# Voxel

A series of voxels in a stack with a single voxel shaded
Illustration of a voxel grid, each containing a color value.

A voxel represents a value on a regular grid in three-dimensional space. Voxel is a combination of "volume" and "pixel" where pixel is a combination of "picture" and "element". As with pixels in a bitmap, voxels themselves do not typically have their position (their coordinates) explicitly encoded along with their values. Instead, the position of a voxel is inferred based upon its position relative to other voxels (i.e., its position in the data structure that makes up a single volumetric image). In contrast to pixels and voxels, points and polygons are often explicitly represented by the coordinates of their vertices. A direct consequence of this difference is that polygons are able to efficiently represent simple 3D structures with lots of empty or homogeneously filled space, while voxels are good at representing regularly sampled spaces that are non-homogeneously filled.

Voxels are frequently used in the visualization and analysis of medical and scientific data. Some volumetric displays use voxels to describe their resolution. For example, a display might be able to show 512×512×512 voxels.

## Rendering

A volume described as voxels can be visualized either by direct volume rendering or by the extraction of polygon iso-surfaces that follow the contours of given threshold values. The marching cubes algorithm is often used for isosurface extraction, however other methods exist as well.

Both ray-tracing and ray-casting, as well as rasterisation, can be applied to voxel data to obtain 2D raster graphics to depict on a monitor.

Another technique for voxels involves Raster graphics where you simply raytrace every pixel of the display into the scene. A typical implementation will raytrace each pixel of the display starting at the bottom of the screen using what is known as a y-buffer. When a voxel is reached that has a higher y value on the display it is added to the y-buffer overriding the previous value and connected with the previous y-value on the screen interpolating the color values. There is a major downside to voxel rasterization when transformation is applied which causes severe aliasing.[1]

Outcast and other 1990's video games employed this graphics technique for effects such as reflection and bump-mapping and usually for terrain rendering. Outcast's graphics engine was mainly a combination of a ray casting (heightmap) engine, used to render the landscape, and a texture mapping polygon engine used to render objects. The "Engine Programming" section of the games credits in the manual has several subsections related to graphics, among them: "Landscape Engine", "Polygon Engine", "Water & Shadows Engine" and "Special effects Engine".[2] Although Outcast is often cited as a forerunner of voxel technology,[3] this is somewhat misleading. The game does not actually model three-dimensional volumes of voxels. Instead, it models the ground as a surface, which may be seen as being made up of voxels. The ground is decorated with objects that are modeled using texture-mapped polygons. When Outcast was developed, the term "voxel engine", when applied to computer games, commonly referred to a ray casting engine (for example the VoxelSpace engine). On the engine technology page of the game's website, the landscape engine is also referred to as the "Voxels engine".[4] The engine is purely software-based; it does not rely on hardware-acceleration via a 3D graphics card.[5]

John Carmack also experimented with Voxels for the Quake III engine.[6] One such problem cited by Carmack is the lack of graphics cards designed specifically for such rendering requiring them to be software rendered, which still remains an issue with the technology to this day.

Comanche was also the first commercial flight simulation based on voxel technology. NovaLogic used the proprietary Voxel Space engine developed for the company by Kyle Freeman [7](written entirely in Assembly language) to create open landscapes.[8] This rendering technique allowed for much more detailed and realistic terrain compared to simulations based on vector graphics at that time.[5]

## Voxel data

A voxel represents a single sample, or data point, on a regularly spaced, three-dimensional grid. This data point can consist of a single piece of data, such as an opacity, or multiple pieces of data, such as a color in addition to opacity. A voxel represents only a single point on this grid, not a volume; the space between each voxel is not represented in a voxel-based dataset. Depending on the type of data and the intended use for the dataset, this missing information may be reconstructed and/or approximated, e.g. via interpolation.

The value of a voxel may represent various properties. In CT scans, the values are Hounsfield units, giving the opacity of material to X-rays.[9]:29 Different types of value are acquired from MRI or ultrasound.

Voxels can contain multiple scalar values, essentially vector (tensor) data; in the case of ultrasound scans with B-mode and Doppler data, density, and volumetric flow rate are captured as separate channels of data relating to the same voxel positions.

While voxels provide the benefit of precision and depth of reality, they are typically large data sets and are unwieldy to manage given the bandwidth of common computers. However, through efficient compression and manipulation of large data files, interactive visualization can be enabled on consumer market computers.

Other values may be useful for immediate 3D rendering, such as a surface normal vector and color.

## Uses

Common uses of voxels include volumetric imaging in medicine and representation of terrain in games and simulations. Voxel terrain is used instead of a heightmap because of its ability to represent overhangs, caves, arches, and other 3D terrain features. These concave features cannot be represented in a heightmap due to only the top 'layer' of data being represented, leaving everything below it filled (the volume that would otherwise be the inside of the caves, or the underside of arches or overhangs).

## Voxel editors

While scientific volume visualization doesn't require modifying the actual voxel data, voxel editors can be used to create art (especially 3D pixel art) and models for voxel based games. Some editors are focused on a single approach to voxel editing while others mix various approaches. Some common approaches are:

• Slice based: The volume is sliced in one or more axes and the user can edit each image individually using 2D raster editor tools. These generally store color information in voxels.
• Sculpture: Similar to the vector counterpart but with no topology constraints. These usually store density information in voxels and lack color information.
• Building blocks: The user can add and remove blocks just like a construction set toy.

### Voxel editors for games

Many game developers use in-house editors that are not released to the public, but a few games have publicly available editors, some of them created by players.

### General purpose voxel editors

There are a few voxel editors available that are not tied to specific games or engines. They can be used as alternatives or complements to traditional 3D vector modeling.

## Extensions

A generalization of a voxel is the doxel, or dynamic voxel. This is used in the case of a 4D dataset, for example, an image sequence that represents 3D space together with another dimension such as time. In this way, an image could contain 100×100×100×100 doxels, which could be seen as a series of 100 frames of a 100×100×100 volume image (the equivalent for a 3D image would be showing a 2D cross section of the image in each frame). Although storage and manipulation of such data requires large amounts of memory, it allows the representation and analysis of spacetime systems.

## References

1. ^ http://www.tnlc.com/eep/voxels.html
2. ^ Outcast manual. Infogrames. 1999. p. 34.
3. ^ Bertuch, Manfred (2009). "'Klötzchenwelten' [Worlds of little blocks] in c't Magazin issue 04/2009". Hannover: Heise Zeitschriften Verlag GmbH & Co. KG. p. 183. ISSN 0724-8679.
4. ^ Engine Technology
5. ^ a b "Voxel terrain engine", introduction. In a coder's mind, 2005.
6. ^ http://www.tomshardware.com/reviews/voxel-ray-casting,2423-2.html
7. ^ http://patents.justia.com/inventor/kyle-g-freeman
8. ^ http://www.flightsim.com/vbfs/content.php?2994-NovaLogic-Awarded-Patent-For-Voxel-Space-Graphics-Engine
9. ^ Novelline, Robert. Squire's Fundamentals of Radiology. Harvard University Press. 5th edition. 1997. ISBN 0-674-83339-2.
10. ^ http://www.extentofthejam.com/pseudo/
12. ^ http://projectorgames.net/blog/?p=168
13. ^ http://www.mobygames.com/company/appeal-sa
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15. ^ "VANGERS". kdlab.com. Retrieved 2009-12-20.
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18. ^ http://trovegame.com
19. ^ http://oortonline.com/
20. ^ http://wonderstruckgames.com/
21. ^ "Project Perfect Mod". Ppmsite.com. 2007-04-04. Retrieved 2012-05-19.
22. ^ "Ken Silverman's Projects Page". Advsys.net. Retrieved 2012-05-19.
23. ^ "Ken Silverman's Voxlap Page". Advsys.net. Retrieved 2012-05-19.
24. ^ "CryEngine2 Sandbox2 Tutorial". Konakona.nbtxathcx.net. Retrieved 2012-05-19.
25. ^ "C4 Engine Features". Terathon.com. Retrieved 2012-05-19.
26. ^ "The Transvoxel Algorithm for Voxel Terrain". Terathon.com. Retrieved 2012-05-19.
27. ^ "Qubicle's Main Page". minddesk.com. Retrieved 2014-07-06.