iPad retina images - Why use two different image sizes?

Question

For iPad Retina (or iPhone Retina as well), why do we need to have two types of image sizes and add a suffix like @2x for the retina version?

Can\'t we just have one ty

Answer 1

Resizing takes time and memory. And resizing while retaining sharpness is hard to do. Which algorithm do you use? Bicubic or bilinear? "But this icon looks better when resized with other-algorithm-here!" How is iOS meant to know that the 1px border you drew is still meant to be a 1px border at half the resolution?