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What is 2.5D Animation?

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Question: What is 2.5D Animation?
You've heard of 2D animation: animation that takes place in two-dimensional space, usually drawn with line art against a flat background with depth only loosely implied by the placement of objects in relation to each other, and by simplified highlights and shadows. You've also heard of 3D animation: animation that takes place in three-dimensional space, with objects able to actually move on three axes rather than on the two allowed by 2D (with the third in 2D only implied by changes in an object's size and motion on two axes). But what is 2.5D? How can animation be halfway between 2D and 3D?
Answer: Basically, 2.5D animation is 2D animation in a 3D space. Sometimes this involves actually moving 2D-animated objects in a 3D space; sometimes it involves using clever tricks of perspective and shadow to make 2D space look like 3D space, although you're still working on a 2D plane. One of my favorite examples of 2.5D animation is the Leningrad Dr. House music video, which is nightmare fuel at its finest (seriously, I can't even understand what it's saying, but those faces will haunt me in the darkest hours of night) - but it's also a stellar example of well-done 2.5D animation.

If you look at the Dr. House video, all of the figures look to be 2D drawings, but they appear to be moving in a 3D space - many even giving the illusion of turning in three dimensions. This is accomplished through clever layering, shadows, perspective animations, morphing, and it looks like some of the animation is done using bone systems. This creates a certain fluidity and changes in depth that make us think we're looking at 2D paper cutouts moving in a 3D space, almost like paper dolls standing on a table.

The 2.5D effect also extends to objects that don't look like paper cutouts, but just typical 2D animations - but they appear to move in a 3D space. This can be as simple as a character's head turning, but it moves with that 3D fluidity that makes it seem like a 3D object with flat shading and outlines to give it a 2D look, and further creates the impression of 3D space around a 2D object. You see this a lot in newer episodes of Family Guy, where they actually do use 3D animations rendered to look like 2D animations. They're 3D objects moving in a 2D space.

Adding shadows to a 2D object can also accomplish the 2.5D effect. I don't mean shading the figure itself; I mean having it cast a shadow on the objects around it. Think of a 2D object standing against a white background. On its own, it's just a drawing on paper. Flat. Two-dimensional. Now add a shadow of that object, stretching into the distance - into the paper, a dimension that technically doesn't exist for this object but that we can imply. Suddenly it's a 2D object standing in a 3D space, because we've created the implication of a third dimension by adding this shadow - which should taper off with perspective to further imply depth.

I've seen people create 2.5D effects in both 2D and 3D programs; in programs such as Flash, you can use the tools to create false perspective, and then animate / tween that perspective changing for a 2.5D effect. You can also create shadows using copies of objects greyed out and arranged properly, with perspective tools appropriately stretching the shape. In 3D programs, you can map 2D objects onto transparent polygons and then animate and position them, with lighting to add shadows.

No matter which way it's done, if you look closely, you'll see 2.5D everywhere.

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