Division RGBA array c++

I need to do filters for images. Library where I need to do this return me array of chars(char*). I write my class ,where I implement operator[][], but I didn`t know how can I do well division for layers(red,green , blue) and work with them. Different filters do different things with layers. For Example one do image red .And I know that I can write cycle in every filter, but maybe exist some another way without repeat cycles ,maybe any pattern ? Help,please.