Add Encoding.flate_jpeg to recognize deflated JPEG images

FlateDecode+DCTDecode compressed images are essentially deflated JPEGs,
typically created by OCRmyPDF's optimizer. This change ensures pdfinfo
correctly identifies them and should_visible_page_image_use_jpg treats
them as JPEG-origin images, allowing JPEG output when appropriate.

src/ocrmypdf/_pipeline.py

@@ -735,7 +735,8 @@ def ocr_engine_direct(
 def should_visible_page_image_use_jpg(pageinfo: PageInfo) -> bool:
     """Determines whether the visible page image should be saved as a JPEG.
 
-    If all images were JPEGs originally, permit a JPEG as output.
+    If all images were JPEGs originally (including FlateDecode+DCTDecode),
+    permit a JPEG as output.
 
     Args:
         pageinfo: The PageInfo object containing information about the page.
@@ -744,7 +745,7 @@ def should_visible_page_image_use_jpg(pageinfo: PageInfo) -> bool:
         A boolean indicating whether the visible page image should be saved as a JPEG.
     """
     return bool(pageinfo.images) and all(
-        im.enc == Encoding.jpeg for im in pageinfo.images
+        im.enc in (Encoding.jpeg, Encoding.flate_jpeg) for im in pageinfo.images
     )
 
 

src/ocrmypdf/pdfinfo/_image.py

@@ -108,10 +108,18 @@ class ImageInfo:
             self._type = 'image'
 
         self._bpc = int(pim.bits_per_component)
-        try:
-            self._enc = FRIENDLY_ENCODING.get(pim.filters[0])
-        except IndexError:
-            self._enc = None
+        if (
+            len(pim.filters) == 2
+            and pim.filters[0] == '/FlateDecode'
+            and pim.filters[1] == '/DCTDecode'
+        ):
+            # Special case: FlateDecode followed by DCTDecode
+            self._enc = Encoding.flate_jpeg
+        else:
+            try:
+                self._enc = FRIENDLY_ENCODING.get(pim.filters[0])
+            except IndexError:
+                self._enc = None
 
         try:
             self._color = FRIENDLY_COLORSPACE.get(pim.colorspace or '')

src/ocrmypdf/pdfinfo/_types.py

@@ -36,6 +36,7 @@ class Encoding(Enum):
     lzw = auto()
     flate = auto()
     runlength = auto()
+    flate_jpeg = auto()
 
 
 FloatRect = tuple[float, float, float, float]