Populate DPI from JFIF cm density

Author: radarhere

Tests/images/jfif_unit_cm.jpg

@@ -181,6 +181,10 @@ def test(xdpi: int, ydpi: int | None = None) -> tuple[int, int] | None:
         assert test(100, 200) == (100, 200)
         assert test(0) is None  # square pixels
 
+    def test_dpi_jfif_cm(self):
+        with Image.open("Tests/images/jfif_unit_cm.jpg") as im:
+            assert im.info["dpi"] == (2.54, 5.08)
+
     @mark_if_feature_version(
         pytest.mark.valgrind_known_error, "libjpeg_turbo", "2.0", reason="Known Failing"
     )

Tests/test_file_jpeg.py

@@ -90,6 +90,9 @@ def APP(self: JpegImageFile, marker: int) -> None:
         else:
             if jfif_unit == 1:
                 self.info["dpi"] = jfif_density
+            elif jfif_unit == 2:  # cm
+                # 1 dpcm = 2.54 dpi
+                self.info["dpi"] = tuple(d * 2.54 for d in jfif_density)
             self.info["jfif_unit"] = jfif_unit
             self.info["jfif_density"] = jfif_density
     elif marker == 0xFFE1 and s[:6] == b"Exif\0\0":