wagtail/docs/advanced_topics/images/renditions.rst

.. _image_renditions:

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:

.. code-block:: python

    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:

.. code-block:: python

    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:

.. code-block:: python

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

See also: :ref:`image_tag`

.. _prefetching_image_renditions:

Prefetching image renditions
----------------------------

.. versionadded:: 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:

.. code-block:: python

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

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

.. code-block:: python

    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:

.. code-block:: python

    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)
        )

.. _image_rendition_methods:

Model methods involved in rendition generation
----------------------------------------------

.. versionadded:: 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:

.. automodule:: wagtail.images.models

.. class:: AbstractImage
    :noindex:

    .. automethod:: get_rendition

    .. automethod:: find_existing_rendition

    .. automethod:: create_rendition

    .. automethod:: generate_rendition_file