Centroid of only specific (dark) pixels in an 8-bit image

Hello,

I have a small program that calculates centroid as attached. But this program currently calculates centroid of whole image and I want to calculate centroid of only dark pixels and ignore everything else. For example, the attached image has a cross mark made with dark gray/black pixels in form of circles. I want the program to find centroid of only dark pixels and ignore whole white background. I need some kind of thresholding/restriction that will allow only pixels with certain value (for e.g. <100 because it is an 8-bit image with values of pixels between 0 and 255) or less pass through the vi that calculates centroid. Can someone please help as I am new with LabVIEW and I am trying to learn?

Thanks

VeMo