@digitalfx @amyworrall DataProviders are one of the avenues I’d explore:
1. Look into CGDataProvider to check if there’s any explicit support for random-access, non-buffered backing.
2. Memory-mapping the file may be an option? This will page in data upon request. (Mac only, not iOS!)
Both depend on whether CG’s TIFF reader tries to consume everything at once.

3. Investigate third-party readers for the file format; some probably support such use cases explicitly.

@pmdj @digitalfx it's GIS I'm doing :)

Essentially I have my own map rendering software. I'm currently working on a heightmap (in Photoshop), and I want to keep it high res in order to be able to crop bits out of it later. But I need my software to be able to read subsets of it in order to handle re-projecting it.

@pmdj @digitalfx Yeah, for this case I don't necessarily need it as a CGImage. (Actually most of my pipeline is CIImages anyway. But this particular operation doesn't need to behave like the rest of what I'm doing.)

At the moment the app is just for me, so no security issues. It may turn into a product one day, but I'd have a LOT of work to do before I'm happy with someone else using it! When it's just me, I can tolerate janky bits :)