HDR / UHDR support

[swissspidy]

Related:

• https://core.trac.wordpress.org/ticket/62285
• Add support for HDR gain maps ImageMagick/ImageMagick#6377
• add support for encoding/decoding ultrahdr images ImageMagick/ImageMagick#7198
• Support for Ultra HDR libvips/libvips#3799
• https://github.com/google/libultrahdr
• https://github.com/MONOGRID/gainmap-js

[gregbenz]

It would be incredible to see JPG gain map support finally land in the WordPress media library (it's partially supported now using "full" size, as that avoids transcoding).

I have several resources related to gain maps listed at https://gregbenzphotography.com/hdr/#developers

[swissspidy]

Oh wow, this looks like such a useful resource, thanks for creating & sharing that!

[gregbenz]

I just added some comparison images to help show why gain maps are so critical (vs just using a high bit-depth format like AVIF or JXL without a gain map and letting the browser tone map on less capable displays): https://gregbenzphotography.com/hdr-photos/hdr-gain-maps-vs-tone-mapping/

[gregbenz]

Apple's v26 updates for iOS, iPadOS, and MacOS will enable HDR browsing in Safari (and all browsers on mobile since this is a WebKit update). That's on the order of ~1.3 billion devices gaining HDR browsing support starting in Q4: https://gregbenzphotography.com/photography-reviews/apple-safari-now-supports-hdr-photography/

It would be very beneficial if media experiments were updated to support uploading / transcoding of key HDR formats. In my testing, using the "full" size in the WP media library works great (no transcoding so the image is not broken). I see no issues once an HDR image has been added to the media library.

The following formats would be key to support:

• JPG with ISO gain map (most commonly used now and 100% backwards compatible).
• 8-12 bit AVIF with ISO gain map (fastest path to AVIF support as base SDR AVIF is widely supported, and this offers the highest possible quality due to both bit depth and the gain map).
• AVIF (no map), ie 10-12-bit with PQ curve (this prioritizes size over quality by eliminating the gain map, resulting in lower quality on SDR or less capable HDR displays)

[ WordPress v6.8 was intended to support the last option (simple AVIF without a map), but needs a bug fix as the output is 8-bit SDR and the CICP color information is stripped: https://core.trac.wordpress.org/ticket/62285#comment:21 ]

Note: the following would be nice but are much lower priority due to limited browser support: HEIF / JXL with or without gain map. PNG would also be lower priority given both limited support and because the files are much larger than AVIF (which supports transparency).

[swissspidy]

Happy to add/test support once it lands in libvips! 👍

[swissspidy]

Initial support just landed in libvips: libvips/libvips#4645

Now just needs a new release & then a new release of wasm-vips. 🤞

[kleisauke]

It looks like https://github.com/google/libultrahdr is licensed under Apache-2.0, would that cause any issues for Gutenberg? I'm asking because I noticed this code and this comment.

[swissspidy]

Great question. Not 100% sure to be honest as IANAL. AFAIK there is some nuance to that, because WP is licensed under "GPLv2 or later", which means a combined work could be distributed under GPLv3. We had a similar question regarding LGPLv3 + GPL v2 or later, and apparently it was not OK. See WordPress/gutenberg#61447 (comment)

We should get some legal advice for this case too.

[kleisauke]

We had a similar question regarding LGPLv3 + GPL v2 or later, and apparently it was not OK.

IANAL, but as I understand it, a library licensed under LGPL-3.0-or-later (SPDX) is compatible with a GPL-2.0-or-later project. The incompatibility applies only with the -only variants (e.g. LGPL-3.0-only or LGPL-2.1-only), which cannot be used in a GPL-2.0-or-later project.