Re: [PNG] Patch up for Chromium eXIf support

On Wed, Jan 31, 2024 at 11:44 AM Chris Lilley <chris@w3.org> wrote:
> Also this one
>   https://wpt.fyi/results/css/css-images/image-orientation/image-orientation-exif-png.html?label=experimental&label=master&aligned

But that's nothing to do with PNG; a PNG image is stored top-left ->
bottom-right.  *That* is in the spec, 4.5.1(b):

>Scanline serialization: the image is serialized a scanline at a time. Pixels are ordered left to right in a scanline and scanlines are ordered top to bottom.

I suggest making it clear in the eXIf description that format
information which contradicts the PNG specification be ignored.  This
has always been an issue; the EXIF data can say things which are
incompatible with the restricted set of PNG encodings and so it is
incumbent on the ENCODER to encode the data in a PNG compatible way.
For example EXIF supports YCbCr data but packing it into the PNG RGB
channels, while possible, is wrong; XYZ is fine in PNG (ideally with
the obvious cHRM chunk) but non-gamma encoded channels are not.  The
same applies to other encoding-specific information from other file
formats; image rotation, the JPEG trick of extending the image to the
next multiple of 8 pixels etc.

Received on Wednesday, 31 January 2024 20:10:04 UTC