Added 'format' template filter

This allows the developer to change the output format of an image placement

wagtail/wagtailimages/image_operations.py

@@ -227,3 +227,14 @@ class JPEGQualityOperation(Operation):
 
     def run(self, willow, image, env):
         env['jpeg-quality'] = self.quality
+
+
+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'")
+
+    def run(self, willow, image, env):
+        env['output-format'] = self.format

wagtail/wagtailimages/models.py

@@ -435,7 +435,23 @@ class Filter(models.Model):
                 else:
                     willow = operation.run(willow, image) or willow
 
-            if original_format == 'jpeg':
+            # Find the output format to use
+            if 'output-format' in env:
+                # 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 unanimated GIFs to PNG as well
+                if original_format == 'gif' and not willow.has_animation():
+                    output_format = 'png'
+
+            if output_format == 'jpeg':
                 # Allow changing of JPEG compression quality
                 if 'jpeg-quality' in env:
                     quality = env['jpeg-quality']
@@ -445,17 +461,10 @@ class Filter(models.Model):
                     quality = 85
 
                 return willow.save_as_jpeg(output, quality=quality, progressive=True, optimize=True)
-            elif original_format == 'gif':
-                # Convert image to PNG if it's not animated
-                if not willow.has_animation():
-                    return willow.save_as_png(output)
-                else:
-                    return willow.save_as_gif(output)
-            elif original_format == 'bmp':
-                # Convert to PNG
+            elif output_format == 'png':
                 return willow.save_as_png(output)
-            else:
-                return willow.save(original_format, output)
+            elif output_format == 'gif':
+                return willow.save_as_gif(output)
 
     def get_cache_key(self, image):
         vary_parts = []

wagtail/wagtailimages/wagtail_hooks.py

@@ -77,6 +77,7 @@ def register_image_operations():
         ('width', image_operations.WidthHeightOperation),
         ('height', image_operations.WidthHeightOperation),
         ('jpegquality', image_operations.JPEGQualityOperation),
+        ('format', image_operations.FormatOperation),
     ]