fix: fallback to 72 dpi when image header is 0 (#1494)

HISTORY.rst

@@ -3,6 +3,14 @@
 Release History
 ---------------
 
+Unreleased
+++++++++++
+
+* **Fixed** – Issues #1497 / #1494: adding an image whose header reports
+  `dpi = 0` now falls back to `72 dpi` instead of raising
+  `ZeroDivisionError` when using `Document.add_picture()`.
+
+
 1.2.0 (2025-06-16)
 ++++++++++++++++++
 

src/docx/image/image.py

@@ -87,19 +87,27 @@ def px_height(self) -> int:
 
     @property
     def horz_dpi(self) -> int:
-        """Integer dots per inch for the width of this image.
-
-        Defaults to 72 when not present in the file, as is often the case.
         """
-        return self._image_header.horz_dpi
+        Horizontal DPI reported for this image.  Returns the header value
+        unless it is `None` **or** `0`, in which case it defaults to
+        72 dpi, matching Word's internal assumption.
+        """
+        dpi = self._image_header.horz_dpi
+        if dpi in (None, 0):
+            return 72
+        return dpi
 
     @property
     def vert_dpi(self) -> int:
-        """Integer dots per inch for the height of this image.
-
-        Defaults to 72 when not present in the file, as is often the case.
         """
-        return self._image_header.vert_dpi
+        Vertical DPI reported for this image.  Returns the header value
+        unless it is `None` **or** `0`, in which case it defaults to
+        72 dpi, matching Word's internal assumption.
+        """
+        dpi = self._image_header.vert_dpi
+        if dpi in (None, 0):
+            return 72
+        return dpi
 
     @property
     def width(self) -> Inches:
@@ -219,16 +227,24 @@ def px_height(self):
 
     @property
     def horz_dpi(self):
-        """Integer dots per inch for the width of this image.
-
-        Defaults to 72 when not present in the file, as is often the case.
         """
-        return self._horz_dpi
+        Horizontal DPI reported for this image.  Returns the header value
+        unless it is `None` **or** `0`, in which case it defaults to
+        72 dpi, matching Word's internal assumption.
+        """
+        dpi = self._horz_dpi
+        if dpi in (None, 0):
+            return 72
+        return dpi
 
     @property
     def vert_dpi(self):
-        """Integer dots per inch for the height of this image.
-
-        Defaults to 72 when not present in the file, as is often the case.
         """
-        return self._vert_dpi
+        Vertical DPI reported for this image.  Returns the header value
+        unless it is `None` **or** `0`, in which case it defaults to
+        72 dpi, matching Word's internal assumption.
+        """
+        dpi = self._vert_dpi
+        if dpi in (None, 0):
+            return 72
+        return dpi

tests/test_zero_dpi.py

@@ -0,0 +1,17 @@
+"""Regression test for issues #1497 and #1494 – ZeroDivisionError when adding a
+JPEG whose header reports 0 × 0 DPI.
+"""
+
+from pathlib import Path
+
+from docx import Document
+
+FIX = Path(__file__).with_name("test_files") / "zero_dpi.jpg"
+
+
+class DescribeZeroDensityJPEG:
+    """Suite covering `Document.add_picture()` with 0-DPI images."""
+
+    def it_handles_zero_dpi(self):
+        doc = Document()
+        doc.add_picture(str(FIX))