title: OpenCV Error: Assertion failed (s >= 0) in setSize - while trying to place an image over a webcam feed
body: <p>I am trying to overlay an image of a hat over the head in a webcam feed and I get the error in the following line - </p>
  
  <pre><code>hat = cv2.resize(imghat, (hatWidth,hatHeight), interpolation = cv2.INTER_AREA)
  </code></pre>
  
  <p>The program runs, but quickly exits and gives the above error. The code segment is given below - </p>
  
  <pre><code># Create the mask for the hat
  imghatGray = cv2.cvtColor(imghat, cv2.COLOR_BGR2GRAY)
  #cv2.imwrite("imghatGray.png", imghatGray)
  
  ret, orig_mask = cv2.threshold(imghatGray, 0, 255, cv2.THRESH_BINARY)
  #cv2.imwrite("orig_mask.png", orig_mask)
  
  # Create the inverted mask for the hat
  orig_mask_inv = cv2.bitwise_not(orig_mask)
  #cv2.imwrite("orig_mask_inv.png", orig_mask_inv)
  
  # Convert hat image to BGR
  # and save the original image size (used later when re-sizing the image)
  imghat = imghat[:,:,0:3]
  orighatHeight, orighatWidth = imghat.shape[:2]
  
  video_capture = cv2.VideoCapture(0)
  
  while True:
  
      ret, frame = video_capture.read()
  
      gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  
      faces = face_cascade.detectMultiScale(gray, 1.3, 5, flags=cv2.cv.CV_HAAR_SCALE_IMAGE)
  
      for (x, y, w, h) in faces:
          cv2.rectangle(frame, (x,y), (x+w, y+h), (255,0,0), 2)
  
          roi_gray = gray[y:y+h, x:x+w]
          roi_color = frame[y:y+h, x:x+w]
  
          hatWidth = w
          hatHeight = hatWidth * orighatHeight / orighatWidth
  
          # Center the hat
          x1 = x -15
          x2 = x + hatWidth + 15
          y1 = y - hatHeight
          y2 = y + 15
  
          # Check for clipping
          if x1 &lt; 0:
              x1 = 0
          if y1 &lt; 0:
              y1 = 0
          if x2 &gt; w:
              x2 = w
          if y2 &gt; h:
              y2 = h
  
          cv2.rectangle(frame, (x1,y1), (x2, y2), (0,255,0), 2)
  
          # Re-calculate the width and height of the hat image
          hatWidth = x2 - x1
          hatHeight = y2 - y1
  
          # Re-size the original image and the masks to the hat sizes
          # calcualted above
          hat = cv2.resize(imghat, (hatWidth,hatHeight), interpolation = cv2.INTER_AREA)
          mask = cv2.resize(orig_mask, (hatWidth,hatHeight), interpolation = cv2.INTER_AREA)
          mask_inv = cv2.resize(orig_mask_inv, (hatWidth,hatHeight), interpolation = cv2.INTER_AREA)
  
          # take ROI for hat from background equal to size of hat image
          roi = roi_color[y1:y2, x1:x2]
  
          # roi_bg contains the original image only where the hat is not
          # in the region that is the size of the hat.
          roi_bg = cv2.bitwise_and(roi,roi,mask = mask_inv)
  
          # roi_fg contains the image of the hat only where the hat is
          roi_fg = cv2.bitwise_and(hat,hat,mask = mask)
  
          # join the roi_bg and roi_fg
          dst = cv2.add(roi_bg,roi_fg)
  
          # place the joined image, saved to dst back over the original image
          roi_color[y1:y2, x1:x2] = dst
  
          break
  </code></pre>
  
  <p>This is the error message shown - </p>
  
  <p><strong>v2.error: /home/vinravun/opencv-2.4.10/modules/core/src/matrix.cpp:116: error: (-215) s >= 0 in function setSize</strong></p>