Generating renditions in Python

Rendered versions of original images generated by the Wagtail {% image %} template tag are called “renditions”, and are stored as new image files in the site’s [media]/images directory on the first invocation.

Image renditions can also be generated dynamically from Python via the native get_rendition() method, for example:

newimage = myimage.get_rendition('fill-300x150|jpegquality-60')

If myimage had a filename of foo.jpg, a new rendition of the image file called foo.fill-300x150.jpegquality-60.jpg would be generated and saved into the site’s [media]/images directory. Argument options are identical to the {% image %} template tag’s filter spec, and should be separated with |.

The generated Rendition object will have properties specific to that version of the image, such as url, width and height, so something like this could be used in an API generator, for example:

url = myimage.get_rendition('fill-300x186|jpegquality-60').url

Properties belonging to the original image from which the generated Rendition was created, such as title, can be accessed through the Rendition’s image property:

>>> newimage.image.title
'Blue Sky'
>>> newimage.image.is_landscape()
True

Prefetching image renditions

New in version 3.0: This following guidance is only applicable in Wagtail versions 3.0 and above.

When using a queryset to render a list of objects with images, you can make use of Django’s built-in prefetch_related() queryset method to prefetch the renditions needed for rendering with a single additional query. For long lists of items, or where multiple renditions are used for each item, this can provide a significant boost to performance.

For example, say you were rendering a list of events (with thumbnail images for each). Your code might look something like this:

def get_events():
    return EventPage.objects.live().select_related("listing_image")

The above can be modified slightly to prefetch the renditions for listing images:

def get_events():
    return EventPage.objects.live().select_related("listing_image").prefetch_related("listing_image__renditions")

If images in your project tend to have very large numbers of renditions, and you know in advance the ones you need, you might want to consider using a Prefetch object to select only the renditions you need for rendering. For example:

from django.db.models import Prefetch
from wagtail.images import get_image_model


def get_events():
    # These are the renditions required for rendering
    renditions_queryset = get_image_model().get_rendition_model().objects.filter(
        filter_spec__in=["fill-300x186", "fill-600x400", "fill-940x680"]
    )

    # `Prefetch` is used to fetch only the required renditions
    return EventPage.objects.live().select_related("listing_image").prefetch_related(
        Prefetch("listing_image__renditions", queryset=renditions_queryset)
    )

Model methods involved in rendition generation

New in version 3.0: The following method references are only applicable to Wagtail versions 3.0 and above.

The following AbstractImage model methods are involved in finding and generating a renditions. If using a custom image model, you can customise the behaviour of either of these methods by overriding them on your model:

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Generating renditions in Python

Prefetching image renditions

Model methods involved in rendition generation