[digikam] [Bug 467675] digiKam fail to export to WEBP format.

[bugzilla_noreply at kde.org]

https://bugs.kde.org/show_bug.cgi?id=467675

--- Comment #38 from caulier.gilles at gmail.com ---
Hi Maik,

I can reproduce now the dysfunction with WEBP under Linux.

- Open a JPEG files in editor
- Run GMIC Qt B&W filter.
- Export to WebP

The resulting image generated by GMIC Qt is fine in editor. GMIC convert from
RGBA to grayscale image. When we re-importing the image data, we re-convert to
DImg as RGBA. Code is located in this function:

https://github.com/cgilles/gmic-qt/blob/master/src/Host/digiKam/editor/host_digikam.cpp#L75

Nothing special here. If i export to another format image is not broken. I
tried TIFF, PGF, JPEG, PNG, etc...

But in fact it's not only relevant of any filter applied to GMIC: all image
process and exported in WEBP are broken. For ex, A JPEG file post-processed in
GMIC with Brightness-Contrast filter and exported as WEBP is also broken.

So something is wrong in WEBP plugin. I tried to disable the saveMetadata() in
DIMg::Qimage plugin to see if it have any effect. It's not better, expected
that alpha channel become the main color and no contents is displayed.

I analyzed the WEBP RGB contents generated and it sound like only the first
column of the image is used to generate all the rest. Open the WEBP image
export to gimp and disable the alpha channel:

https://i.imgur.com/2rL83S6.png

It's a really strange effect.

Note: an image post processed in GMIC and export in PNG and reopen in editor to
be re-exported a WEBP is fine. So for me, something is badly pass to the DImg
container when post processed in GMIC that WEBP codec miss-understood. 

For the moment i have no other ideas in mind.
Best

Gilles

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