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
. Hence, 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
Generating multiple renditions for an image¶
You can generate multiple renditions of the same image from Python using the native get_renditions()
method. It will accept any number of ‘specification’ strings or Filter instances
, and will generate a set of matching renditions much more efficiently than generating each one individually. For example:
image.get_renditions('width-600', 'height-400', 'fill-300x186|jpegquality-60')
The return value is a dictionary of renditions keyed by the specifications that were provided to the method. The return value from the above example would look something like this:
{
"width-600": <Rendition: Rendition object (7)>,
"height-400": <Rendition: Rendition object (8)>,
"fill-300x186|jpegquality-60": <Rendition: Rendition object (9)>,
}
Caching image renditions¶
Wagtail will cache image rendition lookups, which can improve the performance of pages which include many images.
By default, Wagtail will try to use the cache called “renditions”. If no such cache exists, it will fall back to using the default cache.
Prefetching image renditions¶
When using a queryset to render a list of images or objects with images, you can prefetch the renditions needed 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.
Image QuerySets¶
When working with an Image QuerySet, you can make use of Wagtail’s built-in prefetch_renditions
queryset method to prefetch the renditions needed.
For example, say you were rendering a list of all the images uploaded by a user:
def get_images_uploaded_by_user(user):
return ImageModel.objects.filter(uploaded_by_user=user)
The above can be modified slightly to prefetch the renditions of the images returned:
def get_images_uploaded_by_user(user):
return ImageModel.objects.filter(uploaded_by_user=user).prefetch_renditions()
The above will prefetch all renditions even if we may not need them.
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 specifying a set of filters to the prefetch_renditions
method and only select the renditions you need for rendering. For example:
def get_images_uploaded_by_user(user):
# Only specify the renditions required for rendering
return ImageModel.objects.filter(uploaded_by_user=user).prefetch_renditions(
"fill-700x586", "min-600x400", "max-940x680"
)
Non Image Querysets¶
If you’re working with a non Image Model, you can make use of Django’s built-in prefetch_related()
queryset method to prefetch renditions.
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 you know in advance the renditions you’ll need, you can filter the renditions queryset to use:
from django.db.models import Prefetch
from wagtail.images import get_image_model
def get_events():
Image = get_image_model()
filters = ["fill-300x186", "fill-600x400", "fill-940x680"]
# `Prefetch` is used to fetch only the required renditions
prefetch_images_and_renditions = Prefetch(
"listing_image",
queryset=Image.objects.prefetch_renditions(*filters)
)
return EventPage.objects.live().prefetch_related(prefetch_images_and_renditions)
Model methods involved in rendition generation¶
The following AbstractImage
model methods are involved in finding and generating renditions. If using a custom image model, you can customize the behavior of either of these methods by overriding them on your model:
- class wagtail.images.models.AbstractImage
- get_rendition(filter: Filter | str) AbstractRendition ¶
Returns a
Rendition
instance with afile
field value (an image) reflecting the suppliedfilter
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: Filter) AbstractRendition ¶
Returns an existing
Rendition
instance with afile
field value (an image) reflecting the suppliedfilter
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: Filter) AbstractRendition ¶
Creates and returns a
Rendition
instance with afile
field value (an image) reflecting the suppliedfilter
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.
- get_renditions(*filters: Filter | str) Dict[str, AbstractRendition] ¶
Returns a
dict
ofRendition
instances with image files reflecting the suppliedfilters
, keyed by filter spec patterns.Note: If using custom image models, instances of the custom rendition model will be returned.
- find_existing_renditions(*filters: Filter) Dict[Filter, AbstractRendition] ¶
Returns a dictionary of existing
Rendition
instances withfile
values (images) reflecting the suppliedfilters
and the focal point values from this object.Filters for which an existing rendition cannot be found are ommitted from the return value. If none of the requested renditions have been created before, the return value will be an empty dict.
- create_renditions(*filters: Filter) Dict[Filter, AbstractRendition] ¶
Creates multiple
Rendition
instances with image files reflecting the suppliedfilters
, and returns them as adict
keyed by the relevantFilter
instance. Where suitable renditions already exist in the database, they will be returned instead, so as not to create duplicates.This method is usually called by
Image.get_renditions()
, 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: Filter, *, source: File = None) File ¶
Generates an in-memory image matching the supplied
filter
value and focal point value from this object, wraps it in aFile
object with a suitable filename, and returns it. The return value is used as thefile
field value for rendition objects saved byAbstractImage.create_rendition()
.If the contents of
self.file
has already been read into memory, thesource
keyword can be used to provide a reference to the in-memoryFile
, bypassing the need to reload the image contents from storage.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 outfilter
for an instance of a customFilter
subclass of your design.