### Description
Given a filename, render window and optionally a rgba value, take a screenshot of the render window and write it to a file. The extension of the filename determines what writer to use.

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### Implementation
```python


def WriteImage(fileName, renWin, rgba=True):
    """
    Write the render window view to an image file.

    Image types supported are:
     BMP, JPEG, PNM, PNG, PostScript, TIFF.
    The default parameters are used for all writers, change as needed.

    :param fileName: The file name, if no extension then PNG is assumed.
    :param renWin: The render window.
    :param rgba: Used to set the buffer type.
    :return:
    """

    import os

    if fileName:
        # Select the writer to use.
        path, ext = os.path.splitext(fileName)
        ext = ext.lower()
        if not ext:
            ext = '.png'
            fileName = fileName + ext
        if ext == '.bmp':
            writer = vtk.vtkBMPWriter()
        elif ext == '.jpg':
            writer = vtk.vtkJPEGWriter()
        elif ext == '.pnm':
            writer = vtk.vtkPNMWriter()
        elif ext == '.ps':
            if rgba:
                rgba = False
            writer = vtk.vtkPostScriptWriter()
        elif ext == '.tiff':
            writer = vtk.vtkTIFFWriter()
        else:
            writer = vtk.vtkPNGWriter()

        windowto_image_filter = vtk.vtkWindowToImageFilter()
        windowto_image_filter.SetInput(renWin)
        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.SetFileName(fileName)
        writer.SetInputConnection(windowto_image_filter.GetOutputPort())
        writer.Write()
    else:
        raise RuntimeError('Need a filename.')


```