I'd like to help, but I don't follow your description. What would you be looping over? And what would you be summing? It sounds like you only have one NDVI image, so you must not be looping over images. Are you trying to sum all of the NDVI values in pixels that have NDVI above or below some certain value? That's kind of what it sounds like, but I have a hard time thinking of why someone would want to do that. So, probably I just don't understand. If you can give more details, I'll see if I can offer suggestions. - Peg

Yes, I have only one image and I want to add the NDVI values of a specific area. I was wondering if I can create a ROI and then somehow calculate the sum of all the NDVI values in pixels of that ROI which are within a threshold. If I'm still vague please let me know. Thank you.