The continuing confusion around16-bits (or even 12-bits) HEIC images

The previous question was closed so I'll open up a new one.

I've already spent a lot of time and effort trying to make this work because I was hoping to use a higher bit-depth .heic format to store the slides and photographs I scanned in the past years.

Even lossless 16-bit .heic would likely be a lot smaller than 16 bit TIFF or PNG images - that is, if it would work.

As mentioned in all the previous discussions, the Preview app in Mac OS has a greyed-out "16 bits" depth option when exporting a 16-bit TIFF or PNG image to .heic. As mentioned in the previous thread, it looks as if Apple has only partially implemented HEIC support.

But it's more complex than that. When I started looking into this in November '20 I found the libheif project on Github, which was supposedly able to convert 16-bit TIFF or PNG images into .heic format up to 12 bits.

Why only to 12 instead of 16 has never become clear to me, but I reckoned that a12 bits original can already stand some editing before visibly posterizing, so that was already quite useful.

Unfortunately, libheif produced odd results for some combinations of greyscale/RGB and bit depth, like stretching the image horizontally or turning the image completely green. I opened an issue to report these issues and I think they eventually fixed those issues.

At the time I was still on Catalina, and even though Preview couldn't produce these files itself, I could display 12-bit heic images in it just fine (confirmed in Inspector > More info tab > Depth, and also in the Photos app.

But then Big Sur came, and killed that option. In Big Sur, 12-bit .heic images showed up completely white. I tried everything I could think of: updating libheif; trying instead with Imagemagick and Gimp and whatever I could find. But nothing worked, and I eventually just let it rest for a while. At least I hadn't yet started to use that odd 12-bit .heic format yet.

And now, on Monterey, I got curious again and tried if Apple had maybe reverted breaking the display of 12-bit .heic files. Or (not holding my breath) - even gotten around to making the 16-bits version work.

But, no. Though there is one very small improvement: I can now create 10-bit .heic files using either ImageMagic or libheif, and they show up perfectly OK in Preview (including in the Inspector).

Actually, I can even create 12-bit files, but that turns the images uniform dark blue. Interestingly, Preview Inspector does report it as having a depth of 12 bits.

But 10 bits - I don't know. 12 bits was still a significant improvement over 8, but 10 is hardly worth the effort.

But it really puzzles me to no end that in all those two years I haven't been able to find a statement from Apple why they left .heic so crippled in the first place. It doesn't make sense. Maybe it's infringing on some remote legal issue. But they could have at least mentioned that.

And why would they break supporting the half-baked 12-bit option in Big Sur?

So you see, many questions and very little answers yet. But I'll keep an eye open if anything might improve, for it's just too useful to let go.