The factors are 1.25, 1.5, and 2, as far as I know. Note that there are many different pixel densities out there and they don't follow simple integer multiples.

The factors are 1.25, 1.5, and 2, as far as I know. Note that there are many different pixel densities out there and they don't follow simple integer multiples. As for the choice of algorithm, that has nothing to do with the scaling factor.