I attach the image I want to work with.
I define with red vertical lines the width or the space of the area I want to measure the total number of pixels for every column (e.g. cumsum command). The columns will contain the total number of pixels of only the distance between the top white pixel of the curve and the last bottom white pixel. No beyond the two margins. Only this distance. From top white pixel to bottom white pixel, all pixels included in this black area, and only for the space between those two red vertical lines that denote the first and last white pixels of the white little curve line bottom structure.
I am sure it is not hard. Just a bit fuzzy and complicated but other than that I know it can be done.
Thanks for the time spending reading my query.