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@kiding
Last active February 17, 2025 22:52
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Extracting HDR Gain Map from iOS 14.1+ (iPhone 12+) photos
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)
}
@alexfoxy
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@rogino Yeah, no dice with that option unfortunately. I think that the AVCapturePhoto has no HDRGainMap for some reason, I assume it's some configuration issue with the AVCaptureSession, but with no documentation it's pretty hard to work out. The other option is that capturing HDRGainMaps is not a public API, has anyone managed to capture images with a HDRGainMap outside of the default camera app?

One other query I had, do ProRAW images have an HDRGainMap, or only HEIC?

Thank you for the help!

@rogino
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rogino commented Jan 17, 2025

Hmmm, I’m still leaning towards the photo having HDR metadata but it not being read for some reason. I’m running out of ideas though.

If you haven’t already, can you try watch this video and use the sample app to see if the photos captured in your app have HDR data?
https://developer.apple.com/videos/play/wwdc2023/10181/
(They don’t have screenshots of the sample app and I’m on my phone so I might have linked to the wrong one)

@alexfoxy
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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:

Screenshot 2025-01-19 at 16 03 03

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