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103 lines
4.0 KiB
ReStructuredText
103 lines
4.0 KiB
ReStructuredText
.. _image_renditions:
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Generating renditions in Python
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=====================================
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Rendered versions of original images generated by the Wagtail ``{% image %}`` template tag are called "renditions",
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and are stored as new image files in the site's ``[media]/images`` directory on the first invocation.
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Image renditions can also be generated dynamically from Python via the native ``get_rendition()`` method, for example:
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.. code-block:: python
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newimage = myimage.get_rendition('fill-300x150|jpegquality-60')
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If ``myimage`` had a filename of ``foo.jpg``, a new rendition of the image file called
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``foo.fill-300x150.jpegquality-60.jpg`` would be generated and saved into the site's ``[media]/images`` directory.
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Argument options are identical to the ``{% image %}`` template tag's filter spec, and should be separated with ``|``.
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The generated ``Rendition`` object will have properties specific to that version of the image, such as
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``url``, ``width`` and ``height``, so something like this could be used in an API generator, for example:
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.. code-block:: python
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url = myimage.get_rendition('fill-300x186|jpegquality-60').url
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Properties belonging to the original image from which the generated Rendition was created, such as ``title``, can
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be accessed through the Rendition's ``image`` property:
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.. code-block:: python
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>>> newimage.image.title
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'Blue Sky'
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>>> newimage.image.is_landscape()
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True
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See also: :ref:`image_tag`
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.. _prefetching_image_renditions:
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Prefetching image renditions
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----------------------------
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.. versionadded:: 3.0
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This following guidance is only applicable in Wagtail versions 3.0 and above.
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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.
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For example, say you were rendering a list of events (with thumbnail images for each). Your code might look something like this:
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.. code-block:: python
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def get_events():
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return EventPage.objects.live().select_related("listing_image")
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The above can be modified slightly to prefetch the renditions for listing images:
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.. code-block:: python
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def get_events():
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return EventPage.objects.live().select_related("listing_image").prefetch_related("listing_image__renditions")
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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:
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.. code-block:: python
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from django.db.models import Prefetch
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from wagtail.images import get_image_model
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def get_events():
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# These are the renditions required for rendering
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renditions_queryset = get_image_model().get_rendition_model().objects.filter(
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filter_spec__in=["fill-300x186", "fill-600x400", "fill-940x680"]
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)
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# `Prefetch` is used to fetch only the required renditions
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return EventPage.objects.live().select_related("listing_image").prefetch_related(
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Prefetch("listing_image__renditions", queryset=renditions_queryset)
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)
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.. _image_rendition_methods:
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Model methods involved in rendition generation
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----------------------------------------------
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.. versionadded:: 3.0
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The following method references are only applicable to Wagtail versions 3.0 and above.
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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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.. automodule:: wagtail.images.models
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.. class:: AbstractImage
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:noindex:
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.. automethod:: get_rendition
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.. automethod:: find_existing_rendition
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.. automethod:: create_rendition
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.. automethod:: generate_rendition_file
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