Rendering a big image on iOS is one of those things that looks trivial until your app gets killed by the OS mid-export. CGContext, draw, makeImage(), done — except the moment the output gets large, that innocent-looking pipeline quietly asks for gigabytes of RAM and iOS terminates you.
I hit this wall building Mozary, an iOS app that packs 100+ photos into a single giant picture (a photo mosaic). In v1.1.0 I finally killed the "high-resolution export crashes with out-of-memory" bug for good.
The fix: stop putting the canvas in RAM at all. Put it in a memory-mapped file and let the OS page it to disk. RAM usage dropped from "4.8 GB, please die" to a few dozen MB, flat, regardless of output size.
This post is the walkthrough — with the actual Swift. If you've ever seen Core Graphics blow up on a big image (mosaics, collages, stitched panoramas, high-res rendering — same trap), this is for you.
TL;DR
