Support format-webp and allow changing default formatting

docs/advanced_topics/images/image_file_formats.rst

@@ -0,0 +1,45 @@
+Image file formats
+==================
+
+Using the picture element
+-------------------------
+
+The `picture element <https://developer.mozilla.org/en-US/docs/Web/HTML/Element/picture>`_
+can be used with the ``format-<type>`` image operation to specify different
+image formats and let the browser choose the one it prefers. For example:
+
+.. code-block:: python
+
+    {% load wagtailimages_tags %}
+
+    <picture>
+        {% image myimage width-1000 format-webp as image_webp %}
+        <source srcset="{{ image_webp.url }}" type="image/webp">
+
+        {% image myimage width-1000 format-png as image_png %}
+        <source srcset="{{ image_png.url }}" type="image/png">
+
+        {{ image_png }}
+    </picture>
+
+Customizing output formats
+--------------------------
+
+By default all ``bmp`` and ``webp`` images are converted to the ``png`` format
+when no image output format is given.
+
+The default conversion mapping can be changed by setting the
+``WAGTAILIMAGES_FORMAT_CONVERSIONS`` to a dictionary which maps the input type
+to an output type.
+
+For example:
+
+.. code-block:: python
+
+    WAGTAILIMAGES_FORMAT_CONVERSIONS = {
+        'bmp': 'jpeg',
+        'webp': 'webp',
+    }
+
+will convert ``bmp`` images to ``jpeg`` and disable the default ``webp``
+to ``png`` conversion.

docs/advanced_topics/images/index.rst

@@ -7,6 +7,7 @@ Images
 
     renditions
     animated_gifs
+    image_file_formats
     custom_image_model
     changing_rich_text_representation
     feature_detection

wagtail/images/image_operations.py

@@ -246,8 +246,9 @@ class FormatOperation(Operation):
     def construct(self, fmt):
         self.format = fmt
 
-        if self.format not in ['jpeg', 'png', 'gif']:
-            raise ValueError("Format must be either 'jpeg', 'png' or 'gif'")
+        if self.format not in ['jpeg', 'png', 'gif', 'webp']:
+            raise ValueError(
+                "Format must be either 'jpeg', 'png', 'gif', or 'webp'")
 
     def run(self, willow, image, env):
         env['output-format'] = self.format

wagtail/images/models.py

@@ -404,20 +404,23 @@ class Filter:
                 # Developer specified an output format
                 output_format = env['output-format']
             else:
-                # Default to outputting in original format
-                output_format = original_format
-
-                # Convert BMP files to PNG
-                if original_format == 'bmp':
-                    output_format = 'png'
+                # Convert bmp and webp to png by default
+                default_conversions = {
+                    'bmp': 'png',
+                    'webp': 'png',
+                }
 
                 # Convert unanimated GIFs to PNG as well
-                if original_format == 'gif' and not willow.has_animation():
-                    output_format = 'png'
+                if not willow.has_animation():
+                    default_conversions['gif'] = 'png'
 
-                # Convert WEBP files to PNG
-                if original_format == 'webp':
-                    output_format = 'png'
+                # Allow the user to override the conversions
+                conversion = getattr(settings, 'WAGTAILIMAGES_FORMAT_CONVERSIONS', {})
+                default_conversions.update(conversion)
+
+                # Get the converted output format falling back to the original
+                output_format = default_conversions.get(
+                    original_format, original_format)
 
             if output_format == 'jpeg':
                 # Allow changing of JPEG compression quality

wagtail/images/tests/test_image_operations.py

@@ -521,6 +521,16 @@ class TestFormatFilter(TestCase):
 
         self.assertEqual(out.format_name, 'gif')
 
+    def test_webp(self):
+        fil = Filter(spec='width-400|format-webp')
+        image = Image.objects.create(
+            title="Test image",
+            file=get_test_image_file(),
+        )
+        out = fil.run(image, BytesIO())
+
+        self.assertEqual(out.format_name, 'webp')
+
     def test_invalid(self):
         fil = Filter(spec='width-400|format-foo')
         image = Image.objects.create(