Extracting HDR Gain Map from iOS 14.1+ (iPhone 12+) photos

HDRGainMap.swift

import UIKit
import MobileCoreServices.UTCoreTypes

if #available(iOS 14.1, *) {
    let input = Bundle.main.url(forResource: "IMG_0037", withExtension: "HEIC")!
    let output = FileManager().temporaryDirectory.appendingPathComponent("IMG_0037.GAIN_MAP.BMP")
    let source = CGImageSourceCreateWithURL(input as CFURL, nil)!

    // urn:com:apple:photo:2020:aux:hdrgainmap
    let dataInfo = CGImageSourceCopyAuxiliaryDataInfoAtIndex(source, 0, kCGImageAuxiliaryDataTypeHDRGainMap)! as Dictionary
    let data = dataInfo[kCGImageAuxiliaryDataInfoData] as! Data
    let description = dataInfo[kCGImageAuxiliaryDataInfoDataDescription]! as! [String: Int]
    let size = CGSize(width: description["Width"]!, height: description["Height"]!)
    
    let ciImage = CIImage(bitmapData: data, bytesPerRow: description["BytesPerRow"]!, size: size, format: .L8, colorSpace: nil)
    let cgImage = CIContext().createCGImage(ciImage, from: CGRect(origin: CGPoint(x: 0, y: 0), size: size))!
    
    let destRef = CGImageDestinationCreateWithURL(output as CFURL, kUTTypeBMP, 1, nil)!
    CGImageDestinationAddImage(destRef, cgImage, [:] as CFDictionary)
    CGImageDestinationFinalize(destRef)

    print(output)
}

Although I'm late to the party, I must say this thread is the most informative and inspiring resource on the entire internet regarding this specific topic. I've truly learned a lot from all the discussions above and want to sincerely thank every participant and contributor. Inspired by the discussion, it seems I've figured out how to use Apple's public APIs to generate a JPEG with a gain map that renders properly on both iPhone and Mac. It can also be inspected by Adobe's demo app and other third-party software like HoneyView. I've shared the code with detailed documentation, hoping it will be helpful to others: https://github.com/grapeot/AppleJPEGGainMap