Saving DNG (raw photos) taken on other camera to iOS Photo Library

I write an app that reads dng files from my Ricoh GR II and save them to iOS 10's Photo Library, code like this

let photoLibrary = PHPhotoLibrary.shared()
photoLibrary.performChanges({
    PHAssetChangeRequest.creationRequestForAsset(from: image)
}) { (success: Bool, error: Error?) -> Void in
    if success {
        print("Saving photo ok")
    } else {
        print("Error writing to photo library: \(error!.localizedDescription)")
    }
}

And I got the error below:

ImageIO: PluginForUTType:316: file format 'com.adobe.raw-image' doesn't support writing
Error writing to photo library: The operation couldn’t be completed. (Cocoa error -1.)

I guess maybe iOS only support DNG that's taken on iPhone?

Solution

PHAssetChangeRequest.creationRequestForAsset(from: image) starts from a UIImage, so it wouldn't get the original file into your library anyway. A UIImage is the displayable result of reading and decoding an image file; by the time you have a UIImage it doesn't know whether it came from a JPEG or a DNG or a GIF or was rendered at run time via CGBitmapContext or whatever. When you try to save via creationRequestForAssetFromImage:, you're taking that end result and turning it back into a file — whatever kind of file that method wants. (Probably JPEG.)

If you want to put an actual DNG file into the photo library, you'll need to use a Photos framework method that takes original files, not decoded images. Furthermore, since not every Photos client can deal with RAW DNGs, Photos requires that every DNG file you put in the library be accompanied by a JPEG representation that non-RAW-supporting apps (sadly, including the Photos app itself) can see.

Luckily, there's an API for that.

PHPhotoLibrary.shared().performChanges( {
    let creationRequest = PHAssetCreationRequest.forAsset()
    let creationOptions = PHAssetResourceCreationOptions()
    creationOptions.shouldMoveFile = true
    creationRequest.addResource(with: .photo, data: jpegData, options: nil)
    creationRequest.addResource(with: .alternatePhoto, fileURL: dngFileURL, options: creationOptions)
}, completionHandler: completionHandler)

PHAssetCreationRequest is for creating assets from the underlying resources - one or more image files, video files, some combination thereof (for Live Photos), etc. The photo and alternatePhoto resources are how you provide a DNG file and its accompanying JPEG preview. And the shouldMoveFile option is good for if you don't want to blow your device storage from copying the file from your app sandbox into the Photos library storage — good for big resources like DNGs and 4K video.

(The code snippet is from Apple's Photo Capture guide.)

That said, while Apple's RAW processing supports images from all sorts of third-party cameras, it doesn't look like their list includes any Ricoh models. (Not even this kind.)

That doesn't prevent you from storing Ricoh DNGs in the Photos library, though — it just means the only apps that will be able to usefully read them from the library will need their own Ricoh RAW processing support to see anything but the preview JPEG.