Add a tweak to change the image compression quality used by the content server when it makes thumbnails.

resources/default_tweaks.py

@@ -550,3 +550,10 @@ highlight_virtual_library = 'yellow'
 # all available output formats to be present.
 restrict_output_formats = None
 
+#: Set the thumbnail image quality used by the content server
+# The quality of a thumbnail is largely controlled by the compression quality
+# used when creating it. Set this to a larger number to improve the quality.
+# Note that the thumbnails get much larger with larger compression quality
+# numbers.
+# The value can be between 50 and 99
+content_server_thumbnail_compression_quality = 70

src/calibre/library/server/content.py

@@ -18,6 +18,7 @@ from calibre.utils.magick.draw import (save_cover_data_to, Image,
         thumbnail as generate_thumbnail)
 from calibre.utils.filenames import ascii_filename
 from calibre.ebooks.metadata.opf2 import metadata_to_opf
+from calibre.utils.config import tweaks
 
 plugboard_content_server_value = 'content_server'
 plugboard_content_server_formats = ['epub', 'mobi', 'azw3']
@@ -175,8 +176,13 @@ class ContentServer(object):
             cherrypy.response.headers['Last-Modified'] = self.last_modified(updated)
 
             if thumbnail:
-                return generate_thumbnail(cover,
+                quality = tweaks['content_server_thumbnail_compression_quality']
-                        width=thumb_width, height=thumb_height)[-1]
+                if quality < 50:
+                    quality = 50
+                elif quality > 99:
+                    quality = 99
+                return generate_thumbnail(cover, width=thumb_width,
+                        height=thumb_height, compression_quality=quality)[-1]
 
             img = Image()
             img.load(cover)