I'm kind of skeptical with the 'custom software' part.. I agree that due to its timing (as sjgadsby pointed out) that any software they use around that time would have to be 'custom' by nature; but they would not need much 'custom features' to create this feature.

It could only be a custom build of the official camera app.

The only non conventional 'custom feature' that they need may be translucent/flip-book like feature. But that is not mandatory either, due to all the rigs they use and post processing capabilities they already have.