jbig2enc: Add patch to retain DPI

jbig2enc's pdf.py demo program did not properly retain an image's DPI
when creating a PDF; it was hardcoded to 600, regardless of the actual
resolution of the input image. This patch corrects that by intelligently
extracting the DPI from the file being converted.

Closes Homebrew/homebrew#7913.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/jbig2enc.rb

@@ -12,6 +12,8 @@ class Jbig2enc < Formula
   def patches
     # jbig2enc hardcodes the include and libraries paths.
     # Fixing them up for Homebrew
+    # Also contains a patch to pdf.py that retains DPI:
+    # https://github.com/agl/jbig2enc/issues/15
     DATA
   end
 
@@ -24,7 +26,7 @@ end
 
 __END__
 diff --git a/Makefile b/Makefile
-index 0553375..e728c40 100644
+index dbb4556..aac8101 100644
 --- a/Makefile
 +++ b/Makefile
 @@ -1,11 +1,11 @@
@@ -42,3 +44,36 @@ index 0553375..e728c40 100644
  
  libjbig2enc.a: jbig2enc.o jbig2arith.o jbig2sym.o
  	ar -rcv libjbig2enc.a jbig2enc.o jbig2arith.o jbig2sym.o
+diff --git a/pdf.py b/pdf.py
+index cd37c9f..4b4a8e8 100644
+--- a/pdf.py
++++ b/pdf.py
+@@ -11,7 +11,7 @@ import os
+ # Run ./jbig2 -s -p <other options> image1.jpeg image1.jpeg ...
+ # python pdf.py output > out.pdf
+
+-dpi = 600
++#dpi = 600
+
+ class Ref:
+   def __init__(self, x):
+@@ -121,16 +121,16 @@ def main(symboltable='symboltable', pagefiles=glob.glob('page-*')):
+     except IOError:
+       sys.stderr.write("error reading page file %s\n"% p)
+       continue
+-    (width, height) = struct.unpack('>II', contents[11:19])
++    (width, height,xres,yres) = struct.unpack('>IIII', contents[11:27])
+     xobj = Obj({'Type': '/XObject', 'Subtype': '/Image', 'Width':
+         str(width), 'Height': str(height), 'ColorSpace': '/DeviceGray',
+         'BitsPerComponent': '1', 'Filter': '/JBIG2Decode', 'DecodeParms':
+         ' << /JBIG2Globals %d 0 R >>' % symd.id}, contents)
+-    contents = Obj({}, 'q %f 0 0 %f 0 0 cm /Im1 Do Q' % (float(width * 72) / dpi, float(height * 72) / dpi))
++    contents = Obj({}, 'q %f 0 0 %f 0 0 cm /Im1 Do Q' % (float(width * 72) / xres, float(height * 72) / yres))
+     resources = Obj({'ProcSet': '[/PDF /ImageB]',
+         'XObject': '<< /Im1 %d 0 R >>' % xobj.id})
+     page = Obj({'Type': '/Page', 'Parent': '3 0 R',
+-        'MediaBox': '[ 0 0 %f %f ]' % (float(width * 72) / dpi, float(height * 72) / dpi),
++        'MediaBox': '[ 0 0 %f %f ]' % (float(width * 72) / xres, float(height * 72) / yres),
+         'Contents': ref(contents.id),
+         'Resources': ref(resources.id)})
+     [doc.add_object(x) for x in [xobj, contents, resources, page]]