use png instead of pdf due to it's wider support (in the qt console and in firefox)

nltk/tree.py

@@ -689,13 +689,19 @@ class Tree(list):
         childstr = ", ".join(unicode_repr(c) for c in self)
         return '%s(%s, [%s])' % (type(self).__name__, unicode_repr(self._label), childstr)
 
-    def _repr_pdf_(self):
+    def _repr_png_(self):
+        """
+        Draws and outputs in PNG for ipython.
+        PNG is used instead of PDF, since it can be displayed in the qt console and
+        has wider browser support.
+        """
         import os
         import base64
         import subprocess
         import tempfile
         from nltk.draw.tree import tree_to_treesegment
         from nltk.draw.util import CanvasFrame
+        from nltk.internals import find_binary
         _canvas_frame = CanvasFrame()
         widget = tree_to_treesegment(_canvas_frame.canvas(), self)
         _canvas_frame.add_widget(widget)
@@ -703,15 +709,18 @@ class Tree(list):
         # print_to_file uses scrollregion to set the width and height of the pdf.
         _canvas_frame.canvas()['scrollregion'] = (0, 0, w, h)
         with tempfile.NamedTemporaryFile() as file:
-            ps_path = '{0:}.ps'.format(file.name)
-            pdf_path = '{0:}.pdf'.format(file.name)
-            _canvas_frame.print_to_file(ps_path)
+            in_path = '{0:}.ps'.format(file.name)
+            out_path = '{0:}.png'.format(file.name)
+            _canvas_frame.print_to_file(in_path)
             _canvas_frame.destroy_widget(widget)
-            subprocess.call(['epstopdf', ps_path], shell=True)
-            with open(pdf_path, 'rb') as sr:
+            subprocess.call([find_binary('gs', binary_names=['gswin32c.exe', 'gswin64c.exe'], env_vars=['PATH'], verbose=False)] +
+                            '-q -dEPSCrop -sDEVICE=png16m -r90 -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -dSAFER -dBATCH -dNOPAUSE -sOutputFile={0:} {1:}'
+                            .format(out_path, in_path).split(),
+                            shell=True)
+            with open(out_path, 'rb') as sr:
                 res = sr.read()
-            os.remove(ps_path)
-            os.remove(pdf_path)
+            os.remove(in_path)
+            os.remove(out_path)
             return base64.b64encode(res).decode()
 
     def __str__(self):