Ultra HDR in WebP

[grishka]

This question is mostly about the format itself rather than this library, but...

Since I recently got myself a phone that shoots in ultra HDR, I naturally started wanting everything to support it. Before asking other people about it though, it feels fair to first explore adding support to my own project. And this is where I hit a snag: I make heavy use of WebP in my image processing/storage pipeline. I convert all user-uploaded images to high-quality WebP for storage, to save space, and then I serve smaller WebP and JPEG representations of that one file through imgproxy. Both Smithereen itself and imgproxy use libvips for image processing, which is considering adding support for ultra HDR.

However, even when libvips does add that support, it still doesn't appear that WebP is one of the supported formats:

• The Android documentation straight up ignores everything except JPEG
• And this library only supports JPEG, HEIF, and AVIF:

  libultrahdr/ultrahdr_api.h

  Lines 148 to 153 in ea518da

  	/*!\brief List of supported codecs */
  	typedef enum uhdr_codec {
  	UHDR_CODEC_JPG, /**< Compress {Hdr, Sdr rendition} to an {Sdr rendition + Gain Map} using jpeg */
  	UHDR_CODEC_HEIF, /**< Compress {Hdr, Sdr rendition} to an {Sdr rendition + Gain Map} using heif */
  	UHDR_CODEC_AVIF, /**< Compress {Hdr, Sdr rendition} to an {Sdr rendition + Gain Map} using avif */
  	} uhdr_codec_t; /**< alias for enum uhdr_codec */

So what's the best course of action for me here? I did give AVIF a try a while ago, but was unsatisfied with the glacially slow encoding speed (it took hundreds of milliseconds per image on an Apple M1), so unless something has changed about that, switching to AVIF isn't really a viable option.