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bug#39994: 27.0.90; Broken image-converter probe for imagemagick

From: Lars Ingebrigtsen
Subject: bug#39994: 27.0.90; Broken image-converter probe for imagemagick
Date: Sun, 02 Aug 2020 09:56:33 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Juri Linkov <juri@linkov.net> writes:

> Since now it's pretest time, I gave it more testing, and found more problems:
> 1. AFAIR one of the goals for creating image-converter.el
> was to handle such rare image formats as WEBP,
> but I tried to open a webp file, and image-converter failed
> because it doesn't recognize WEBP.
> There is no WEBP mentioned in the output of "identify -list format".
> After installing `apt-get install webp`, another command
> "identify -list delegate" reports its support with:
>   Delegate                Command
> -------------------------------------------------------------------------------
>          webp =>          "dwebp' -pam '%i' -o '%o"
>           png<= webp      "cwebp' -quiet %Q '%i' -o '%o"

My imagemagick says:

$ identify -list format | grep -i webp
     WEBP* WEBP      rw-   WebP Image Format (libwebp 0.6.1[0208])

So I guess that you have a too-old imagemagick installation?  I don't
really think this is an image-converter.el bug, though -- it's best
effort, and if you don't have external programs to display these things,
then it fails.

> 2. After adding manually webp to image-converter-regexp,
> there is another problem: image-converter--convert-magick
> calls the command with
>   (apply #'call-process (car command)
>          nil t nil
> where the arg 't' means to mix standard error output with ordinary output,
> but ImageMagick outputs some info messages to stderr, e.g.:
>   Decoded /tmp/magick-20114vaPD-fxUjRW4. Dimensions: 320 x 214 . Format: 
> lossy. Now saving...
>   Saved file /tmp/magick-20114h1Jh0D04beDR
> thus breaking the image output.

Yes, it's a pain that we can't direct stderr to its own buffer.  Is
there any reason why?  We don't want to write this stuff to a file
(which is allowed), because of the problems with clean-up.

> 3. Visiting an image file from an archive signals the error
>   Cannot display image: (IMAGE-FORMAT should be a symbol like ‘image/png’)

Do you have a test case for this?

> 4. Exif fails to visit images with the error:
>   Cannot display image: (sequencep 122)
> Shouldn't exif code be called with ignore-errors, so its errors won't
> affect the image displaying?
> Test case:
>   exif --output=blackz.jpg --tag=Artist --ifd=0 --set-value='z' 
> test/data/image/black.jpg

I seem to recall fixing this, and this test case doesn't fail for me (in
Emacs 28.1).

(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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