alt Hacker NewsDPI (or PPI) is an absolute measurement. Scale factor is intentionally relative. Different circumstances will want to have different scale factor : dpi ratios; most software do not care if certain UI element is exactly x mm in size, but instead just care that their UI element scale matches the rest of the system.
Basically scale factor neatly encapsulates things like viewing distance, user eyesight, dexterity, and preference, different input device accuracy, and many others. It is easier to have human say how big/small they want things to be than have gazillion flags for individual attributes and then some complicated heuristics to deduce the scale.
I disagree, I don't want a relative metric. You're saying scale factor neatly encapsulates viewing distance, eyesight, preference, but compared to what? Scale is meaningless if I don't have a reference point. If I have two different size monitors you have now created a metric where a scale of 2x means something completely different. So to get things look the same I either have to manually calculate DPI or I have to manually try and error until it looks right. Same thing if I change monitors, I now have to try until I get the desired scale, while if I had DPI I would not have to change a thing.
> It is easier to have human say how big/small they want things to be than have gazillion flags for individual attributes and then some complicated heuristics to deduce the scale.
I don't understand why I need gazillion flags, I just set desired DPI (instead of scale). But an absolute metric is almost always better than a relative metric, especially if the relative point is device dependent.