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It seems the image you provide is with alpha channel, where as you said the white region is complete transparent(alpha = 0), so just do the following.

• Load source with alpha, you should set flags = -1 on imread().

• Now split image in to four channel.

  Mat splitedBGRA[4];
  split(src,splitedBGRA);
  

Note that OpenCV channel order is BGRA.

• Now threshold alpha channel, you should use THRESH_BINARY_INV.

• Find contour and calculate bounding box for each contour.

See the result I got with the above algorithm.