I don't know if this is the best way, but the below algorithm provides one way - based on IDL's LABEL_REGION function - that does not require many code lines: ; Make some simple dummy 400 x 400 image data seed = 1 temp = byte(randomu(seed,10,10) * 10) image = rebin(temp, 400, 400) window, XSIZE=400, YSIZE=400, /FREE, TITLE='Original Image' tvscl, image ; Let's make a threshold that considers ~10% of image to be dead pixels threshold = 1b deadPixelIndices = where(image le threshold) ; Make a binary image from these indices ; LABEL_REGION will need dead pixels to be the only values gt 0 deadPixelMask = bytarr(400, 400) deadPixelMask[deadPixelIndices] = 255b window, XSIZE=400, YSIZE=400, /FREE, TITLE='Dead Pixels In White' tvscl, deadPixelMask ; This example produces 12 larger regions. Let's add one small 1-pixel ; region at the edge of the mask deadPixelMask[150,0] = 255b tvscl, deadPixelMask ; LABEL_REGION does not find regions or parts of regions that are ; on any outside edge of the image. That is why we make this extra ; image object maskWithExtraBorder = bytarr(402,402) maskWithExtraBorder[1,1] = deadPixelMask ; Center mask in new image result = label_region(maskWithExtraBorder, /ALL_NEIGHBORS) nDeadPixelRegions = max(result) ; 13 Best of luck, James Jones