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Re: [csswg-drafts] [css-images-3] Allow impls to not respect exif data if it's after the image data (#4929)

From: Jon Sneyers via GitHub <sysbot+gh@w3.org>
Date: Thu, 29 Oct 2020 15:58:35 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-718849793-1603987114-sysbot+gh@w3.org>
In the JPEG XL file format, you can put the Exif data where you want, but for web use cases, we define an abbreviated 'file format' that is just a 'naked' JPEG XL codestream without any metadata.

All render-impacting metadata is defined in the JXL codestream, and if the optional Exif (or XMP) metadata says something that conflicts with what the codestream says, then a decoder has to ignore the Exif/XMP. The codestream is the single source of truth. This includes image dimensions, bit depth, number of channels, image orientation, intrinsic dimensions, and color space. In the codestream these things are part of the header, and are always put before the actual image data in the bitstream. Otherwise you cannot do meaningful progressive decoding.

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