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)
}

The HDR effect does not show up properly in Apple Photos on MacOS.

From what I observe, it seems that the macOS Photos app gets the render formula wrong.
I am able to reproduce a similar result by deliberately changing * to + somewhere in the formula.

Therefore, I guess even Apple engineers themselves don’t have documentations about the exact design.

Any idea what metadata is missing?

It might be the HDRGainMapVersion that's missing. From Apple's documentation:

Before you begin, check whether the image’s metadata contains the HDRGainMapVersion key. If this key exists in the metadata, the gain map is valid and you can continue calculating the headroom.

I did not know about this piece of metadata when I was creating the sample image. It seems, on macOS, this is actually required for Photos to recognize the image as HDR.

I stopped playing with this script. Adobe Lightroom Classic got the ability to edit HDR images recently, and I got a mirrorless camera Canon EOS R6 Mark II which is capable of shooting 10-bit HEIF. This fact appeared to be useless for me — the effect is barely visible on my iPhone 12 PM but it is present. Old good camera RAW (or multiple shots of a single scene) is still a valid starting point. I don't have nor Apple XDR display, neither Apple Silicon-based MBP with the XDR display, or an iPad Pro 12.9 with the XDR display in order to edit images in the HDR range. However, I have the Dell U3223QE display that is HDR400 certified, and yes, I saw the difference there in HDR mode on the display and enabled HDR in macOS. Though it's not sufficient for confident editing workflow, just a chance to try.

One more thing I'd like to add is that HEIF is now dead for me as a file format. I didn’t manage to wait for its wide spreading. Apple finally added AVIF support in macOS Sonoma, and Adobe Lightroom Classic is able to export HDR images to AVIF. That's enough for my current needs.

Adobe, Android 14, Chrome, Edge, Brave, and Opera all support a JPG gain map standard published by Adobe/Google. That's the ideal solution now (already widely supported, backwards compatible with any viewer, can export with control over the SDR rendition via Lightroom / Adobe Camera RAW, and the spec is part of a draft ISO specification). https://gregbenzphotography.com/hdr-images/jpg-hdr-gain-maps-in-adobe-camera-raw/

Longer term, an AVIF gain map will be even better once the key pieces are in place to support encoding and viewing (this already has support in Chrome under a dev flag).

@andsimakov Shoot RAW and process for HDR. The results are much better than capturing directly as HEIF. I'd only shoot that format for very narrow situations such as sports (ie, the same scenarios where you would use JPG over RAW now for SDR). The HDR AVIF export from Lightroom is great for sharing in the Photos app on Apple devices, but the JPG gain map is better for the web (as it gives you significant control over the SDR rendition, rather than using automatic tone mapping - and it offers much better support for FireFox and Edge which don't support HDR AVIF at this time).

@gregbenz thank you for that link. It seems I missed that article. It looks interesting! Yes, JPEG + Gain Map is now the most versatile solution if it is supported by something other than Apple XDR displays and OSes before them. I didn't know about the FireFox browser. My use case is quite narrow. It's image sharing from Apple iCloud to Apple Photos across Apple devices of my family + Apple TV. My most important goal towards new formats — the thing I wanted to get from HEIF and finally got from AVIF — compressing efficiency. I just want to save regular SDR 8-bit files in AVIF in order to save space in Apple iCloud and to postpone upgrading to the next more expensive tier. 10 bits and EDR/HDR is the next priority for me.

@andsimakov Eventually, I would expect widespread adoption of AVIF and gain maps. For now, JPG gain maps in the browser and Android 14, and HDR AVIF on iOS (or browse in LR iOS).

I'm not aware of a way to show HDR photos on Apple TV, but I haven't yet tested to see if HDR AVIF works there too with the latest TV OS. You cannot use AirPlay 2. HDMI output from at least iPhone 15 should work, but I haven't confirmed (definitely works from MacOS).

AVIF is getting really close to being a reasonable mainstream standard for SDR (MS Edge just needs to drop the dev flag and we should be in great shape for generally widespread usage). AVIF also offers other SDR benefits: transparency and reduced risk of banding (10-bit SDR).

My buddy views my photos from a shared album on Apple TV and Philips OLED TV and enjoys the luma effect on some of them.

@andsimakov Are you seeing HDR with HEIC? I just tried this morning using a shared Photos album with this week's latest iOS / TVOS, and as soon as a valid HDR AVIF is added to a shared album, it loses its magic (looks like clipped SDR). So I've lost the HDR before I could even open the photo on Apple TV. AirPlay doesn't work either for HDR. I've just been bypassing the Apple TV and using an HDMI connection to the TV to show photos from a computer.

@gregbenz oh, I just have to trust his words. I showed him pairs SDR/EDR images in the same album, and he told there was a difference. I cannot confirm it on my own, because I still don't have OLED TV 🤷🏻‍♂️ But I have to add those images were produced with Radiance+ iOS app. Lightroom wasn't capable of editing HDRs at that time yet. So my words are irrelevant for HDR AVIFs from Lightroom.

@andsimakov Gotcha. If he saw both, probably worked. Apple has supported EXR for a while, but I don't recall Radiance working. Will have to try that.

I can't believe I managed to resurrect such an old thread on something so esoteric. Bravo! I love this community!

Any idea what metadata is missing?

It might be the HDRGainMapVersion that's missing. From Apple's documentation:

Before you begin, check whether the image’s metadata contains the HDRGainMapVersion key. If this key exists in the metadata, the gain map is valid and you can continue calculating the headroom.

I did not know about this piece of metadata when I was creating the sample image. It seems, on macOS, this is actually required for Photos to recognize the image as HDR.

@frankschlegel Thanks for the pointer! I poked around and as usual with Apple, the documentation is maddening. There's literally no mention of it in any Apple docs besides the one you linked. I even greped all the headers and there's literally only these two:

$ rg -t h -i hdrgain
Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/CoreImage.framework/Headers/CIImage.h
188:CORE_IMAGE_EXPORT CIImageOption const kCIImageAuxiliaryHDRGainMap NS_AVAILABLE(11_0, 14_1);

Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/ImageIO.framework/Headers/CGImageProperties.h
817:IMAGEIO_EXTERN const CFStringRef kCGImageAuxiliaryDataTypeHDRGainMap   IMAGEIO_AVAILABLE_STARTING(11.0, 14.1);

# (Identical copies in iPhoneSimulator, TVOS, etc).

The only other mentions of HDRGainMapVersion on Google are related to this thread - it's about adding it to the XMP in the auxiliary gain map thread on exiftool. I poked around and found that HDRGainMapVersion is an integer, either 65536 (0x10000) as in that thread, or 131072 (0x20000) in some newer files. The stored format 1278226488 is the integer value of kCVPixelFormatType_OneComponent8, which makes sense since the gain map is a JPEG that decodes to an 8-bit grayscale image encoded with a Rec. 709 transfer function.

Any luck on your side getting Apple Photos to recognize a bespoke image?

Adobe, Android 14, Chrome, Edge, Brave, and Opera all support a JPG gain map standard published by Adobe/Google. That's the ideal solution now (already widely supported, backwards compatible with any viewer, can export with control over the SDR rendition via Lightroom / Adobe Camera RAW, and the spec is part of a draft ISO specification). https://gregbenzphotography.com/hdr-images/jpg-hdr-gain-maps-in-adobe-camera-raw/

Longer term, an AVIF gain map will be even better once the key pieces are in place to support encoding and viewing (this already has support in Chrome under a dev flag).

@andsimakov Shoot RAW and process for HDR. The results are much better than capturing directly as HEIF. I'd only shoot that format for very narrow situations such as sports (ie, the same scenarios where you would use JPG over RAW now for SDR). The HDR AVIF export from Lightroom is great for sharing in the Photos app on Apple devices, but the JPG gain map is better for the web (as it gives you significant control over the SDR rendition, rather than using automatic tone mapping - and it offers much better support for FireFox and Edge which don't support HDR AVIF at this time).

@gregbenz I agree with you completely. I wrote several JPEGs using the Adobe/Google format and it works well in Lightroom/Chrome/Android, as expected. But as the others on this thread have pointed out, Apple doesn't yet support it and we have no idea when they will. And most of my friends and family only use Apple devices so it'd be nice to have a format that works for them on say, iMessage or Safari.

AVIF seems to be only semi-working on both Apple and Google phones, so I wouldn't count on it as a universal solution for a while.

Any luck on your side getting Apple Photos to recognize a bespoke image?

@jiawen Indeed. It took me a while, but I was able to generate another test image that also works on macOS:

The following was needed to make it work:

• In the XMP metadata, the HDRGainMapVersion key needed to be present. (I set it to 65536)
• A gain map auxiliary image of type urn:com:apple:photo:2020:aux:hdrgainmap.
• The {MakerApple} metadata tags 33 and 48 need to have meaningful values according to Apple's documentation. (I set them to 0.8 and 0.0 for maximum effect.)
• And, new to me, the image must be larger than 360 pixels in each dimension. For smaller images, the HDR effect simply doesn't show. This took me the longest to find out. 😅

The first and last point were needed on macOS, but iOS displayed HDR even without them.

I also found, on both platforms, that only Photos is capable of displaying gain map HDR images. The HDR effect appears nowhere else, neither in Messages, Preview, Files, etc.

either 65536 (0x10000) as in that thread, or 131072 (0x20000) in some newer files

Hu, I haven't seen 131072 yet. Do you have an example of where you've seen it?

@jiawen Did you write code to do that, or just use software like Adobe LR / ACR? ImageMagick has an open request for contributors who may be able to help support gain maps. ImageMagick/ImageMagick#6377. ImageMagick support would be key to support for gain maps in the WordPress media library, etc.

Another resource for this group (if the interest isn't just limited to HEIC / Swift) is the HDR JPG gain map library published by Google: https://github.com/google/libultrahdr. This is built to the Google / Adobe gain map spec (which is the basis of a draft ISO specification and is a de facto standard with support in Lightroom, Adobe Camera RAW, Chrome, Edge, Brave, and Opera already).

Hi @frankschlegel - this is amazing! I confirmed that your JPEG renders as HDR in Apple Photos my iPhone 15 Pro Max (running iOS 17.1) and M1 Macbook Pro (running macOS Sonoma). Unsurprisingly, it does not render correctly in Apple Photos on my M1 Mac Studio running macOS Ventura when connected to an Apple XDR display (you see a difference if you toggle HDR on/off in Preferences. But it doesn't show the HDR text. It only shows a flat field of bright white vs less bright white.). Surprisingly, it does render as HDR in Chrome (v118)! Even on Ventura. It looks like Google decided to parse one of the fields, probably the XMP you added.

Amazing!

Hu, I haven't seen 131072 yet. Do you have an example of where you've seen it?

I got the following in the aux image's XMP on a JPEG taken with an iPhone 15 Pro's rear camera. I extracted it using exiftool -MPImage2 -b my_image.jpg > aux.jpg followed by exiftool -xmp -b aux.jpg.

<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 6.0.0">
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
      <rdf:Description rdf:about=""
            xmlns:HDRGainMap="http://ns.apple.com/HDRGainMap/1.0/"
            xmlns:apdi="http://ns.apple.com/pixeldatainfo/1.0/">
         <HDRGainMap:HDRGainMapVersion>131072</HDRGainMap:HDRGainMapVersion>
         <HDRGainMap:HDRGainMapHeadroom>5.651999</HDRGainMap:HDRGainMapHeadroom>
         <apdi:NativeFormat>1278226488</apdi:NativeFormat>
         <apdi:AuxiliaryImageType>urn:com:apple:photo:2020:aux:hdrgainmap</apdi:AuxiliaryImageType>
         <apdi:StoredFormat>1278226488</apdi:StoredFormat>
      </rdf:Description>
   </rdf:RDF>
</x:xmpmeta>

@jiawen Did you write code to do that, or just use software like Adobe LR / ACR? ImageMagick has an open request for contributors who may be able to help support gain maps. ImageMagick/ImageMagick#6377. ImageMagick support would be key to support for gain maps in the WordPress media library, etc.

Another resource for this group (if the interest isn't just limited to HEIC / Swift) is the HDR JPG gain map library published by Google: https://github.com/google/libultrahdr. This is built to the Google / Adobe gain map spec (which is the basis of a draft ISO specification and is a de facto standard with support in Lightroom, Adobe Camera RAW, Chrome, Edge, Brave, and Opera already).

I wrote bespoke code to do this. I would love to open source this code as it is simultaneously useful, esoteric, yet not too difficult when you use the right abstractions. But my manager probably wants me to spend my time elsewhere. I think my employer can be convinced to release this portion of our code and maintain it, but someone else will need to plumb it into ImageMagick as I lack expertise there.

@jiawen That's great. If you are able to share it, I think just getting it as it currently stands to ImageMagick via ImageMagick/ImageMagick#6377 may be of great value to help expedite those efforts.

I got the following in the aux image's XMP on a JPEG taken with an iPhone 15 Pro's rear camera.

@jiawen Very interesting! It seems Apple introduced a new version with the iPhone 15, including the new HDRGainMapHeadroom XMP field. I wonder what this does... It's not documented in their gain map documentation so far.

Surprisingly, it does render as HDR in Chrome (v118)!

Also in Brave. It seems support for Apple's format was added together with the support for Adobe's format to the Chrome engine. Very nice!

I wonder where this is all going. On the developer side, Apple seems to advocate for exporting HDR images in what they call "ISO HDR" format (basically an HDR video still frame). In fact, that's the only official way for exporting HDR images. There are no simple APIs for writing an SDR image + HDR gain map.

@gregbenz You are very deep into this topic. Do you have an idea what will be the dominant standard in the future? This "ISO HDR" format seems to be the obvious candidate, as it's simply the same as HDR video. But it comes with the big disadvantage that there is no clearly defined SDR representation for those images. It's up to the displaying application how to tone-map those to SDR. The gain map approach is much more explicit.

I don’t know that I’d quite say ISO HDR is the same as a video still frame, not in a literal sense anyhow. Video tends to define a 100 nits white point, photography is using 203. There are certainly many commonalities (sadly including the lack of a common standard for tone mapping).

The “ISO HDR” refers to encoding of an HDR image. There is another ISO draft for gain maps. These are complimentary, as an ISO gain map can use an ISO HDR as the base image. You can use one without the other, but they work best together (if the base image in a gain map is HDR).

Gain maps are the obvious way forward. They offer a much better experience for viewing on any display which lacks the full HDR headroom encoded in the image. It’s vastly better than tone mapping.

JPG gain map is the obvious choice right now. Browsers representing ~75% of viewers (once fully updated) support it on systems with HDR display. And backwards compatibility is 100% (if your browser / image software doesn’t support HDR or gain maps, you gracefully fall back to a nice SDR).

However, JPG requires large files for high quality. There is a draft AVIF gain map standard (Adobe has shared sample files and Chrome/Brave support it). This is clearly the way things “should” go.

I say “should” because best doesn’t always win. As of now, Microsoft Edge (and the file explorer) doesn’t support any AVIF, and that needs to change for AVIF to be viable. Assuming that happens (they have support under a dev flag), we still need an encoder and more browsers to add support. It’s part of the same draft ISO proposal for gain maps and I hope to see it happen.

Another theoretical candidate would be JXL (JPEG XL). As browser support is zero and Chrome pulled support, it doesn’t seem promising anytime soon.

If it were all up to me: we’d use JPG gain maps for now, transition to AVIF gain maps as soon as MS Edge supports AVIF (encoded with an SDR base image for compatibility), then finally get to the ideal AVIF encoded with an HDR base image when support for AVIF gain maps is widely available.

The other thing I’d like to see is encoder support to let the artist provide the SDR rendition as well. This has many benefits, which I outlined in this feature request for Photoshop (please upvote it if you agree): https://community.adobe.com/t5/photoshop-ecosystem-ideas/allow-full-user-control-of-the-sdr-rendition-in-an-hdr-gain-map-export/idi-p/14205440#M19464

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