HDR JXL gain map - using base HDR image w/diff methodology vs AVIF?

Report · Oct 15, 2024

I'm excited by the addition of better support for the upcoming ISO standard for HDR via gain maps on AVIF and JXL in LrC 14. However, I'm trying to reconcile the resulting files exported (via these instructions) with my understanding of gain maps. I'm also testing them in the linked Gain Map Demo app.

It's clear that "maximize compatibility" means "include a gain map." The increased file size alone shows this—both AVIF and JXL ~double in size when exported this way, which makes sense. However, I'm not sure why the AVIF and JXL are behaving differently in my testing. Everything below is with "maximize compatibility" turned on—I'm testing gain maps, not pure HDR files.

- It appears that the AVIF file is an SDR base plus gain map to get to HDR. I can verify this with how it's displayed in Apple software on various devices, and by comparing the SDR display rendition to what I specify in Lightroom. This is kind of what I'd expect—this is how a traditional JPG+gain map works.

- I'm not sure what's happening with the JXL. "Maximize compatibility" generates a file that still seems to use the HDR rendition as the base image. I.e., on an SDR display, I see whatever software's own tone mapping of the HDR image—I'm not seeing the true SDR rendition.

Is it possible that the JXL is using an "HDR minus gain map" methodology—if that's even included in the standard? If that's the case, I'd have to assume I don't have any other software that can handle rendering this type of gain map.

It's just generally weird to me that AVIF is output as base SDR and JXL is base HDR, though.

So, maybe it's actually a bug and the JXL "maximize compatibility" output isn't actually structuring the file correctly?

I'm just not sure, but something seems odd, and wondering if anyone with more knowledge can shed some light!

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FWIW, the Gain Map demo app sees the AVIF+gain map as SDR-only—no gain map detected, even though it's there. It sees the JXL as HDR-only—whether or not "maximize compatibility" is checked. I realize that app may be out of date for where the standards are at this point, but it verifies what I'm seeing as the "base" image for both, at least.

I'm on MacOS Sequoia 15.0.1, M1 MBP 13" 2020, LrC 14.0. I see this in both P3 and Rec 2020 color profile outputs.

Report · Nov 30, 2024

Had the same problem -> updating the Gain map demo app to the version 17.0.1 or higher fixed it for me.

Report · Nov 30, 2024

That fixed it on the AVIF side for me, yes.

JXLs exported with "maximum compatibility" from LrC still appear to use HDR as a base rendition, however. I just exported a JXL, maximum compatibility, and opened in the gain map. It's analyzed as a 16-bit JPEG XL - HDR + Gain Map. (not SDR + Gain Map)

The Base rendition is actually an HDR image itself. Check out the histograms below—the first is "Base." I'd expect this to be SDR-only. Instead, it's HDR, and matches the histogram and appearance of the image in HDR in LrC.

The second is "Base + Full Gain Map"—which brings the histogram down into SDR territory, and matches the histogram and appearance of the image in "Preview for SDR Display" in LrC.

Because "Base" is HDR, if software doesn't support JXL gain maps, we have to rely on the software's own tone mapping of an HDR JXL.

This is all the opposite of what I'd expect.

With AVIF and JPG, the base is SDR and the gain map pushes the SDR into the HDR range. That's what I'd expect. So I don't understand what's going on with these JXLs.

Report · Nov 30, 2024

Ah, as far as I know its not a bug, it's just the way JXL works. From what I know JXL has as base a HDR photo which is mapped to SDR while with AVIF it's the opposite.

„8d tests ISO encoding for JXL (base HDR image).

This test is very different from the other gain map tests, as the image is encoded with a base HDR image, and you must therefore force the display to SDR in order to determine whether the gain map has been applied.“ - https://gregbenzphotography.com/hdr/

Report · Dec 01, 2024

Ah! OK, it's that gain map model. Less ideal for backwards compatibility given that gain maps have to be supported in order to properly render SDR, but makes sense.