The question of supporting multiple screens has been asked to death, but I haven't seen much discussion in regards to game development (with hitboxes, collision checking, etc).

Currently, my game is running in "compatibility mode" producing very poor visuals on higher end devices due to upscaling. I'm looking for tips and recommendations for what others have done to make their games graphics look good across all screen sizes.

Do developers include 2 copies of each resource (medium and high densities) or are high density resources simply scaled down for lower density devices?

Are density-independent pixels used in your calculations?