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Describing a world in data

Just like modern games, early games like Ant Attack required data that described in some meaningful way how the landscape was to appear. The eerie city landscape of "Antchester" (shown in the following screenshot) was constructed in memory as a 128 x 128 byte grid, the first 128 bytes defined the upper-left wall, and the 128 byte row below that, and so on. Each of these bytes described the vertical arrangement of blocks in lower six bits, for game logic purposes the upper two bits were used for game sprites.

Heightmaps are common ground

The arrangement of numbers in a grid pattern is still extensively used to represent terrain. We call these grids "maps" and they are popular by virtue of being simple to use and manipulate. A long way from "Antchester", maps can now be measured in megabytes or Gigabytes (around 20GB is needed for the whole earth at 30 meter resolution). Each value in the map represents the height of the terrain at that location.

These kinds of maps are known as heightmaps. However, any information that can be represented in the grid pattern can use maps. Additional maps can be used by 3D engines to tell it how to mix many textures together; this is a common terrain painting technique known as "splatting". Splats describe the amount of blending between texture layers. Another kind of map might be used for lighting, adding light, or shadows to an area of the map. We also find in some engines something called visibility maps which hide parts of the terrain; for example we might want to add holes or caves into a landscape. Coverage maps might be used to represent objects such as grasses, different vegetation layers might have some kind of map the engine uses to draw 3D objects onto the terrain surface. GROME allows us to create and edit all of these kinds of maps and export them, with a little bit of manipulation we can port this information into most game engines. Whatever the technique used by an engine to paint the terrain, height-maps are fairly universal in how they are used to describe topography.

The following is an example of a heightmap loaded into an image viewer. It appears as a gray scale image, the intensity of each pixel represents a height value at that location on the map.

This map represents a 100 square kilometer area of north-west Afghanistan used in a flight simulation.

GROME like many other terrain editing tools uses heightmaps to transport terrain information. Typically importing the heightmap as a gray scale image using common file formats such as TIFF, PNG, or BMP. When it's time to export the terrain project you have similar options to save.

This commonality is the basis of using GROME as a tool for many different engines. There's nothing to stop you from making changes to an exported heightmap using image editing software. The GROME plugin system and SDK permit you to make your own custom exporter for any unsupported formats. So long as we can deal with the material and texture format requirements for our host 3D engine we can integrate GROME into the art pipeline.

Texture sizes

Using textures for heightmap information does have limitations. The largest "safe" size for a texture is considered 4096 x 4096 although some of the older 3D cards would have problems with anything higher than 2048 x 2048. Also, host 3D engines often require texture dimensions to be a power of 2. A table of recommended dimensions for images follow:

512 x 512 provides efficient trade-off between resolution and performance and is the default value for GROME operations.

If you're familiar with this already then great, you might see questions posted on forums about texture corruption or materials not looking correct. Sometimes these problems are the result of not conforming to this arrangement. Also, you'll see these numbers crop up a few times in GROME's drop-down property boxes. To avoid any potential problems it is wise to ensure any textures you use in your projects conform to these specifications. One exception is Unreal Development Kit ( UDK) in which you'll see numbers such as 257 x 257 used.

If you have a huge amount of terrain data that you need to import for a project you can use the texture formats mentioned earlier but I recommend using RAW formats if possible. If your project is based on real-world topography then importing DTED or GeoTIFF data will extract geographical information such as latitude, longitude, and number of arc seconds represented by the terrain.

Digital Terrain Elevation Data (DTED)

A file format used by geographers and mappers to map the height of a terrain. Often used to import real-world topography into flight simulations. Shuttle Radar Topography Mission (SRTM) data is easily obtained and converted.

The huge world problem

Huge landscapes may require a lot of memory, potentially more than a 3D card can handle. In game consoles memory is a scarce resource, on mobile devices transferring the app and storing is a factor. Even on a cutting edge PC large datasets will eat into that onboard memory especially when we get down to designing and building them using high-resolution data. Requesting actions that eat up your system memory may cause the application to fail. We can use GROME to create vast worlds without worrying too much about memory. This is done by taking advantage of how GROME manages data through a process of splitting terrain into "zones" and swapping it out to disk. This swapping is similar to how operating systems move memory to disk and reload it on demand. By default whenever you import large DTED files GROME will break the region into multiple zones and hide them. Someone new to GROME might be confused by a lengthy file import operation only to be presen ted with a seemingly empty project space.

When creating terrain for engines such as Unity, UDK, Ogre3D, and others you should keep in mind their own technical limitations of what they can reasonably import.

Most of these engines are built for small scale scenes. While GROME doesn't impose any specific unit of measure on your designs, one unit equals one meter is a good rule of thumb. Many third-party models are made to this scale. However it's up to the artist to pick a unit of scale and importantly, be consistent.

Keep in mind many 3D engines are limited by two factors:

• Floating point math precision
• Z-buffer (depth buffer) precision

Floating point precision

As a general rule anything larger than 20,000 units away from the world origin in any direction is going to exhibit precision errors. This manifests as vertex jitter whenever vertices are rotated and transformed by large values. The effects are not something you can easily work around. Changing the scale of the object shifts the error to another decimal point. Normally in engines that specialize in rendering large worlds they either use a camera-relative rendering or some kind of paging system. Unity and UDK are not inherently capable of camera-relative rendering but a method of paging is possible to employ.

Depth buffer precision

The other issue associated with large scene rendering is z-fighting. The depth buffer is a normally invisible part of a scene used to determine what part is hidden by another, depth-testing. Whenever a pixel is written to a scene buffer the z component is saved in the depth buffer. Typically this buffer has 16 bits of precision, meaning you have a linear depth of 0 to 65,536. This depth value is based on the 3D camera's view range (the difference between the camera near and far distance). Z-fighting occurs when objects appear to be co-planer polygons written into the z-buffer with similar depth values causing them to "fight" for visibility. This flickering is an indicator that the scene and camera settings need to be rethought. Often the easy fix is to increase the z-buffer precision by increasing the camera's near distance. The downside is that this can clip very near objects.

GROME will let you create such large worlds. Its own Graphite engine handles them well. Most 3D engines are designed for smaller first and third-person games which will have a practical limit of around 10 to 25 square kilometers (1 meter = 1 unit). GROME can mix levels of detail quite easily, different regions of the terrain have their own mesh density. If for example you have a map on an island, you will want lots of detail for the land and less in the sea region. However, game engines such as Unity, UDK, and Ogre3 Dare are not easily adapted to deal with such variability in the terrain mesh since they are optimized to render a large triangular grid of uniform size. Instead, we use techniques to fake extra detail and bake it into our terrain textures, dramatically reducing the triangle count in the process. Using a combination of Normal Maps and Mesh Layers in GROME we can create the illusion of more detail than there is at a distance.

Normal map

A Normal is a unit vector (a vector with a total length of one) perpendicular to a surface. When a texture is used as a Normal map, the red, green, and blue channels represent the vector (x,y,z). These are used to generate the illusion of more detail by creating a bumpy looking surface. Also known as bump-maps.

Summary

In this article we looked at heightmaps and how they allow us to import and export to other programs and engines. We touched upon world sizes and limitations commonly found in 3D engines.

Resources for Article :

Further resources on this subject:

• Photo Manipulation with GIMP 2.6 [Article]
• Setting up a BizTalk Server Environment [Article]
• Creating and Warping 3D Text with Away3D 3.6 [Article]