DPI is a tuple (#2472)

* DPI is a tuple

* Some EXIF only contains an X resolution for DPI

* Refactor

* Test with no DPI in EXIF

* Handle EXIF with no DPI

* Created with: exiftool "-*resolution*"= photoshop-200dpi.jpg

* Test when not in EXIF, DPI==72,72

* Use X resolution for Y, default to 72,72 dpi

* Created with: exiftool -exif:ResolutionUnit=cm photoshop-200dpi.jpg

* Test for EXIF with dpcm instead of dpi

* Convert dpcm to dpi, and default to inches if unit unknown

PIL/JpegImagePlugin.py

@@ -119,8 +119,17 @@ def APP(self, marker):
 
     # If DPI isn't in JPEG header, fetch from EXIF
     if "dpi" not in self.info and "exif" in self.info:
-        x_resolution = self._getexif()[0x011A]
+        exif = self._getexif()
-        self.info["dpi"] = x_resolution[0] / x_resolution[1]
+        try:
+            resolution_unit = exif[0x0128]
+            x_resolution = exif[0x011A]
+            dpi = x_resolution[0] / x_resolution[1]
+            if resolution_unit == 3: # cm
+                # 1 dpcm = 2.54 dpi
+                dpi *= 2.54
+            self.info["dpi"] = dpi, dpi
+        except KeyError:
+            self.info["dpi"] = 72, 72
 
 
 def COM(self, marker):

Tests/test_file_jpeg.py

@@ -506,7 +506,27 @@ class TestFileJpeg(PillowTestCase):
         im = Image.open("Tests/images/photoshop-200dpi.jpg")
 
         # Act / Assert
-        self.assertEqual(im.info.get("dpi"), 200)
+        self.assertEqual(im.info.get("dpi"), (200, 200))
+
+    def test_dpi_from_dpcm_exif(self):
+        # Arrange
+        # This is photoshop-200dpi.jpg with EXIF resolution unit set to cm:
+        # exiftool -exif:ResolutionUnit=cm photoshop-200dpi.jpg
+        im = Image.open("Tests/images/exif-200dpcm.jpg")
+
+        # Act / Assert
+        self.assertEqual(im.info.get("dpi"), (508, 508))
+
+    def test_no_dpi_in_exif(self):
+        # Arrange
+        # This is photoshop-200dpi.jpg with resolution removed from EXIF:
+        # exiftool "-*resolution*"= photoshop-200dpi.jpg
+        im = Image.open("Tests/images/no-dpi-in-exif.jpg")
+
+        # Act / Assert
+        # "When the image resolution is unknown, 72 [dpi] is designated."
+        # http://www.exiv2.org/tags.html
+        self.assertEqual(im.info.get("dpi"), (72, 72))
 
 
 if __name__ == '__main__':