how to convert a grayscale image to a sequence of bytes

how do i convert the image into a sequence of bytes and then get the hash value using SHA3 256 bits.

Answer 1

[Image Analyst]

For the first question:

sequenceOfBytes = grayImage(:);

[Walter Roberson]

Or alternately, in case it is not already uint8:

sequenceOfBytes = typecast(grayImage, 'uint8');