How to store a very large image on an iPhone/iPad

What's the best way to store a very large image for an iOS app? I want an app to be able to view images that might be hundreds of megabytes, perhaps as much as a gigabyte as jpeg. I need to be able to store the image and retrieve selected areas for display.

Currently the images are cut into 512x512 pixel tiles and stored as jpeg files in a directory tree with tens of thousands of tiles (actually an image pyramid including downsamples).

Ignoring the question of displaying the image, I'm interested in the most efficient, manageable way to store this data on the device: files, like they currently are, in an sqlite database or something else?

Second part to the question. Is there a limit to the amount of data an app can store, or can a开发者_运维技巧n app keep importing data up to the storage limit of the device. I'm asking here about data that an app imports after it's installed.

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The solution to this is to pre tile the enormous image so the tiles can be quickly retrieved from the file system on an as needed basis. One problem with very large images is that most solutions require the whole image to be rendered into a context, consuming vast amounts of memory. On a system like iOS, where memory is limited, the way to solve this is to use a library like libjeg or libjpegturbo to render an image a line at a time, then save the pixels into a raw file. The downside to doing this directly is that when you need one tile, you need to jump all over the file system finding each row of a tile. Thus a better solution is to not only incrementally scan, but incrementally tile too. You can use mmap to map the file into just the area you need, so you can really minimize memory consumption. That said, you can thrash the Unified Buffer Queue on iOS so badly the app crashes, or even the whole system!

If you are curious about how to implement the above solution, there is a freely available project on github - PhotoScrollerNetwork - that does all the above.

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A sample from Apple: PhotoScroller

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What about splitting into parts. Then it can be gathered by your application if needed