Support saving PDF with different X and Y resolution.

Add a `dpi` parameter to the PDF save function, which accepts
a tuple with X and Y dpi.
This is useful for converting tiffg3 (fax) images to pdf,
which have split dpi like (204,391), (204,196) or (204,98).

Tests/test_file_pdf.py

@@ -80,6 +80,48 @@ def test_resolution(tmp_path):
     assert size == (61.44, 61.44)
 
 
+def test_dpi(tmp_path):
+    im = hopper()
+
+    outfile = str(tmp_path / "temp.pdf")
+    im.save(outfile, dpi=(75, 150))
+
+    with open(outfile, "rb") as fp:
+        contents = fp.read()
+
+    size = tuple(
+        float(d)
+        for d in contents.split(b"stream\nq ")[1].split(b" 0 0 cm")[0].split(b" 0 0 ")
+    )
+    assert size == (122.88, 61.44)
+
+    size = tuple(
+        float(d) for d in contents.split(b"/MediaBox [ 0 0 ")[1].split(b"]")[0].split()
+    )
+    assert size == (122.88, 61.44)
+
+
+def test_resolution_and_dpi(tmp_path):
+    im = hopper()
+
+    outfile = str(tmp_path / "temp.pdf")
+    im.save(outfile, resolution=200, dpi=(75, 150))
+
+    with open(outfile, "rb") as fp:
+        contents = fp.read()
+
+    size = tuple(
+        float(d)
+        for d in contents.split(b"stream\nq ")[1].split(b" 0 0 cm")[0].split(b" 0 0 ")
+    )
+    assert size == (122.88, 61.44)
+
+    size = tuple(
+        float(d) for d in contents.split(b"/MediaBox [ 0 0 ")[1].split(b"]")[0].split()
+    )
+    assert size == (122.88, 61.44)
+
+
 @mark_if_feature_version(
     pytest.mark.valgrind_known_error, "libjpeg_turbo", "2.0", reason="Known Failing"
 )

docs/handbook/image-file-formats.rst

@@ -1497,6 +1497,11 @@ The :py:meth:`~PIL.Image.Image.save` method can take the following keyword argum
     image, will determine the physical dimensions of the page that will be
     saved in the PDF.
 
+**dpi**
+    A tuple of (x_resolution, y_resolution), with inches as the resolution
+    unit. If both the ``resolution`` parameter and the ``dpi`` parameter are
+    present, ``resolution`` will be ignored.
+
 **title**
     The document’s title. If not appending to an existing PDF file, this will
     default to the filename.

src/PIL/PdfImagePlugin.py

@@ -53,7 +53,12 @@ def _save(im, fp, filename, save_all=False):
     else:
         existing_pdf = PdfParser.PdfParser(f=fp, filename=filename, mode="w+b")
 
-    resolution = im.encoderinfo.get("resolution", 72.0)
+    x_resolution = y_resolution = im.encoderinfo.get("resolution", 72.0)
+
+    dpi = im.encoderinfo.get("dpi")
+    if dpi:
+        x_resolution = dpi[0]
+        y_resolution = dpi[1]
 
     info = {
         "title": None
@@ -214,8 +219,8 @@ def _save(im, fp, filename, save_all=False):
                 stream=stream,
                 Type=PdfParser.PdfName("XObject"),
                 Subtype=PdfParser.PdfName("Image"),
-                Width=width,  # * 72.0 / resolution,
-                Height=height,  # * 72.0 / resolution,
+                Width=width,  # * 72.0 / x_resolution,
+                Height=height,  # * 72.0 / y_resolution,
                 Filter=filter,
                 BitsPerComponent=bits,
                 Decode=decode,
@@ -235,8 +240,8 @@ def _save(im, fp, filename, save_all=False):
                 MediaBox=[
                     0,
                     0,
-                    width * 72.0 / resolution,
-                    height * 72.0 / resolution,
+                    width * 72.0 / x_resolution,
+                    height * 72.0 / y_resolution,
                 ],
                 Contents=contents_refs[page_number],
             )
@@ -245,8 +250,8 @@ def _save(im, fp, filename, save_all=False):
             # page contents
 
             page_contents = b"q %f 0 0 %f 0 0 cm /image Do Q\n" % (
-                width * 72.0 / resolution,
-                height * 72.0 / resolution,
+                width * 72.0 / x_resolution,
+                height * 72.0 / y_resolution,
             )
 
             existing_pdf.write_obj(contents_refs[page_number], stream=page_contents)