Python: Can not reshape numpy.array

Question

Im using LSB to embed a message in an image

Idea: for example: message in bit is: 01010 then i will all 100... to this message -> new message: 010101000....

Here is my code:

# num_lsbs: number of bits replaced: 1bit or 2bits, ...
# cover_image_file: original image
# stego_image_file: image file after embeded

def lsb(cover_img_file, msg_file, num_lsbs, stego_image_file):
    #1. Read cover_img_file 
    cover_img = imread(cover_img_file) # cover_img: numpy 

    #2.1 Read msg_file
    file_obj = open(msg_file, 'r')
    msg = file_obj.read()
    file_obj.close()

    #2.2 Conver msg to msg_bits
    msg_bits = bitarray.bitarray()
    msg_bits.fromstring(msg)

    #2.3 Add 100... to msg bit
    max_num_bits = num_lsbs * cover_img.size 
    # Check if there are enough spaces
    if (len(msg_bits) > max_num_bits - 1):
        print "Not enough spaces to embed"
        return

    msg_bits.extend('1' + '0' * (max_num_bits - len(msg_bits) - 1)) 

    #2.4 Convert msg_bits to msg img
    str01 = msg_bits.to01()

    msg_img = np.array ([int(str01[i:i + num_lsbs], 2) for i in range (0, len(msg_bits),
                                                               num_lsbs)], dtype = np.uint8)
    msg_img.reshape(cover_img.shape)
    print '\n\n--------Check shape--------\n'
    print cover_img.shape, cover_img.dtype
    print msg_img.shape, msg_img.dtype

              ^
              |
              |
        #<<I can not reshape here>>
    #3. Embed msg img to cover img
    stego_img = ((cover_img >> num_lsbs) << num_lsbs) + msg_img

    ........

I can not reshape my msg_img I want its shape is like cover_img shape

Here is the error:

<ipython-input-2-82ce863f48cb> in embed_lsb(cover_img_file, msg_file, num_lsbs, stego_image_file)
     41 
     42     #3. Embed msg img to cover img
---> 43     stego_img = ((cover_img >> num_lsbs) << num_lsbs) + msg_img
     44 
     45     #4. Save the stego img file

ValueError: operands could not be broadcast together with shapes (414,500,3) (621000,)

Can anyone help me Thank in advance!!

print your shape check messages before attempting to reshape, it should help you understand what's wrong. — Julien
– Julien, Commented Oct 22, 2015 at 4:30

Julien · Accepted Answer · 2015-10-22 06:32:45Z

2

Ok. So your reshaping didn't work because you are using reshape() which creates a new (reshaped) array, which is then forgotten. What you need to do is:

msg_img = msg_img.reshape(cover_img.shape)

or better:

msg_img.shape = cover_img.shape

edited Oct 22, 2015 at 6:32

answered Oct 22, 2015 at 5:56

Julien

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