How does Instagram or Hipstamatic do photo filters?

How are these iphone apps making these filters work with photos? I have been curious about this for a while. Is there a library that does this?

What app do you use for Instagram filters?

If you like to keep things simple, stick to Instagram and Canva. If you want to create a film look, then try VSCO. If you love filters and advanced editing tools, your choices include Lightroom, Camera +2, and Snapseed. If you like experimenting, then consider using Afterlight.

What is hipstamatic photography?

It uses the phone's camera to allow the user to shoot photographs, to which it applies a number of software filters to make the images look as though they were taken with a vintage film camera. The user can choose among a number of effects which are presented in the application as simulated lenses, films and flashes.

This is a really cool explanation on what it takes to do a filter: http://taptaptap.com/blog/creating-a-camera-plus-fx/

As explained on the Camera+ blog.