From 0e6e5afe8dcbe76bd09c2bb6d23dc8f305726c91 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Tue, 10 Mar 2020 10:00:35 +0530
Subject: [PATCH] MOBI Output: Improve conversion of PNG images with
 transparency to GIF

---
 src/calibre/ebooks/mobi/utils.py |  9 ++-------
 src/calibre/utils/img.py         | 32 +++++++++++++++++++++++++++-----
 2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/src/calibre/ebooks/mobi/utils.py b/src/calibre/ebooks/mobi/utils.py
index faf7e5a6b5..0121777702 100644
--- a/src/calibre/ebooks/mobi/utils.py
+++ b/src/calibre/ebooks/mobi/utils.py
@@ -10,7 +10,7 @@ import struct, string, zlib, os
 from collections import OrderedDict
 from io import BytesIO
 
-from calibre.utils.img import save_cover_data_to, scale_image, image_to_data, image_from_data, resize_image
+from calibre.utils.img import save_cover_data_to, scale_image, image_to_data, image_from_data, resize_image, png_data_to_gif_data
 from calibre.utils.imghdr import what
 from calibre.ebooks import normalize
 from polyglot.builtins import unicode_type, range, as_bytes, map
@@ -417,13 +417,8 @@ def to_base(num, base=32, min_num_digits=None):
 def mobify_image(data):
     'Convert PNG images to GIF as the idiotic Kindle cannot display some PNG'
     fmt = what(None, data)
-
     if fmt == 'png':
-        from PIL import Image
-        im = Image.open(BytesIO(data))
-        buf = BytesIO()
-        im.save(buf, 'gif')
-        data = buf.getvalue()
+        data = png_data_to_gif_data(data)
     return data
 
 # Font records {{{
diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index bf10b2b7af..71d1966138 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -67,6 +67,32 @@ def load_jxr_data(data):
 
 # }}}
 
+# png to gif {{{
+
+
+def png_data_to_gif_data(data):
+    from PIL import Image
+    img = Image.open(BytesIO(data))
+    buf = BytesIO()
+    if img.mode in ('p', 'P'):
+        transparency = img.info.get('transparency')
+        if transparency is not None:
+            img.save(buf, 'gif', transparency=transparency)
+        else:
+            img.save(buf, 'gif')
+    elif img.mode in ('rgba', 'RGBA'):
+        alpha = img.split()[3]
+        mask = Image.eval(alpha, lambda a: 255 if a <=128 else 0)
+        img = img.convert('RGB').convert('P', palette=Image.ADAPTIVE, colors=255)
+        img.paste(255, mask)
+        img.save(buf, 'gif', transparency=255)
+    else:
+        img = img.convert('P', palette=Image.ADAPTIVE)
+        img.save(buf, 'gif')
+    return buf.getvalue()
+
+# }}}
+
 # Loading images {{{
 
 
@@ -140,11 +166,7 @@ def image_to_data(img, compression_quality=95, fmt='JPEG', png_compression_level
         w.setQuality(90)
         if not w.write(img):
             raise ValueError('Failed to export image as ' + fmt + ' with error: ' + w.errorString())
-        from PIL import Image
-        im = Image.open(BytesIO(ba.data()))
-        buf = BytesIO()
-        im.save(buf, 'gif')
-        return buf.getvalue()
+        return png_data_to_gif_data(ba.data())
     is_jpeg = fmt in ('JPG', 'JPEG')
     w = QImageWriter(buf, fmt.encode('ascii'))
     if is_jpeg: