Keep format when transposing images

Fix #1755.

tests/draw/test_image.py

@@ -2,7 +2,8 @@
 
 import pytest
 
-from ..testing_utils import assert_no_logs, capture_logs
+from ..testing_utils import (
+    FakeHTML, assert_no_logs, capture_logs, resource_filename)
 
 centered_image = '''
     ________
@@ -578,3 +579,15 @@ def test_image_exif_image_orientation(assert_same_renderings):
         ''',
         tolerance=25,
     )
+
+
+@assert_no_logs
+def test_image_exif_image_orientation_keep_format():
+    # Regression test for https://github.com/Kozea/WeasyPrint/issues/1755
+    pdf = FakeHTML(
+        string='''
+          <style>@page { size: 10px }</style>
+          <img style="display: block; image-orientation: 180deg"
+               src="not-optimized-exif.jpg">''',
+        base_url=resource_filename('<inline HTML>')).write_pdf()
+    assert b'DCTDecode' in pdf

weasyprint/images.py

@@ -135,6 +135,8 @@ def get_image_from_uri(cache, url_fetcher, optimize_size, url,
                 else:
                     # Store image id to enable cache in Stream.add_image
                     image_id = md5(url.encode()).hexdigest()
+                    # Keep image format as it is discarded by transposition
+                    image_format = pillow_image.format
                     if orientation == 'from-image':
                         if 'exif' in pillow_image.info:
                             pillow_image = ImageOps.exif_transpose(
@@ -148,6 +150,7 @@ def get_image_from_uri(cache, url_fetcher, optimize_size, url,
                         if flip:
                             pillow_image = pillow_image.transpose(
                                 Image.Transpose.FLIP_LEFT_RIGHT)
+                    pillow_image.format = image_format
                     image = RasterImage(pillow_image, image_id, optimize_size)
 
     except (URLFetchingError, ImageLoadingError) as exception: