For the iOS app I am writing I want to incorporate some aspects of ambient occlusion for photo manipulation. Can someone suggest a GLSL approach that will work. Realtime performance is not super critical. Under a second compute time is fine.
Generally, ambient occlusion requires 3D data to be calculated. If you are planning to add some depth information then I would suggest the use of screen space ambient occlusion; SSAO.
Using screen space ambient occlusion should fit better into your pipeline. Since you want to manipulate photos, I presume you already have a method for adding post process filters.
What you will need to do is render the 3D data to a depth buffer. There are many sources which detail how to do this. If you can't find any, try looking up deferred shading techniques. These rely on filling a depth buffer for post processing so there should be a good example out there for you.