One of the "special" options in your homepage is the "flat bottom" -- don't the tubs normally come with flat bottoms? Or do they normally come with something on them?
Japanese traditional tubs have a flat bottom.
Anyway due to requirements and "commonsense practice" of many countries we are building the tubs with a slight gradient towards the drain.
This is to avoid the small water deposit that would stay even after you drained the tub.
Please note that we can apply the gradient only if the drain hole is towards the edge of the tub (not in the center)
Kobe Kazamidori chapter
japan@bartokdesign.com