diff --git a/changelog.d/2142.feature b/changelog.d/2142.feature
new file mode 100644
index 0000000000..e21e8325e1
--- /dev/null
+++ b/changelog.d/2142.feature
@@ -0,0 +1 @@
+Improve quality of thumbnails for 1-bit/8-bit color palette images.
diff --git a/synapse/rest/media/v1/thumbnailer.py b/synapse/rest/media/v1/thumbnailer.py
index c995d7e043..8cf415e29d 100644
--- a/synapse/rest/media/v1/thumbnailer.py
+++ b/synapse/rest/media/v1/thumbnailer.py
@@ -82,13 +82,21 @@ class Thumbnailer(object):
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, width, height, output_type):
"""Rescales the image to the given dimensions.
Returns:
BytesIO: the bytes of the encoded image ready to be written to disk
"""
- scaled = self.image.resize((width, height), Image.ANTIALIAS)
+ scaled = self._resize(width, height)
return self._encode_image(scaled, output_type)
def crop(self, width, height, output_type):
@@ -107,13 +115,13 @@ class Thumbnailer(object):
"""
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))
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