D: The Hybrid Solution - From 2D to 3D: The Quest for the Lost Dimension

Chapter 3: From 2D to 3D: The Quest for the Lost Dimension

2.5 D: The Hybrid Solution

Let’s revisit our example shot. We have already concluded that because of the camera movement it cannot be done in 2D, so we must construct it in 3D. If we want to mimic reality as close as possible, we need to create every tree leaf and every blade of grass as separate 3D entities. When you think of the lawn extending around and behind the house or the trees in the far background, it is obvious that there is no way this can be done—such a scene, with billions of objects, is totally impractical to model and render. It is also a gigantic overkill—we cannot really see that much detail in the distance anyway. In fact, even creating separate trees in the distance is an overkill—as I’ve mentioned earlier, there would hardly be any noticeable parallax and perspective shift between those trees because of their relative distance from the camera. So is there really a need to build the entire scene in 3D? What if there was a way to use the 2D workflow within a 3D scene?

Such a technique is called 2.5D, or camera projection. Of course, there’s no such thing as 2.5D, but this moniker appropriately describes this technique—essentially a compromise between 2D and 3D, a smart and efficient way to get the best of each workflow. The idea is simple: the elements are created in 2D but are separated into several layers according to distance. Then each layer is projected on a flat card (or a very simple model) that is placed in three-dimensional space, at the correct distance from the camera. When the scene is rendered through the virtual camera, there will be accurate parallax between the different layers since they are in fact placed at different distances from the lens.

Obviously, this technique is not suitable for animated characters or foreground objects, especially if there is a strong perspective shift. But it works wonderfully well for backgrounds and distant objects, and even foreground elements that do not have much depth in them (a wall, for example). Rendering is extremely fast because there are no detailed 3D objects and no CG lighting, only images on cards (or simple geometry), and you get the benefit of being able to use real footage or photos, and avoid having to build and light everything from scratch.

This example of a matte painting in progress shows how different elements added to the footage are projected on separate cards. Notice that the cards are not yet arranged in the correct order—the debris element should be just in front of the plane, and the electricity pole should go in the back.

Courtesy of fxphd/Eduardo Abon.

Technically, the reason why the 2D elements are projected on cards (or simple geometry) is easy to understand if you examine the real-world difference between painting something on a screen or projecting onto it. If you paint on the screen, the image is “stuck” to it. Move the screen away from the camera, and the image will become smaller. Bring it closer and the image will grow bigger.

Move it sideways and the image moves too. But if you use a projector to project an image on the screen, the image will not change at all as you move the screen around (basically, the screen will move

“through” the image). The projection method therefore allows the artists to create a 2D matte painting in a 2D software such as Photoshop, and then transfer it to a 3D environment. The artist can move around the projection geometry without affecting the projected imagery, which enables proper set up of the scene for accurate parallax and depth without destroying the original look of the 2D matte painting.

The 2.5D workflow is widely used in visual effects, especially for environments. Matte painters (see Chapter 5) harness this technique to create extremely detailed and believable surroundings that have depth and realistic parallax without ever getting into any 3D work. The results can often be more convincing than a full 3D build, because what is lost in terms of fully accurate parallax and perspective shift is gained by the use of photographic material (incidentally, this technique is also prevalent in video games. Take any car race game, for example—you’ll easily notice that most of the background objects are not 3D, but rather 2.5D cards).

Putting It All Together

The reality of VFX work is that many shots are done as a combination of 2D, 3D, and 2.5D. There is no reason to stick rigidly to one workflow when in fact a combination can give the best results with the least amount of resources and time. So let’s return to our example shot once again, but this time, rather than look at it from a strictly 2D/3D standpoint, we’ll come up with a smarter plan that combines all options, starting from the foreground and moving towards the background. . .

Foreground: The strong perspective shift necessitates 3D in the foreground, so the front wall and fence are built in 3D. However, this does not mean that the models need to be very elaborate, because a lot of detail can be achieved with proper textures (see Chapter 5 for further discussion of texturing and shading). It also makes sense to create the big foreground tree as a full 3D element, because this will generate some fine parallax between tree branches and leaves, which will boost the believability of the shot. In an ideal world, the grass on the lawn (or at least the foreground part of it) would be created in 3D, but this will require a very dense model. We can get away with a 2.5D projection of a lawn image on the ground plane, because the human eye is more forgiving with very small detail and will probably not pick up the lack of internal parallax and perspective shift in that area.

Mid-ground: The main element in the mid-ground is the house itself. Most of the detail on the façade can be 2D; the only areas that need to be depicted in 3D are those that are either protruding from the façade or recessed from it, like the balcony and windows. Other elements such as bushes and nearby structures can be done as 2.5D projections on cards.

The foreground fence is built as a 3D element to achieve correct perspective shift.

Sons of Liberty © Stephen David Entertainment, History Channel, A+E Studios. Visual effects by Brainstorm Digital.

The model of the house is very simple. Most of the detail will come from the textures. The importance here is to build the protruding parts (like the balcony) and the sunken areas (like the windows) in 3D, because this is where parallax and perspective shift will be most noticeable.

Sons of Liberty © Stephen David Entertainment, History Channel, A+E Studios. Visual effects by Brainstorm Digital.

The base wall texture is added to the house.

Sons of Liberty © Stephen David Entertainment, History Channel, A+E Studios. Visual effects by Brainstorm Digital.

More texture detail is added to the house while the adjacent structure is also built with simple geometry.

Background: All the distant elements can be simple 2.5D projections. There is really no need to add any 3D detail, as long as the projections cards are placed at the correct distance from the camera based on the camera tracking information.

Lawn grass, bushes, distance trees and other elements are added as 2.5D projections on cards.

Finally, the large foreground tree is added as a full 3D model. This allowed us to achieve subtle but necessary parallax within the tree itself, and to add some breeze animation to the leaves.

This analysis of a specific shot and the plan that ensues are typical procedures within the VFX workflow.

They are absolutely necessary in order to ensure proper usage of resources, time, and money. In an ideal world, doing everything in 3D could be great (assuming all the 3D work, from modeling to lighting, is top notch, of course). But in the realities of filmmaking, a pragmatic approach that takes into consideration practical limitations often leads to better end results.

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