Table of Contents
A detail object is a small mesh drawn in large numbers onto the terrain around the camera position. They are separated from ordinary models for efficiency. The selection and placement of these objects is largely automatic — it is based on the texture used by the underlying terrain.
Examples of detail objects are: grasses, ferns, bulrushes, pebbles, and rocks. The user does not interact with detail objects (for example, no collision detection is performed).
BigWorld by default has about 60,000 triangles of flora active at
any time, of which about 15,000 will be drawn at any one time. You may
change the size of the vertex buffer used by changing the value of the
vb_size tag in your flora configuration file (which is
specified by the environment/floraXML tag in your
configuration file
<res>/resources.xml —
for details, see File resources.xml.).
Because of the multitude of objects involved, it is impractical to place detail objects by hand. Instead, BigWorld has Ecotypes.
Ecotypes define objects that are automatically placed within the world, guided by terrain texture. They are defined in XML, and have the following properties:
-
One or more units (Moo visuals).
-
One or more textures, which are used to match the terrain to an ecotype.
-
Sound tag, for character footsteps.
For example, a grass ecotype could contain two separate meshes: one for low-height grass, and another for mid-height grass.
The set of ecotypes currently active is managed by the Flora class, which also manages the individual active detail objects, allocating and de-allocating them as required.
Each terrain block can have four textures, and each of these textures can have one associated ecotype. This allows for four different ecotypes per terrain block. Hence, within the detail sphere (50-metre radius from the camera) up to 16 different ecotypes may be visible.
Detail objects are not placed precisely within the terrain blocks — this would create square regions of detail objects. Detail objects 'jitter' their position before querying the terrain for its detail mapping. This effectively antialiases the quantised detail map.
Flora is calculated and stored in terrain files by the World Editor. For more details, see the document Content Creation Manual (accessible in World Editor, Model Editor, and Particle Editor via the Help → Content Creation menu item, or by pressing F1).
The detail objects are mapped directly from terrain textures. Visually this is correct, since in real life the colour on the terrain is actually made from the colour of millions of detail objects (blades of grass, etc...). Thus, a green 'grassy' terrain texture can be made to map perfectly to a grass ecotype.
In BigWorld, whenever a texture is used, its accompanying detail objects are displayed.
Drawing detail objects is an extremely performance-critical area, as all routines are run thousands of times per frame.
Vertex buffers are the perfect solution. The detail objects are kept transformed in a large vertex buffer. Thus, 1,000 pieces of grass at 6 vertices each means a world-space vertex buffer of 6,000 vertices. This allows all grass to be drawn using one call to the video card, using one transform matrix. 3D accelerators are extremely fast performing this kind of batched rendering.
As the player moves around the world, he sees the detail objects in the immediate vicinity (50m). In reality, this means that the active set of detail objects must change according to the camera position. Detail objects that are just coming into view are faded in, using alpha blending.
The flora configuration XML file
<flora>.xml (for
details on this file's grammar, see the document File Grammar Guide's section <flora>.xml) can define
the size of the vertex buffer either statically or as a set of options
selectable by the user via the FLORA_DENSITY graphics
setting. For details, see FLORA_DENSITY.
In a multi-thousand object system, frame coherency must be utilised aggressively. The obvious choice for frame coherency is the vertex transforms. Each detail object is kept transformed in a vertex buffer in world space, until its terrain tile (or 'flora block') leaves the detail radius, at which point another terrain tile will have moved into the detail radius. Then the new tile's detail objects will be transformed and placed in the freed up vertex buffer memory.
On average, detail objects have shown around 97% coherency between frames, making for a huge saving in transformation cost.
In order to create an immersive, living, breathing world, most things must animate.
Animating detail objects add greatly to the illusion of the world. The BigWorld client uses 3D Perlin noise based on (wx, wz, time) to create believable procedural animation to the detail objects. Perlin noise was chosen because it is a cheap way to create space-time coherent noise. The implementation performs the animation efficiently in the vertex shader.
When using simplified meshes (which is crucial in a multi-thousand object ornamentation system), lighting becomes challenging. Consider a criss-cross grass object rising vertically out of the terrain. If lit using standard procedures, the criss-cross objects would flare up in the evening sun.
In the BigWorld client, lighting for detail objects is performed using the light map calculated from the actual terrain. Thus, it picks up terrain shadows as well. This is a mostly correct solution, because the mesh-based terrain itself is really a simplified version of the millions of detail objects in existence.
The XML flora file defines among other things, the light map to be
used, plus the ecotypes and noise generation functions. For details on
this file's grammar, see the document File Grammar Guide's section <flora>.xml.