title: cv::UMat::convertTo in python
body: <p>I want to convert a <code>UMat</code> to a different type. </p>
  
  <p>I already tried using the <code>UMat</code> method <code>convertTo</code> which got me following error:</p>
  
  <p><code>AttributeError: 'cv2.UMat' object has no attribute 'convertTo'</code>.</p>
  
  <p>I also tried using the <code>numpy</code> method <code>np.float32</code> which appearently doesn't work with <code>UMat</code>:</p>
  
  <p><code>TypeError: float() argument must be a string or a number, not 'cv2.UMat'</code></p>
  
  <p>I would use a numpy array but I need it in the <code>UMat</code> format for other methods.</p>
  
  <p><br>
  This is the code in question:</p>
  
  <pre><code>array255 = cv2.UMat(int(height), int(width), type=cv2.CV_8UC1, s=(255))
  
  ...
  
  hsv_frame = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
  
  ...
  
  lhsv = cv2.split(hsv_frame)
  s = cv2.subtract(array255, lhsv[1])
  v = cv2.subtract(array255, lhsv[2])
  #s = s.convertTo(cv2.CV_32F)
  #v = v.convertTo(cv2.CV_32F)
  s = np.float32(s)
  v = np.float32(v)
  </code></pre>