title: Python Wand convert pdf to black image
body: <p>I'm trying to convert some pdf files to jpg through Wand in Python:</p>
  
  <pre><code>from wand.image import Image as Img
  from wand.color import Color
  
      def importPdf(self):
          filename, _ = QtWidgets.QFileDialog.getOpenFileName(self, "Open File",
                                                              QtCore.QDir.currentPath())
          print(filename)
          if not filename:
              print('error')
              return
          with Img(filename=filename,format='jpeg', resolution=300) as image:
              image.compression_quality = 99
              image.save(filename='file.jpeg')
              self.open_picture()
  </code></pre>
  
  <p>My problem is that it results is a black screeen. The conversion works fine with png, but I cannot perform the OCR (via tesseract on the png).
  I think it comes from a kind of transparent layer, but I have not found the way to remove it, though I did several things such as </p>
  
  <pre><code>image.alpha_channel = False # made the same with True
  image.background_color = Color('White')
  </code></pre>
  
  <p>before saving the file.
  I'm using Imagemagick V6.9, because V7 fails with Wand.</p>