- Index LSB of intensity value and append it to a growing list
of bits
- After this operation, a string of bits is obtained, a part of which
(or all of it) is the bitstream of the coded message.
- Decode the entire string of bits using the bit_to_message function
Inputs:
image: a two-dimensional list of pixels, in zero-based row-major
order. Each pixel is a three element tuple of integers in the
range [0,255]
Result:
A string of ASCII characters, decoded from image using steganography
scheme described above.
Examples:
>>> decode(encode(test_image,''))
''
>>> decode(encode(test_image,'hi'))
'hi'
>>> decode(encode(test_image,'hello'))
'he'
"""
bitstream = ''
for row in image:
for pixel in row:
for intensity in pixel:
# Use get_bit function from bits.py library
# to select the LSB of each intensity value
bitstream += bits.get_bit(intensity,0)
# Decode message using bits_to_message function
message = bits.bits_to_message(bitstream)
return message
def decode_ext(image, num_bits):
"""
Very slight change from original decode function;
addition of extra nested 'for' loop allows iteration over
one intensity value to extract multiple code bits, as
determined by num_bits input
"""
if not(0 < num_bits <= 8):
print ('Number of bits must be an integer between 1 and 8\
inclusive')
return None
bitstream = ''
for row in image:
for pixel in row:
for intensity in pixel:
# Extra 'for' loop so more than one bit can be
# selected from each intensity value
for pos in range(num_bits):
bitstream += bits.get_bit(intensity,pos)
message = bits.bits_to_message(bitstream)
return message
def round_trip(message):
"""
- Used to test bits_to_message and message_to_bits functions
- Should always return True
"""
return bits.bits_to_message(bits.message_to_bits(message)) == message
