Question

I need to take UIImages that are being fed in a video stream, all of this is on the iPad with limited memory, save them to the file system quickly while the stream is still feeding, then process them after a “recording” session. I need to save the UIImages coming in quickly to avoid interrupting the feed which will still be viewing on the iPad. I’m thinking of saving each frame to a separate file then afterward reading these files sequentially and combining them into a .mov file.

The tricks are: how to save the UIImages quickly, maybe raw data, then when processing the movie, append each UIImage file to it to make a seamless movie file? I will need to do some processing of each frame like scaling and transforms before appending.

Any advice would be greatly appreciated.

Answer 1

Depending on how big your images are, you could let the new coredata “use external storage” attribute do this for you.
Here is the explanation what it does copied from another answer of mine:

Since we are on IO5 now, you no longer need to write images to disk neccessarily.
You are now able to set “allow external storage” on an coredata binary attribute. According to apples release notes it means the following:

Small data values like image thumbnails may be efficiently stored in a
database, but large photos or other media are best handled directly by
the file system. You can now specify that the value of a managed
object attribute may be stored as an external record – see
setAllowsExternalBinaryDataStorage: When enabled, Core Data
heuristically decides on a per-value basis if it should save the data
directly in the database or store a URI to a separate file which it
manages for you. You cannot query based on the contents of a binary
data property if you use this option.

There are several advantages using this approach.
First coredate is saving the files at least as fast as you could when writing to the file system. But if there are any small images which apply to the conditions described above, it’ll be much faster because they will be saved directly in the coredata sqlite file.

Further with iOS 5 it is very easy possible to work on separate managed contexts and perform changes on a child context in background. If finished successfully you can merge this child context into your main managed object context and do the processing you need.

[child performBlock:^{
[childsave:&parentError]; //do this in background on child context
}];

There is a NSPrivateQueueConcurrentType for creating “child-moc” – see [apple documentation][1]

And at least you can work with coredata objects which enables you to cache, limit and optimize further processing after your download completed

[1]: https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdConcurrency.html#//apple_ref/doc/uid/TP40003385 for more info