Whereas image_reduire generates the small image that will properly fit inside a given rectangle, image_passe_partout generates the largest image that fills up that same rectangle.
When followed by a image_recadre with the same dimensions, it makes it possible to generate thumbnails with pre-defined and mandated dimensions without deforming them.
Example
[(#FICHIER
|image_passe_partout{70,70}
|image_recadre{70,70,center}
|image_aplatir{jpg,ffffff}
|inserer_attribut{class,spip_logos})]
See also
Filters