Merge pull request #38 from ajpmaclean/add_python_snippets

Adding Python snippets.

src/Cxx/Snippets.md

 ### Description
-Snippets are chuncks of code that can be cut (*snipped*) and pasted into examples. We want each example tobe stand-alone, so we do not keep the snippet code in a library.
+Snippets are chunks of code that can be cut (*snipped*) and pasted into examples. We want each example to be stand-alone, so we do not keep the snippet code in a library.
 
 ### Available snippets
 

src/Python/Snippets.md

+### Description
+Snippets are chunks of code that can be cut (*snipped*) and pasted into examples. We want each example to be stand-alone, so we do not keep the snippet code in a library.
+
+### Available snippets
+
+#### [GetProgramParameters](/Python/Snippets/GetProgramParameters.md)
+Get the program parameters.
+
+#### [ReadPolyData](/Python/Snippets/ReadPolyData.md)
+Uses the appropriate vtkPolyData reader to read any vtkPolyData file.
+
+#### [WritePNG](/Python/Snippets/WritePNG)
+Takes a screen shot from the render window and writes it to a png file.
+
+

src/Python/Snippets/GetProgramParameters.md

+### Description
+Get the program parameters.
+
+This particular snippet requires a file name and an optional figure number.
+
+To use the snippet, click the *Copy to clipboard* at the upper left of the code blocks.
+
+### Implementation
+``` python
+
+def get_program_parameters():
+    import argparse
+    description = 'What the program does.'
+    epilogue = '''
+        An expanded description of what the program does.
+   '''
+    parser = argparse.ArgumentParser(description=description, epilog=epilogue)
+    parser.add_argument('filename', help='A required filename.')
+    parser.add_argument('figure', default=0, type=int, nargs='?', help='An optional fugure number.')
+    args = parser.parse_args()
+    return args.filename, args.figure
+
+
+```
+
+### Typical usage
+``` python
+
+file_name, figure = get_program_parameters()
+
+```
\ No newline at end of file

src/Python/Snippets/ReadPolyData.md

+### Description
+Given a filename, uses the appropriate vtkPolyData reader to read any vtkPolyData file.
+
+To use the snippet, click the *Copy to clipboard* at the upper left of the code blocks.
+
+### Implementation
+``` python
+
+def ReadPolyData(file_name):
+    import os
+    path, extension = os.path.splitext(file_name)
+    extension = extension.lower()
+    if extension == ".ply":
+        reader = vtk.vtkPLYReader()
+        reader.SetFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    elif extension == ".vtp":
+        reader = vtk.vtkXMLpoly_dataReader()
+        reader.SetFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    elif extension == ".obj":
+        reader = vtk.vtkOBJReader()
+        reader.SetFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    elif extension == ".stl":
+        reader = vtk.vtkSTLReader()
+        reader.SetFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    elif extension == ".vtk":
+        reader = vtk.vtkpoly_dataReader()
+        reader.SetFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    elif extension == ".g":
+        reader = vtk.vtkBYUReader()
+        reader.SetGeometryFileName(file_name)
+        reader.Update()
+        poly_data = reader.GetOutput()
+    else:
+        # Return a None if the extension is unknown.
+        poly_data = None
+    return poly_data
+
+
+```

src/Python/Snippets/WritePNG.md

+### Description
+Given a filename, render window and optionally a rgba value, itakes a screen shot from the render window and writes it to a png file.
+
+To use the snippet, click the *Copy to clipboard* at the upper left of the code blocks.
+
+### Implementation
+``` python
+
+def WritePNG(fileName, ren_win, rgba = True):
+    """
+    Write the render window view to a png file.
+    :param fileName:
+    :param ren_win: The render window.
+    :param rgba: Used to det the buffer type.
+    :return:
+    """
+    """
+    Save a screenshot.
+    :param fileName:
+    :param renWin:
+    :return:
+    """
+    windowto_image_filter = vtk.vtkWindowToImageFilter()
+    windowto_image_filter.SetInput(ren_win)
+    windowto_image_filter.SetScale(1)  # image quality
+    if rgba:
+        windowto_image_filter.SetInputBufferTypeToRGBA()
+    else:
+        windowto_image_filter.SetInputBufferTypeToRGB()
+    # Read from the front buffer.
+        windowto_image_filter.ReadFrontBufferOff()
+        windowto_image_filter.Update()
+
+    writer = vtk.vtkPNGWriter()
+    writer.SetFileName(fileName)
+    writer.SetInputConnection(windowto_image_filter.GetOutputPort())
+    writer.Write()
+
+
+```