Film developed as an art form in the late 19th century with the invention of technologies like still photography, celluloid film strips, and early devices like the kinetoscope that allowed successive photos to be viewed as moving images. Over time, film evolved into a collaborative medium incorporating the work of directors, actors, cinematographers, editors and other roles. Directors work with ideas, images and effects to conceptualize scenes and express their vision, though they rely on entire production teams. Different film genres also emerged like silent comedies, gangster films, and others in response to technological advances and audience demand.