World Management

There is a number of optimization techniques and practices relating to world management. They are used to decrease the rendering load without losing much of the image quality.

Smooth alpha-blended levels of details (LODs) are used to decrease geometry complexity of 3D objects, when these objects move away from the camera, making it possible to lighten the load on the renderer.

UNIGINE offers two mechanisms of LODs switching:

• Disable one LOD and enable another at the specified distance defined by two values: maximum visibility distance of the first LOD (Max Visibility) and minimum visibility distance of the second LOD (Min Visibility).

  

• Smoothly fade one LOD into another at the specified interval defined by two values: minimum fade distance of the first LOD (Min Fade) and maximum fade distance of the first LOD/minimum fade distance of the second LOD (Max Fade).

  

Surface LODs are adjusted in the LOD section of the Node tab of the Parameters window. It provides the following parameters:

Within distance of surface visibility

Within fade distance

Suppose, we have two LODs: high-poly surface_lod_0 and low-poly surface_lod_1, and we need to organize smooth transition between these two LODs.

1. We want the switching distance at 50 units. For that, we need to "dock" visibility distances of surfaces to each other:
   • Our first LOD surface surface_lod_0 should be always presented when the camera is close to the object. So, the minimum visibility distance is set to -inf. And 50 units from the camera will be the maximum visibility distance for it.
   • Directly following it, comes the second LOD surface surface_lod_1. It is visible from 50 units (which is the minimum visibility distance) - and up to infinity (the maximum visibility distance = inf).
2. Now the LOds are switched but sharply rather then smoothly. To be smoothly blended, the symmetrical fade-out (for the 1st LOD) and fade-in (for the 2nd LOD) distances are set. Let's say, the fading region should be 5 units.
   • For the 1st LOD to fade-out, its maximum fade distance is set to 5.
   • For the 2nd LOD to fade-in, its minimum fade distance is also set to 5.

In the result, the LODs will be changed in the following manner:

Reference Object#

There is one more LOD-related parameter: reference object to measure the distance to switch LODs. It specifies whether the distances should be measured to the surface itself or to any of the surfaces or nodes up the hierarchy branch. There are two reference objects for each surface:

Min Parent

Minimum parent is a reference object to measure the minimum visibility distance from: 0 means the surface itself 1 means the reference object is found one level up in the hierarchy: First, the surface hierarchy is stepped up (if any). Then the hierarchy of nodes is considered. For example, if the surface has no parent surfaces, 1 would mean the object it belongs to. As for higher numbers, they will simply move the reference object up through all the surface parents and then all the node parents.

Max Parent

Maximum parent is a reference object to measure the maximum visibility distance from. The same principle is used to count it.

Let's take a model of the house, for example. When the camera is close by, the high-poly detail surfaces are seen, such as the door arch, stone corners, round window and roof tiles. When we move away, all these surfaces should be simultaneously changed by one united low-poly LOD surface.

Notice

Merging several surfaces into one reduces the number of objects to draw, hence, it reduces the number of DIP requests and speeds up rendering.

High-poly model

Low-poly model to be used as distant LOD

The problem is, all this detail surfaces have different bounding boxes. So if their distances are checked for themselves (0 as min and max parents), we can have a situation when LODs of different parts of the house are turned on (or off) unequally, because their bounding boxes will be closer to the camera. This may cause z-fighting artifacts. Here the distant corner has not yet switched to a more detailed LOD, while the close one is drawn twice: as a high-poly corner LOD and at the same time the united low-poly house LOD.

Measuring LODs distance to surfaces can cause artifacts: two LODs switched on at once

If we set a bounding box of the whole house to be a reference object (min and max parent to 1), all surfaces will switch on simultaneously no matter what side we approach the house from.

One more option is to use different reference objects when check. For example, the lower bound (minimum distance) is checked for the surface itself, and the upper bound (maximum distance) is checked for the parent. This may sound complicated, so take a look at the pictures below. The first picture shows a ring, which is split into surfaces according to levels of details.

Levels of details of a ring. Surfaces on the left are parents to surfaces on the right

Here, surfaces from the rightmost column will be displayed, when the camera is very close to them. The leftmost surfaces will be displayed, when the camera is very far from them. Merging of several surfaces into one reduces the number of objects to draw, hence, reduces the number of DIP requests and speeds up rendering.

Note that all of the minimum distances here are measured to the surface itself, but almost all of the maximum distances are measured to another reference object, a parent. One more picture will help you to understand, why it is so.

An example of using measurements of distances to different reference objects.

A star is the camera; it doesn't matter now what exactly the camera will be looking at. On both images, required surfaces are drawn according to the camera position and distances from the camera to the corresponding reference objects. For example, on the left image, the upper left part of the ring is a single surface, the upper right part is split in two separate surfaces, and the bottom part is also a single surface. On the right image, the whole upper part is divided into the smallest possible sectors.

Here, distances are measured to different reference objects to properly "turn off" smaller single sectors and display a larger sector instead. The maximum distance is calculated to the parent sector, because the distances to the neighboring subsectors may differ too much. The minimum distance is calculated to the current sector, because we need to show it, if the camera is too close to it.

A bound object represents a spherical or cubical volume enclosing the whole node, used for describing node's size and location. In UNIGINE, this can be an axis-aligned bounding box or a sphere. Bounds are defined only for the nodes that have visual representation or their own size. The following "abstract" objects do not have bounds at all and therefore are excluded from the spatial tree:

• Dummy Node
• Node Reference
• Node Layer
• World Switcher
• World Transform Path
• World Transform Bone
• World Expression
• Dummy Object (if it has no body assigned)

This approach significantly reduces the size of the tree and improves performance due to saving time on bound recalculation when transforming such nodes.

The following types of bounds are used:

• Local Bounds — bound objects with local coordinates which do not take into account physics and children: BoundBox and BoundSphere.
• World Bounds — same as local ones, but with world coordinates: WorldBoundBox and WorldBoundSphere.
• Spatial Bounds — bound objects with world coordinates used by the spatial tree, and therefore taking physics into account (shape bounds, etc.): SpatialBoundBox and SpatialBoundSphere.

And their hierarchical analogues (taking into account all children) to be used where hierarchical bounds are required (they are slow, but offer correct calculations):

• Local Hierarchical Bounds — bound objects with local coordinates taking bounds of all node's children into account: HierarchyBoundBox and HierarchyBoundSphere.
• World Hierarchical Bounds — same as local ones, but with world coordinates: HierarchyWorldBoundBox and HierarchyWorldBoundSphere.
• Spatial Hierarchical Bounds — hierarchical bound objects used by the spatial tree, and therefore taking physics into account (shape bounds, etc.): HierarchySpatialBoundBox and HierarchySpatialBoundSphere.

The techniques that are appropriate for indoor scenes, are not efficient when it comes to managing the vast landscapes. Rendering speed directly depends on the number of entities and polygons drawn in the scene, as well as physics computed for the objects, the count of which is usually very high in the outdoor scenes. So the main goal of managing is to render only the regions that are seen while culling all the rest. If the world cannot be narrowed down to a set of closed areas, the approach called space partitioning becomes relevant.

Space partitioning in Unigine is implemented using adaptive axis-aligned BSP trees.

Binary Space Partitioning#

To provide effective management of the scene on the one hand and good tree balancing on the other, separate trees are created for different node types:

• World tree handles all sectors, portals, occluders, triggers and clusters.
• Objects tree includes all objects except for collider objects and the ones with the Immovable flag enabled.
• Collider objects form a separate tree to facilitate collision detection and avoid the worst case scenario of testing all the objects against all other objects. This tree contains collider objects. It is clear, that objects can intersect only if they are situated and overlap in the same region of the scene. This tree allows to drastically reduce the number of pair-wise tests and accelerate the calculation.
• Clutter objects are also separated as they are intended to be used in great numbers, which can disturb the balance of the main object tree.
  Notice

  This spatial tree includes static objects with the Immovable flag enabled to optimize node management.
• Light tree handles all light sources.
• Decal tree handles decals.
• Player tree handles all types of players.
• Physical node tree handles all physical forces.
• Sound tree handles all sound sources.

Mesh Partitioning#

When the human eye views a scene, objects in the distance appear smaller than objects close by - this is known as perspective. While orthographic projection ignores this effect to allow accurate measurements, perspective definition shows distant objects as smaller to provide additional realism.

A viewing frustum (or a view frustum) is a field of view of the virtual camera for the perspective projection; in other words, it is the part of the world space that is seen on the screen. Its exact shape depends on the camera being simulated, but often it is simply a frustum of a rectangular pyramid. The planes of the viewing frustum that are parallel to the screen are called the near plane and the far plane.

As the field of view of the camera is not infinite, there are objects that do not get inside it. For example, objects that are closer to the viewer than the near plane will not be visible. The same is true for the objects beyond the far plane, if it is not placed at infinity, or for the objects cut off by the side faces. As such objects are invisible anyway, one can skip their drawing. The process of discarding unseen objects is called viewing frustum culling.

When the human eye looks at a scene, objects in the distance appear smaller than objects close by. Orthographic projection ignores this effect to allow the creation of to-scale drawings for construction and engineering.

With an orthographic projection, the viewing volume is a rectangular parallelepiped, or more informally, a box. Unlike perspective projection, the size of the viewing volume doesn't change from one end to the other, so distance from the camera doesn't affect how large an object appears.

Another popular practice is to remove objects that are completely hidden by other objects. For example, there is no need to draw a room behind a blank wall or flowers behind an entirely opaque fence. This technique is called occlusion culling. The particular cases of occlusion culling are the following:

• Occluders
• Potentially visible sets that divide the space in a bunch of regions, with each region containing a set of polygons that can be visible from anywhere inside this region. Then, in real-time, the renderer simply looks up the pre-computed set given the view position. This technique is usually used to speed up binary space partitioning.

Occluders#

Hardware Occlusion Queries#

Updating each frame a huge number of objects (e.g. smoke, explosions, or crowd simulation) located far away from the camera that are hardly distinguishable or observed as a mass is a waste of resources. To improve performance and avoid the excessive load, simulation of particles, water, cloth, or ropes, and playback of skinned mesh animations can be updated with a reduced framerate.

Periodic update allows saving a great deal of performance. The list of objects that can be optimized this way includes the following:

• Particle Systems
• Skinned Meshes
• Dynamic Meshes (only with a rope, a cloth, or a water body assigned)
• World Expressions
• World Transform Paths

For each of them the Update Distance Limit can be set — a distance from the camera within which the object should be updated.

In addition to that, for Particle Systems, Skinned and Dynamic Meshes, three update rate (FPS) values can be set, which specify how often the simulation should be updated when the object is visible, when only its shadow is visible, or when it is not visible at all.

This feature is enabled with default settings ensuring optimum performance and can be adjusted per-object in UnigineEditor or via API at run time giving you flexibility in optimization.

Data streaming is an optimization technique intended to reduce spikes caused by loading of graphic resources. It supposes that not all the data is loaded into random access memory (RAM) at once. Instead, only the required data is loaded and transferred to the GPU in separate asynchronous threads, and all the rest is loaded progressively, on demand.

Streaming system provides asynchronous loading of the following data to RAM:

• All textures, including cubemaps, voxel probe maps and shadow maps of baked shadows.
• ObjectMeshStatic, ObjectMeshClutter and ObjectMeshCluster.

Streaming system uses texture cache composed of minimized copies generated for all textures with user-defined resolution and stored in the data/.cache_textures folder. These copies are used instead of the originals while they are loaded.

For more information on streaming configuration please refer to the Asynchronous Data Streaming article.

Nodes are updated in the multi-threaded mode, which can substantially increase performance. For example, this can be very handy when a big number of particle systems are rendered in the world.

• All nodes that have one root in the nodes hierarchy are updated in one thread. To parallel the jobs on nodes update, make sure that they do not use the same parent.
• Each Node Reference is handled as a root node without any parents (regardless of their position in node hierarchy). For example, this means that particle systems contained in node references are always optimized for multi-threaded update.