Hello benyun. Heat map allows to unite neighboring areas those would otherwise remain separate ‘islands’. Skipping heat map would affect the final output of `cv2.groupRectangles`. It may not be obvious with the given frame where the hot points represent two clearly pronounced islands, but this is not always the case — there may be more ‘islands’ related to the same vehicle and not uniting them would affect the final bounding boxes. More on that see at p.6 here.