2D shapes can be separated by distance but only on the surface plane. In computer graphics this is measured in horizontal and vertical units - known as X and Y values respectively. These can be used to plot exactly how far across and up an element is on the screen.
A well known example of 2D animation is The Simpsons by Matt Groening.
3D animation on the other hand has the added 'third dimension' of 'depth'. This means shapes can have depth themselves (as opposed to being just flat) and can also be separated by forward/backward distances not just the up/down, left/right distances of 2D. This dimension is described as 'Z' depth i.e. how far away the object is from the screen.
A famous example of 3D animation is "Toy Story" from Disney-Pixar.
2.5D animation is somewhere in-between, an alternative sometimes referred to as 'postcards in space'. This is because while the shapes can also be separated by depth, the images themselves are flat layers - not true 3D models. This 'compositing' technique is often used in documentaries to animate archive photographs in a more interesting way than just panning across like a rostrum camera. This effect is achieved by manipulating an optical phenomenon known as the "parallax effect". It breaks still images into sequenced layers and moves the camera between them to create the illusion they have been shot in motion.
An example of 2.5D animation is a movie about Producer Robert Evans "The kid stays in the picture".
Tip: If you are having problems visualising the differences between the types of animation described above it might be helpful to think of 2D as a picture of a King printed on a playing card. 3D as the King piece on a chess board and 2.5D as playing cards arranged into a 'house of cards'.
Stop motion animation is also worth a mention. All the types mentioned above can be created digitally - directly within a computer. Stop motion is different because it requires a real physical camera to record actual tangible objects. This technique can produce some charming results but it is very labour intensive. Tiny, incremental movements are made then individually photographed. The process is repeated many times - each with progressive step movements - which generate only seconds of moving 'film'.
A popular example of Stop-Motion is the 'claymation' "Chicken Run" by Aardman Animation.
You can see examples of the styles mentioned above by clicking the images above to access their case studies or browse through from the drop down menu under the "Work" menu heading.
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