media/thumbnailer: Better quality for 1-bit / 8-bit color palette images

synapse/rest/media/v1/thumbnailer.py

@@ -50,9 +50,17 @@ def aspect(self, max_width, max_height):
         else:
             return ((max_height * self.width) // self.height, max_height)
 
+    def _resize(self, width, height):
+        # 1-bit or 8-bit color palette images need converting to RGB
+        # otherwise they will be scaled using nearest neighbour which
+        # looks awful
+        if self.image.mode in ["1", "P"]:
+            self.image = self.image.convert("RGB")
+        return self.image.resize((width, height), Image.ANTIALIAS)
+
     def scale(self, output_path, width, height, output_type):
         """Rescales the image to the given dimensions"""
-        scaled = self.image.resize((width, height), Image.ANTIALIAS)
+        scaled = self._resize(width, height)
         return self.save_image(scaled, output_type, output_path)
 
     def crop(self, output_path, width, height, output_type):
@@ -68,17 +76,13 @@ def crop(self, output_path, width, height, output_type):
         """
         if width * self.height > height * self.width:
             scaled_height = (width * self.height) // self.width
-            scaled_image = self.image.resize(
-                (width, scaled_height), Image.ANTIALIAS
-            )
+            scaled_image = self._resize(width, scaled_height)
             crop_top = (scaled_height - height) // 2
             crop_bottom = height + crop_top
             cropped = scaled_image.crop((0, crop_top, width, crop_bottom))
         else:
             scaled_width = (height * self.width) // self.height
-            scaled_image = self.image.resize(
-                (scaled_width, height), Image.ANTIALIAS
-            )
+            scaled_image = self._resize(scaled_width, height)
             crop_left = (scaled_width - width) // 2
             crop_right = width + crop_left
             cropped = scaled_image.crop((crop_left, 0, crop_right, height))