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]
!!Known Issues
Hiccups when loading page

!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>
zip(
  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.
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); //request a specified LOD
  void increaseLodLevel(bool synchronous); //increase the current LOD by 1
  void decreaseLodLevel(); //decrease the current LOD by 1
  ...
}
{CODE}
In fact, they are just proxies for TerrainLodManager, which holds three LOD status: HighestLodPrepared, HighestLodLoaded, TargetLodLevel.
Every LOD request will finally call the updateToLodLevel(int lodLevel, bool synchronous). It acts like:
* Update the TargetLodLevel.
* If TargetLodLevel is higher than HighestLodPrepared, prepare it/them
* If TargetLodLevel is higher than HighestLodLoaded, load it/them
* If TargetLodLevel is lower than HighestLodLoaded, unload it/them
Currently we won't unprepare LODs. If wanted, the rule is:
* If TargetLodLevel is lower than HighestLodPrepared, unprepare it/them
!!Asynchronous LOD Loading
Asynchronously-loading of LOD data is the core feature of this project, the logic is very simple, as shown below.
{img fileId="2136" rel="box[g]"}
It's also the biggest source of bugs. My work is to make it safe and stable.
!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.
__2012-06-10__
*Alpha release.
__2012-06-25__
*Write some wiki.
__2012-07-02__
*Almost finished the Implementation Design section of the wiki.
*Use LodManager though no file bound.
*Fix an "Empty CPU Buffer" exception.
__2012-07-10__
*After 2 big fixes, it works quite better, now.
*But if set paging ON, it still has some bugs.