Traditional animation is sometimes called hand-drawn animation or cel animation and, for most of the 20th Century, many popular animated films were created this way. It was a lengthy process. Thousands of pictures were drawn entirely by hand on acetate sheets, or cels, with each cel being slightly different from the one before it. Each cel was photographed onto a separate frame of film so that when the filmreel was played, the animation moved. This form of animation could also be combined with live-action video by placing the cels on top of the film. This technique was popular in the late 80s and early 90s, and was used in films such as Space Jam and Who Framed Roger Rabbit. Traditional animation takes a lot of artistic skill and has many different artistic styles: Disney’s films are very recognisable and considered quite realistic, whilst Studio Ghibli characters have a distinctive anime look. More stylistic drawings were used for many cartoon programmes, such as The Flintstones, and the Beatles’ Yellow Submarine used a pop-art style that was popular at the time it was made. The music video for the song “Take On Me” by A-ha is a good example of another style of traditional animation called rotoscoping, which used a live-action recording as a template for animation. In this video, a very simple pencil-sketch style was used. In fact, any style of art can be turned into animation. Although this traditional animation became unnecessary when digital techniques were invented, some modern animators, such as those who worked on the 2010 film The Illusionist, still choose to use this form.