Exporting HDR PQ from Photos gives unexpected results (Sequoia 15.1)

Thanks Jeff. Yes, the monitor that I'm using supports HDR (see below for details). As for sharing, I use the iCloud Shared Album feature which automatically converts the image to a low resolution JPG file. This is type of image is adequate for sharing since most people use a small screen to view.

BTW, editing with Canon's DPP program also produces dark image when viewed with Preview. My next step is to transfer the edited image to an iPhone XS Max running iOS 18.1. I'll do this via a file transfer rather than Photos.

Supported monitor colour spaces

Thanks for the suggestion. My understanding is that HDR PQ implies the use of the Rec. 2100 colour space (the HDR PQ transfer curve (EOTF) is from SMPTE ST 2084).

Below is a useful list of the Coding-independent code points (ITU-T H.273) triplets typically used for HDR support in video. QuickTime uses this nomenclature when listing "video details". The camera's HDR PQ format can be characterized as (9-16-0).

As for TIFF files, I tend to avoid their use because of the large file size. In addition to supported 10-bit HDR, HEIC files are very space efficient (i.e. better compression than JPG).

Possible progress ...

"Headroom" is defined the ratio of the luminance of the HDR image’s brightest white to the luminance of standard dynamic range (SDR) white, in the image’s native colour space.

For the case of the HEIC HDR PQ image exported from Photos with the "Original" colour profile, the headroom shown below is 4.926 (2.3 stops) while for the Display P3 colour profile the headroom is 1 (0 stops). My suspicion is that this explains why the (Original colour profile) image is darker (than the image edited in Photos) when displayed in Preview.

Can anyone confirm my suspicion and is there a workaround for this?

- Pie Lover

Exported with "Original" colour profile

According to the results below of an online HDR test, at reasonable brightness, my HDR monitor supports a headroom of about 1.5 to 2 stops. I suspect that macOS is lowering the brightness of the (Original colour profile) image by up to 1 stop in order to match the capability of the display. The display, itself, is only capable of a maximum brightness of about 400 Nits.

- Pie Lover

According to the excerpt reproduced below, macOS applications can support HDR PQ content (BT.2020 colour space with the PQ transfer function):

I found another article on GitHub about HDR that contains following statement:

Tonemapping Content to a Display

When HDR content is rendered to a display, it is transformed by a tone mapping operation to produce an image that fits within the display's HDR headroom.

This information suggests that incorrect headroom for Canon HDR PQ images may be a possible cause of the issue that I describe. Can someone point me to how HDR headroom is encoded for an image in HEIC format? I'd also like to know if a software utility exists which can alter the headroom value.

- Pie Lover

As a workaround, I've found that increasing the exposure of the HDR PQ HEIC in Preview by about 1/2 of full-scale for my set-up gives approximately the same brightness as the display of the edited file in Photos. See below for an example (displayed in PNG).

As the saying goes "your mileage may vary". :-)

- Pie Lover

Thanks for the reply. It's a still image produced by a Canon EOS R5 (Mark I) using the HDR PQ option with RAW+HEIC outputs. As far as I know, HDR PQ is the only one available for HEIC (there's no HDR HLG option).

As a next step, I'm thinking of making similar edits to the original in Canon's own DDP program for macOS. If successful, I can compare the edited file to the one exported from Photos.

- Pie Lover

Just got off the phone with Apple Support. According to the Senior Advisor that I spoke with, Canon's HDR PQ HEIC version is not supported on macOS Sequoia (or likely iOS 18). I find this disappointing because iPhone HDR PQ images seem to display perfectly well in Preview.

Down the road, perhaps Canon will adopt the new "ISO HDR" standard championed by Apple (and others) for HDR PQ.

- Pie Lover

After some additional investigation, I have concluded that while a value of 4.926 is the correct headroom for an HDR HLG file, it should not be applied to an HDR PQ file. The correct value for HDR PQ is 49.26 (= 10000/203). I'm hoping that this will be corrected in a future maintenance update of Sequoia.

HDR HLG provides 2.3 stops of headroom relative to SDR whereas HDR PQ provides 5.6 stops of headroom.

- Pie Lover

iPhone 15 Pro HDR PQ mage has a headroom of 4.926. I suspect that the macOS software incorrectly assumes that a Canon HDR PQ image uses the same value.

- Pie Lover