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

See also: How to use images in templates

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:

class wagtail.images.models.AbstractImage
get_rendition(filter: Union[wagtail.images.models.Filter, str]) wagtail.images.models.AbstractRendition

Returns a Rendition instance with a file field value (an image) reflecting the supplied filter value and focal point values from this object.

Note: If using custom image models, an instance of the custom rendition model will be returned.

find_existing_rendition(filter: wagtail.images.models.Filter) wagtail.images.models.AbstractRendition

Returns an existing Rendition instance with a file field value (an image) reflecting the supplied filter value and focal point values from this object.

If no such rendition exists, a DoesNotExist error is raised for the relevant model.

Note: If using custom image models, an instance of the custom rendition model will be returned.

create_rendition(filter: wagtail.images.models.Filter) wagtail.images.models.AbstractRendition

Creates and returns a Rendition instance with a file field value (an image) reflecting the supplied filter value and focal point values from this object.

This method is usually called by Image.get_rendition(), after first checking that a suitable rendition does not already exist.

Note: If using custom image models, an instance of the custom rendition model will be returned.

generate_rendition_file(filter: wagtail.images.models.Filter) django.core.files.base.File

Generates an in-memory image matching the supplied filter value and focal point value from this object, wraps it in a File object with a suitable filename, and returns it. The return value is used as the file field value for rendition objects saved by AbstractImage.create_rendition().

NOTE: The responsibility of generating the new image from the original falls to the supplied filter object. If you want to do anything custom with rendition images (for example, to preserve metadata from the original image), you might want to consider swapping out filter for an instance of a custom Filter subclass of your design.