Level of Detail

Revision as of 20:04, 10 February 2013

Level of Detail (LOD) refers to a technique where several variants of an object are created, ranging from "high quality and high performance cost" through to "low quality and low performance cost". Each variant should look as close to possible as identical with the exception that overall object quality is lost as you go down the scale. These variants are then packaged together as a single asset with instructions to the game engine to use the most appropriate variant based on how far the object is zoomed out on the screen. When the player is close to the object, the "high detail LOD" variant will be displayed, showing the object in its full glory. As the player moves away from the object, the game can swap from the detailed version to the lower-detail-but-faster-to-render variant since the player is too far away to see the details anyway.

TBD: Image of several LOD meshes seen from close up, then the same meshes shown at the appropriate LOD ranges.

LOD in Textures

All textures are subject to LOD, and the process is simple and automatic. Each texture already has a number of Mip Maps which are automatically generated by Content Manager. The highest map is used whenever one texel represents one or more pixels on the screen. Once the object is far enough away that one pixel represents two texels, a half-size map is used, and so on. This is handled transparently by the GPU with no action required on the part of the content creator.

In cases where no visible object is currently using the higher mip level(s), they may be fully unloaded to conserve RAM. When an object again nears the player, the higher mip levels will be reloaded.

Benefits

• Higher GPU performance because the active texture maps are smaller.
• Lower RAM usage.
• Avoids moire effects on noisy textures.
• Don't have to load the full-detail version of all textures immediately, but can bring them level-by-level as the player approaches an object.

Disadvantages

• Can blur texture details to some extent (mitigated by higher Anisotropy settings on the GPU.)
• 33% increased ram usage for a fully-loaded texture.
• User can sometimes see objects up-close which have not yet loaded their high-detail textures.

Gotchas

• Since a single copy of the texture is shared between all relevant objects in the scene, having a single object at close range prevents unloading the higher texture mips, even if all other objects are distant. The GPU will still render using the appropriate mip levels, but the game will be forced to keep the high-detail mips loaded in this case.
• Since the selection of texture LOD depends on the texel-to-pixel ratio on display, it is possible for a single mesh to use different mip levels simultaneously. To some extent, this is deliberate- distant parts of the mesh may be at lower detail than closer parts of the mesh- but it's also possible for a content creator to worsen this effect by scaling the texture differently on different parts of the mesh. For example, if a 2048x2048 texture is stretched over the entirety of a distant object, then typically the game could select a 256x256 or lower version of the map for rendering. However, if the object also includes a black band around its circumference that is mapped from a single texel of that 2048x2048 texture, then you suddenly have a case where that one texel is mapped to a large number of pixels, requiring the texture to be loaded at highest detail. The game applies some logic to attempt to mitigate such cases, but the content creator should avoid such significant variations in mapping scale where possible.

LOD in Meshes

Mesh-based LOD is a more hands-on technique, and is what is usually referred to by the unqualified term "Level of Detail" since texture LOD is mostly automatic. There are several mesh LOD schemes currently employed in Trainz depending on the type of asset being used. All methods boil down to selecting one quality variant from a small list provided by the content creator. The creator will typically build the highest-detail mesh first- still ensuring the number of polygons is not overkill for the object being created- then create successively smaller mesh variants, each one typically dropping to 20-50% of the polygon count of the previous variant.

When creating LOD variants, there are a few things to keep in mind:

• The content creator's aim is to ensure that each LOD variant looks like the immediately higher variant. They should ideally also look like the highest-level variant, but it's inevitable that some detail will be lost and it's more important to minimise each transition than to minimise the overall appearance change.
• The human eye will pick up on silhouette and shading changes much more than changes to the internal detail of the object. Try to avoid changing the silhouette more than necessary.
• If you take a flattish object made by many small polygons which are nearly but not quite in a plane, and replace that with a single plane, the angle of light from your object will change. This will often result in the variant darkening or lightening as compared to its higher-quality sibling. The normal map can be used to neutralise this effect by ensuring that the lower-detail variants still maintain the appropriate surface normals, even though the geometry lacks the appropriate polygons.

Spline LOD

TBD.

LOD on Moving or Animated Objects

TBD.

LOD on Static Scenery Objects

TBD.