What format for writeHEIFRepresentation preserves HDR?

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Created Oct ’24

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In the WWDC 24 session "Use HDR for dynamic image experiences in your app" it's noted this is how you save edits for Adaptive HDR:

SDR + HDR: writeHEIFRepresentation(of: sdrImage, to: url, colorSpace: p3Space, options: [.hdrImage: hdrImage])
SDR + Gain: writeHEIFRepresentation(of: sdrImage, to: url, colorSpace: p3Space, options: [.hdrGainMapImage: gainImage])

This won't compile because the format argument is missing. What format should be used?

In the WWDC 23 session "Support HDR images in your app" RGBAf, RGBAh, and RGBA16, and RGB10 were mentioned but I'm not sure which one to use.

If relevant, I'm editing photos from the user's photo library, so the image was probably taken on iPhone but perhaps not. Thanks!

Answered by FrankSchlegel in 810705022

Both APIs are used for writing a Gain Map HDR image, i.e, an SDR RGB image that contains an auxiliary single-channel gain map image that contains the HDR information. You can and should use an 8-bit format for this kind of image, e.g., RGBA8.

The format Apple mentioned in the 2023 video (they called it ISO HDR) is for storing an HDR image directly (without an SDR representation). For that, you'd need more than 8 bit because the range of color values in an HDR image is much larger.

However, it seems the industry is moving towards the SDR + gain map standard introduced by Adobe last year, which Apple is now also adopting. I would assume that they won't pursue the ISO HDR format much further, as it's not as compatible and takes more space.

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FrankSchlegel OP

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Accepted Answer

Both APIs are used for writing a Gain Map HDR image, i.e, an SDR RGB image that contains an auxiliary single-channel gain map image that contains the HDR information. You can and should use an 8-bit format for this kind of image, e.g., RGBA8.

The format Apple mentioned in the 2023 video (they called it ISO HDR) is for storing an HDR image directly (without an SDR representation). For that, you'd need more than 8 bit because the range of color values in an HDR image is much larger.

However, it seems the industry is moving towards the SDR + gain map standard introduced by Adobe last year, which Apple is now also adopting. I would assume that they won't pursue the ISO HDR format much further, as it's not as compatible and takes more space.

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