The algorithm to perform discrete cosine transform is

break image into 8*8 block of pixels

from left to right top to bottom subtract 128 from each block of pixels

apply dct to each block

..

the algorithm continues however I have a few questions

1)how to break the image into 8*8 block of pixels.What is the meaning of this statement and how should i proceed about doing it?

2)subtract 128 from what value?

3)how to apply dct?

Thank you