That is because you haven't defined CELL_SIZE. You need to define CELL_SIZE, yourself.

In article, I didn't define the CELL_SIZE as other images may have different cell sizes. It is assumed here that, you would know the value and code it yourself.

In case you don't know the value, you'll have to figure out a way to find the size of each cell.

In the end, I'd advice you to undertand my code and write your own version of code, as it would make you capable to create robust solutions for this problem and your future endeavours.