Finding out number of pixels in white area in binary image as well as number of pixels of ROI in original image.

Hello, I have segmented image in which my region of interest (ROI) was white color cotton. Now I want to compare the number of pixels in segmented area i.e. total number of pixels in white blob in binary image with actual number of pixels of ROI in actual image. How I can do that. Following figure can clear the point.

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As we can see from original image, my ROI was white color cotton circled in red boundry. When I segmented this image I got binary image as shown. As we can noticed there are some missing areas in binary image as compare to original area. So, I want to count the number of pixels in original image of ROI and number of pixels of white blob in binary image. So that I can calculate difference in actual pixels of ROI and actual segmented number of pixels.

Thank You.