It would be very nice to know either the exact DPI or an rough estimate of it. On a retina iPad Pro, 100 pixels is extremely small, while on an older Android phone, it is quite visible. One solution would be to draw using percentages, but a widget that is 10% height on an old Android phone looks normal, but on a huge iPad 10% would be absurdly large.

I don't see anything in the Allegro docs for this, so for now I am writing platform-specific code to determine the DPI for every platform I support, and using that to determine how large to draw things. But knowing this value would be quite useful for rendering, especially for HUD / menu drawing.