History: SoC2012 Improve and Demo the Terrain System

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            {maketoc}

!Project Information
Proposal and discussion thread: [http://www.ogre3d.org/forums/viewtopic.php?f=13&t=69553|http://www.ogre3d.org/forums/viewtopic.php?f=13&t=69553]
Mercurial repository: [https://bitbucket.org/xiaoxq/ogre_soc_tpi/overview|https://bitbucket.org/xiaoxq/ogre_soc_tpi/overview]
The basement project: [http://www.ogre3d.org/tikiwiki/SoC2011+Terrain+Paging+Improvements|SoC2011 Terrain Paging Improvements]
!Implementation Design
!!New Terrain File Format
To be able to load LOD asynchronously, we should save height/delta data separately according to the lowest level it belongs to. So the terrain file format has been adjusted as follows:
{BOX(class="")}<pre>
&lt;TERRAIN_CHUNK>
  &lt;TERRAINGENERALINFO_CHUNK>
    uint8 align; uint16 Size; Real WorldSize; uint16 MaxBatchSize; uint16 MinBatchSize; Vector3 Pos;
  &lt;/TERRAINGENERALINFO_CHUNK>

  &lt;TERRAINLODDATA_CHUNK>zip( LOD{n-1} HeightData, DeltaData; )&lt;/TERRAINLODDATA_CHUNK>
  ...
  &lt;TERRAINLODDATA_CHUNK>zip( LOD{0} HeightData, DeltaData; )&lt;/TERRAINLODDATA_CHUNK>

  LayerInstanceList Layers;
  LayerBlendData;

  &lt;TERRAINDERIVEDDATA_CHUNK> NormalData; &lt;/TERRAINDERIVEDDATA_CHUNK>
  &lt;TERRAINDERIVEDDATA_CHUNK> ColourMap; &lt;/TERRAINDERIVEDDATA_CHUNK>
  &lt;TERRAINDERIVEDDATA_CHUNK> LightMap; &lt;/TERRAINDERIVEDDATA_CHUNK>
  &lt;TERRAINDERIVEDDATA_CHUNK> CompositeMap; &lt;/TERRAINDERIVEDDATA_CHUNK>
  QuadTree;
&lt;/TERRAIN_CHUNK></pre>
{BOX}
The new TERRAIN_CHUNK_ID is 2, but it's not finished yet - by now only LOD data is compressed, which makes the terrain file 3 times larger than previous.
The LOD saving process happens at TerrainLodManager::saveLodData();
When some LOD is requested, just open the file, skip the TERRAINGENERALINFO_CHUNK chunk, and skip the unmatched TERRAINLODDATA_CHUNK chunk, then read the height/delta data into memory. These are done at TerrainLodManager::readLodData();

!!LOD State Machine
Previously, the terrain has only 3 states: Not Prepared, Fully Prepared, Fully Loaded. But now LODs are loaded asynchronously, so the terrain may be only partially prepared/loaded. Every LOD gets its own 3-states, as shown below:
{img fileId="2135" rel="box[g]"}

!!LOD Requests Interface and Implementation
The state machine looks somewhat complicated. Luckily we'll hide the details in TerrainLodManager. The interface for users is very simple:
{CODE(wrap="1", colors="c++")}
class Terrain
{
  ...
  void load(int lodLevel, bool synchronous);
  void increaseLodLevel(bool synchronous = false);
  void decreaseLodLevel();
  ...
}
{CODE}
You can request a specified LOD, or just ask to increase/decrease the current LOD by 1. In fact, they are only proxies of TerrainLodManager.
TerrainLodManager holds three LOD status: HighestLodPrepared, HighestLodLoaded, TargetLodLevel.
Every LOD request will finally reach the updateToLodLevel(int lodLevel, bool synchronous), which do 
modify the TargetLodLevel.
!Guide for Users

!Guide for Developers

!Introduction to the Endless World Sample

!Weekly Progress
!!2012-04-29
*Create repository by importing from kuxv's, and create this wiki page.
*A small fix to make it compilable on my x64 system. Then it runs quite well.
*Merge new changesets from trunk.
*Confirm my development environment: Thinkpad T410s( Intel i5, Nvidia NVS 3100M, 8G memory ), Ubuntu 12.04 64bit. I also have a Win7 64bit, but only for testing.
!!2012-05-06
*Some code cleanup.
!!2012-05-14
*Keep on warming up with the code.
*Start drawing a sequence diagram to show the work flow of LOD and paging.
!!2012-05-20
*Finish the sequence diagram, and get to understand most of the previous implementation ideas.
*Have found some possible buggy point. See them above for detail.
!!2012-05-27
*Start reviewing the code, I think I have understood the idea deeply.
*Small refactoring, extract out a TerrainLodManager, which makes the Terrain component as clear as the original.(80% completed)
!!2012-06-03
*Almost finish extracting the LodManager from Terrain.
*Going to read height/delta once for all.