To be able to move one pixel at a time, you need to declare your world with pixel-sized cells (cellsize of 1) and multiply the width and height by 80, as well. You can still place everything into the world 80 pixels apart from each other by multiplying their placement x and y coordinates by 80 and adding 40 (to account for centering within the 'cell'. Once you do that, the Smoothmove Class will work correctly (if implemented appropriately).

I had some advantages from having a World with a bigger cell size like when my actor add an object on his location it was automatically placed on the right cell. With a one pixel world I don't know what kind of function to use to replace the object automatically

There should not be any change in the code for an actor to add an object on his location unless you are using offsets from the actors location (just multiply the offsets by the cellsize you were using, if that is the case).

Dat Philippe ^^ Anyway the point is that user should be able to place object using the mouse, but those object have specified places, using my big cells, and then should move. You could imagine a chess game where pieces move fluently and smoothly. Is There à way to do that ? Like two superposed worlds, with different cellsizes, or à special method ?

The following should 'snap' the object placed by the user into its proper location (using a imaginary cellsize of 80x80). the (n/80)*80 part gets the multiple of 80 for where it was placed; the +40 centers the object into that 80x80 imaginary cell.