The image scale effect allows one to scale an image while preserving the aspect ratio. So, only 1 of the new dimensions need to be specified.
The implementation of this effect assumes that as soon as this effect is applied, the current dimensions are known. Subsequently, the arithmetic to preserve aspect ratio is done in non image toolkit specific code and the results are passed to the resize effect.
Though the assumption that intermediate dimensions are known may hold for the GD toolkit that is part of core, it does not hold for the Imagemagick toolkit. The Imagemagick toolkit would just pass the 1 available parameter to the imagemagick processing but cannot do so as the effect is changed into a resize effect before being processed by toolkit specific code.
This error will reveal itself if the scale effect is chained after another effect that cannot compute resulting dimensions, like random rotate.
Issue ✨ Combine image scale and resize effects and add preserve aspect ratio option to scale Needs work might (will) be influenced by this patch. But as this is a bug that needs backporting to D7, I filed it as a separate issue.
Needs work
9.5
After being applied to the 8.x branch, it should be considered for backport to the 7.x branch. Note: This tag should generally remain even after the backport has been written, approved, and committed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
This is still an issue in D9.5
Images displayed with a scale effect, in the image style configuration, do not have dimensions displayed in the image tag
Trying to get those in a preprocess function
$image = \Drupal::service('image.factory')->get(DRUPAL_ROOT . $variables['img_element']['#uri']);
$width = $image->getWidth();
Which also has it empty.