EMF: support negative bounding box coordinates (#2249)

* EMF: support negative bounding box coordinates

Similar to placeable WMF, bounding box coordinates
should be interpreted as signed integer, otherwise
opening EMF file with negative (x0,y0) fails.

* Basic load tests for WMF and EMF formats

* WMF/WMF tests: just test open(), not load()

Not sure why load() fails on Debian build. Well, at least we can test
open().

* WMF/EMF: Unpack signed integers using unpack()

* WMF/EMF: Compare to reference PNG rendering

* EMF/WMF comparison: use assert_image_similar()

* Use similarity epsilon 0.5 for WMF, as vector rendering looks different across Windows platforms

* Trigger rebuild

PIL/WmfImagePlugin.py

@@ -14,6 +14,10 @@
 #
 # See the README file for information on usage and redistribution.
 #
+# WMF/EMF reference documentation:
+# https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-WMF/[MS-WMF].pdf
+# http://wvware.sourceforge.net/caolan/index.html
+# http://wvware.sourceforge.net/caolan/ora-wmf.html
 
 from PIL import Image, ImageFile, _binary
 
@@ -56,16 +60,9 @@ if hasattr(Image.core, "drawwmf"):
 # --------------------------------------------------------------------
 
 word = _binary.i16le
-
-
-def short(c, o=0):
-    v = word(c, o)
-    if v >= 32768:
-        v -= 65536
-    return v
-
+short = _binary.si16le
 dword = _binary.i32le
-
+_long = _binary.si32le
 
 #
 # --------------------------------------------------------------------
@@ -121,13 +118,13 @@ class WmfStubImageFile(ImageFile.StubImageFile):
             # enhanced metafile
 
             # get bounding box
-            x0 = dword(s, 8)
-            y0 = dword(s, 12)
-            x1 = dword(s, 16)
-            y1 = dword(s, 20)
+            x0 = _long(s, 8)
+            y0 = _long(s, 12)
+            x1 = _long(s, 16)
+            y1 = _long(s, 20)
 
             # get frame (in 0.01 millimeter units)
-            frame = dword(s, 24), dword(s, 28), dword(s, 32), dword(s, 36)
+            frame = _long(s, 24), _long(s, 28), _long(s, 32), _long(s, 36)
 
             # normalize size to 72 dots per inch
             size = x1 - x0, y1 - y0

PIL/_binary.py

@@ -31,23 +31,41 @@ else:
 # TODO: replace with more readable struct.unpack equivalent
 def i16le(c, o=0):
     """
-    Converts a 2-bytes (16 bits) string to an integer.
+    Converts a 2-bytes (16 bits) string to an unsigned integer.
 
     c: string containing bytes to convert
     o: offset of bytes to convert in string
     """
     return unpack("<H", c[o:o+2])[0]
 
+def si16le(c, o=0):
+    """
+    Converts a 2-bytes (16 bits) string to a signed integer.
+
+    c: string containing bytes to convert
+    o: offset of bytes to convert in string
+    """
+    return unpack("<h", c[o:o+2])[0]
+
 
 def i32le(c, o=0):
     """
-    Converts a 4-bytes (32 bits) string to an integer.
+    Converts a 4-bytes (32 bits) string to an unsigned integer.
 
     c: string containing bytes to convert
     o: offset of bytes to convert in string
     """
     return unpack("<I", c[o:o+4])[0]
 
+def si32le(c, o=0):
+    """
+    Converts a 4-bytes (32 bits) string to a signed integer.
+
+    c: string containing bytes to convert
+    o: offset of bytes to convert in string
+    """
+    return unpack("<i", c[o:o+4])[0]
+
 
 def i16be(c, o=0):
     return unpack(">H", c[o:o+2])[0]

Tests/test_file_wmf.py

@@ -0,0 +1,31 @@
+from helper import unittest, PillowTestCase
+from PIL import Image
+from io import BytesIO
+
+class TestFileWmf(PillowTestCase):
+
+    def test_load_raw(self):
+
+        # Test basic EMF open and rendering
+        im = Image.open('Tests/images/drawing.emf')
+        if hasattr(Image.core, "drawwmf"):
+            # Currently, support for WMF/EMF is Windows-only
+            im.load()
+            # Compare to reference rendering
+            imref = Image.open('Tests/images/drawing_emf_ref.png')
+            imref.load()
+            self.assert_image_similar(im, imref, 0)
+
+        # Test basic WMF open and rendering
+        im = Image.open('Tests/images/drawing.wmf')
+        if hasattr(Image.core, "drawwmf"):
+            # Currently, support for WMF/EMF is Windows-only
+            im.load()
+            # Compare to reference rendering
+            imref = Image.open('Tests/images/drawing_wmf_ref.png')
+            imref.load()
+            self.assert_image_similar(im, imref, 0.5)
+
+
+if __name__ == '__main__':
+    unittest.main()