The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).
Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).
With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.
In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".
What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.
Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.
Adding to this:
atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.
Consider:
@property(atomic, copy) NSString *firstName;
@property(atomic, copy) NSString *lastName;
@property(readonly, atomic, copy) NSString *fullName;
In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.
To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.
If you want to do it real-time in PHP, there is a way. Check out the solution presented here: it was mentioned on News.YC a month or two ago.
Pittsburgh Pattern Recognition, as you mention, seems to have some pretty good technology, and they have an SDK available: http://www.pittpatt.com/products/ftr_sdk/sdk_users_guide.html. They also have a web demo, so if you wanted to be sneaky, you could probably hack something up that would just feed images into their demo. They'd probably stop you eventually, though - better to contact them directly.
Best Answer
In General:
Face Detection: Detect the face in the image. It searches general human face like segment in the whole image. Output may be one or more than one. The output will be a rectangle or rectangles on the faces in the image.[Paul viola method]
Face Recognition: Recognize input face from the already trained database with highest match score. A single face should be given as input, and the output will be a name, or class name or unknown face. [PCA, LDA]