In Python, how can I convert a RGB array to binary and get the most significant bit?

Question

Im using scipy to read a image and extract the RGB array like this

img_b = misc.imread('google.png')
img_b_blue = img_b[:, :, 0]
img_b_blue = img_b_blue.ravel()

Now I need to convert the img_b_blue array to binary and get the most significant bit.

I can convert using map:

img_b_blue_bin = map(bin, img_b_blue)

But it comes as string in the format '0b11110001'. Theres a way to using map and convert to binary without the 'b'? And how can I get the most significant bit?

Answer 1

You can get the most significant bit by right-shifting 7 bits. The result is an integer array.

img_b_blue_bin = img_b_blue >> 7

Alternatively, probably clearer in your use case, is compare with 128. Higher = white, lower = black. The result is a boolean array.

img_b_blue_bin = img_b_blue >= 128

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Similarly, the (n+1)-th least significant bit can be found using (x >> n) & 1 , eg the 2nd most significant bit:

img_b_blue_2nd_msb = (img_b_blue >> 6) & 1

the least significant bit:

img_b_blue_lsb = img_b_blue & 1

See How am I getting a single bit from an int? for how this works for a single integers.