magick:flip does not work, passes extra cli argument

[melkamar]

First off, thanks for a great library!

I encountered an issue when using the magick:flip argument of {% asset %}.

Full description

ImageMagick works correctly, because using magick:resize works as expected:

<div class="image ratio bg-cover"
     {% for image in post.image %}
     style="background-image: url({% asset '{{ image }}' @path magick:resize=512x512 %}) ;"
     {% endfor %}>
</div>

However, if I add the magick:flip argument, the compilation fails with:

  Liquid Exception: `magick mogrify -resize 512x512 -flip /site/.jekyll-cache/assets/proxied/5f93e6.jpeg` failed with error: mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/560. in index.html

First I thought that there is no delegate to handle jpeg, but I checked and it is enabled. Also, this works with the resize, so ImageMagick knows how to handle jpegs.

What is weird is the empty string in the error report:

`... for this image format `'

So I tried changing the HTML code for:

style="background-image: url({% asset '{{ image }}' @path magick:resize=512x512 magick:flip=foobar %}) ;"

And true enough, the error message changed to

  Liquid Exception: `magick mogrify -resize 512x512 -flip foobar /site/.jekyll-cache/assets/proxied/a3b287.jpeg` failed with error: mogrify: unable to open image 'foobar': No such file or directory @ error/blob.c/OpenBlob/3497. mogrify: no decode delegate for this image format `' @ error/constitute.c/ReadImage/560. in index.html

Note the command it is trying to execute:

magick mogrify -resize 512x512 -flip foobar /site/.jekyll-cache/assets/proxied/a3b287.jpeg

The root cause

jekyll-assets is trying to pass n argument to the -flip parameter and that's making ImageMagick fail. My guess is that when there is no arg provided for magick:flip, then the CLI contains an empty string which gets picked up by ImageMagick and makes it fail.

Executing the magick cli command manually works as expected:

magick mogrify -resize 512x512 -flip /site/.jekyll-cache/assets/proxied/a3b287.jpeg

Package versions

jekyll-assets (3.0.12)
mini_magick (4.10.1)
ImageMagick 7.0.8-68 Q16 x86_64 2019-12-18 https://imagemagick.org

Suggested fix

The -flip parameter should not take any positional argument. Maybe this is a legacy expectation? ImageMagick now has -flip and -flop.