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)
}

Just to report back on this. I have managed to get the HDRGainMap from a AVCapturePhoto with the following:

let data = photo.fileDataRepresentation()!
let gainMap = CIImage(data: data, options: [.auxiliaryHDRGainMap : true])

The key thing was that the capture device was configured correctly. From an Apple DTS Engineer:

Make sure your capture device's activeFormat reports true for isHighestPhotoQualitySupported. (or use the ".photo" sessionPreset on the capture session).

The last hurdle I'm encountering is that this still returns nil on an iPhone 13. I'm assuming the older hardware does not support it, but it's strange because I can access some sort of HDRGainMap via the CGImageSourceCopyAuxiliaryDataInfoAtIndex(source, 0, kCGImageAuxiliaryDataTypeHDRGainMap)! as Dictionary method with a photo captured by the default camera app on the iPhone 13. It does appear different to the iPhone 16 Pro however, so I assume there is some difference in hardware output. See below:

I found this while searching for something else, but like to add a comment which may help others hitting the following.

In case anyone is trying this and just gets nil instead of the gainmap:
let gainmap = CGImageSourceCopyAuxiliaryDataInfoAtIndex(source, 0, kCGImageAuxiliaryDataTypeHDRGainMap)! as Dictionary

Try this as most HDR JPEGs today are already using the more standard ISO gainmap format:
let isogainmap = CGImageSourceCopyAuxiliaryDataInfoAtIndex(source, 0, kCGImageAuxiliaryDataTypeISOGainMap)! as Dictionary