Summer of Code 2008: Different LOD Strategies

Student: Christopher Bowen (Lostera)

Mentor: Andres Carrera (Lioric)

Location: TBD

Status: In Progress

Repository: svn.ogre3d.org/svnroot/ogre/branches/soc08-lod

More Information: Ogre Forums Topic

Motivation

In determining what level of complexity to use in representing 3D objects, OGRE currently implements only one approach, a level of detail strategy based on entity distance from the camera. In this implementation, each level of detail for a mesh has an associated reference distance. However, there are several drawbacks to this approach. It does not account for changes in camera field of view (for instance, when zooming) or viewport size. Multiple methods have been proposed to solve these issues. Of these, the simplest and fastest is to estimate the pixel count by projecting the bounding sphere, replacing the reference distance for each level of detail with a reference pixel count.

Components

  • Extend the level of detail system to allow for multiple strategies while continuing support for assets using the current strategy. This would provide a flexible framework for implementing alternative strategies.
  • Within this new system, implement a pixel count approximation strategy which provides more appropriate level of detail selection for a variety of viewing conditions. This strategy would estimate an entity’s pixel count by projecting the bounding sphere; each level of detail for a mesh would have an associated reference pixel count.
  • Change the automatic level of detail generation to utilize the new strategy.
  • If the community concurs, I would also like to provide a means to address the drawbacks for assets using the old system by allowing the user to set the assumed field of view and viewport size upon which the reference distances were based. From this information, an appropriate correction bias would be calculated to automatically correct for other field of view angles and viewport sizes.

Tentative Schedule

4/14-5/26
Discuss with community the desired mechanism for allowing multiple strategies. In particular, decide if an abstract LOD strategy class is desirable even if it is associated with a performance penalty. The primary benefit of this method would be the ability to add additional strategies from plug-ins through the use of factories. The simpler and faster implementation would be a basic conditional in the LOD calculation. This step will be completed before coding begins.
5/26-6/21
Modify the mesh and entity subsystems in the current LOD system to allow for multiple strategies. The precise timeline for this step depends on the design chosen by the community and will be supplied before coding begins. This will be the longest individual step in the process as a lot of existing code relies heavily on the current implementation. Upon completion of this phase, I will make the first release available for testing and design evaluation.
6/22-6/28
Develop and implement a test plan for the completed changes and make corrections for any identified problems.
6/29-7/5
After testing on new system is complete, I will implement the new strategy. Unless precision issues arise, this will be a fairly simple operation and a working release should quickly be made available for testing.
7/6-7/12
Once the new strategy is in place and functional, existing parts of the engine can be made to utilize it. In particular, the automatic LOD generation should be modified to use the improved strategy.
7/13-7/19
Develop and implement additional testing for the added features and make corrections for any identified problems.
7/20-7/31
With these modifications in place, new assets are no longer subject to the drawbacks of the previous system, but older assets, while usable, retain the problems. This can be remedied by implementing a method to automatically bias the LOD based on the reference field of view angle and viewport size used when determining reference distances.
8/1-8/18
Develop and implement final testing, make corrections, clean up code, and finalize documentation.

Progress

5/26
Added LodStrategy class and corresponding functionality for mesh level of detail. Implemented old (distance based) strategy. Initial work begun on new mesh serializer.
6/1
Incorporated LodStrategy into Material and modified both serializers to handle this information. LodStrategy is now utilized for material LOD.
6/7
Merged in LodListener work and modified to function with new system.
6/14
Added some additional LodListener functionality and extended LodStrategy usage to instanced geometry.
6/16
Extended LodStrategy usage to static geometry (with a few changes to instanced geometry).
6/19
Added handling for new and changed fields in XMLConverter.
6/22
Better support for lod strategies in XMLConverter and a few other bugfixes.
6/30
Implemented pixel count approximation strategy.
7/02
Added default strategy to strategy manager as suggested on the forums.
7/12
Added reference view functionality for distance lod strategy.
7/19
Additional testing performed.
7/26
Added user value transformation functionality, so squared distances are no longer required in many places.
8/9
Added support for separate lod camera (mostly just merging in Sinbad's work).
8/15
Cleaned up code and improved documentation.

Porting Notes

Unfortunately, this project makes fairly large changes to the way Ogre handles LOD, and while I've tried to maintain backwards compatibility where practical, there are a number of changes to keep in mind when porting to the new system.

  • Removed Mesh::getLodIndexSquaredDepth.
  • Mesh::getLodIndex now requires a squared distances when using DistanceLodStrategy.
  • Renamed MeshLodUsage::fromDepthSquared to value.
  • Renamed LodDistanceList to LodValueList and LodDistanceIterator to LodValueIterator.
  • Material::getLodIndex now requires squared distances when using DistanceLodStrategy.
  • Renamed Material::mLodDistances to mLodValues.
  • Renamed Material::getLodDistanceIterator to getLodValueIterator
  • Removed Material::getLodIndexSquaredDepth as it no longer applies.
  • Renamed Entity::_getMeshLodFactorInverse to Entity::_getMeshLodFactorTransformed as this method does not neccessarily return the inverse anymore (but will when using DistanceLodStrategy).
  • Renamed InstancedGeometry::BatchInstance::mCamDistanceSquared to mLodValue.
  • Renamed InstancedGeometry::LODBucket::mSquaredDistance to mLodValue.
  • Renamed InstancedGeomety::LODBucket::getSquaredDistance to getLodValue.
  • Renamed StaticGeometry::Region::mCamDistanceSquared to mLodValue.
  • Renamed StaticGeomety::LODBucket::mSquaredDistance to mLodValue.
  • Renamed StaticGeomety::LODBucket::getSquaredDistance to getLodValue.
  • Added LodStrategy argument to MeshSerializerImpl::writeLodSummary.



Possible Additions

While the scope of this project was initially limited to improving the accuracy of lod calculation, there have been other lod related features suggested. Time permitting, these features may be implemented once the requirements outlined in the initial proposal are fulfilled.

  • Lod listeners: This has already been completed, but was not part of the original proposal. This listener interface allows notification and some measure of control for lod changes.
  • Lod relative camera specification: This has also now been completed by Sinbad and merged in. Allow lod calculations to be performed relative to a camera other than the current one. This would be useful primarily for shadowing to ensure that entities in the shadow map are rendered at the same lod as in the viewport. For anyone interested in pursuing this feature now, the implementation would probably involve the addition of a MovableObject::_notifyCurrentLodCamera or similar method parallel to MovableObject::_notifyCurrentCamera. Any reference to _notifyCurrentCamera would probably require an additional call to _notifyCurrentLodCamera, particularly in SceneNode::_findVisibleObjects. SceneManager::_renderScene, SceneManager::_findVisibleObjects, and SceneNode::_findVisibleObjects would probably all require an additional camera parameter. Current calls to SceneManager::_renderScene would need to be supplemented with an lod camera, usually the same camera as is currently being passed to it except when rendering a shadow map. I'm sure other modifications would be necessary, but this should get anyone interested started.

<HR>
Creative Commons Copyright -- Some rights reserved.


THE WORK (AS DEFINED BELOW) IS PROVIDED UNDER THE TERMS OF THIS CREATIVE COMMONS PUBLIC LICENSE ("CCPL" OR "LICENSE"). THE WORK IS PROTECTED BY COPYRIGHT AND/OR OTHER APPLICABLE LAW. ANY USE OF THE WORK OTHER THAN AS AUTHORIZED UNDER THIS LICENSE OR COPYRIGHT LAW IS PROHIBITED.

BY EXERCISING ANY RIGHTS TO THE WORK PROVIDED HERE, YOU ACCEPT AND AGREE TO BE BOUND BY THE TERMS OF THIS LICENSE. THE LICENSOR GRANTS YOU THE RIGHTS CONTAINED HERE IN CONSIDERATION OF YOUR ACCEPTANCE OF SUCH TERMS AND CONDITIONS.

1. Definitions

  • "Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with a number of other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.
  • "Derivative Work" means a work based upon the Work or upon the Work and other pre-existing works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which the Work may be recast, transformed, or adapted, except that a work that constitutes a Collective Work will not be considered a Derivative Work for the purpose of this License. For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work for the purpose of this License.
  • "Licensor" means the individual or entity that offers the Work under the terms of this License.
  • "Original Author" means the individual or entity who created the Work.
  • "Work" means the copyrightable work of authorship offered under the terms of this License.
  • "You" means an individual or entity exercising rights under this License who has not previously violated the terms of this License with respect to the Work, or who has received express permission from the Licensor to exercise rights under this License despite a previous violation.
  • "License Elements" means the following high-level license attributes as selected by Licensor and indicated in the title of this License: Attribution, ShareAlike.

2. Fair Use Rights

Nothing in this license is intended to reduce, limit, or restrict any rights arising from fair use, first sale or other limitations on the exclusive rights of the copyright owner under copyright law or other applicable laws.

3. License Grant

Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive, perpetual (for the duration of the applicable copyright) license to exercise the rights in the Work as stated below:

  • to reproduce the Work, to incorporate the Work into one or more Collective Works, and to reproduce the Work as incorporated in the Collective Works;
  • to create and reproduce Derivative Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission the Work including as incorporated in Collective Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission Derivative Works.
  • For the avoidance of doubt, where the work is a musical composition:
    • Performance Royalties Under Blanket Licenses. Licensor waives the exclusive right to collect, whether individually or via a performance rights society (e.g. ASCAP, BMI, SESAC), royalties for the public performance or public digital performance (e.g. webcast) of the Work.
    • Mechanical Rights and Statutory Royalties. Licensor waives the exclusive right to collect, whether individually or via a music rights society or designated agent (e.g. Harry Fox Agency), royalties for any phonorecord You create from the Work ("cover version") and distribute, subject to the compulsory license created by 17 USC Section 115 of the US Copyright Act (or the equivalent in other jurisdictions).
    • Webcasting Rights and Statutory Royalties. For the avoidance of doubt, where the Work is a sound recording, Licensor waives the exclusive right to collect, whether individually or via a performance-rights society (e.g. SoundExchange), royalties for the public digital performance (e.g. webcast) of the Work, subject to the compulsory license created by 17 USC Section 114 of the US Copyright Act (or the equivalent in other jurisdictions).


The above rights may be exercised in all media and formats whether now known or hereafter devised. The above rights include the right to make such modifications as are technically necessary to exercise the rights in other media and formats. All rights not expressly granted by Licensor are hereby reserved.

4. Restrictions

The license granted in Section 3 above is expressly made subject to and limited by the following restrictions:

  • You may distribute, publicly display, publicly perform, or publicly digitally perform the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. If You create a Collective Work, upon notice from any Licensor You must, to the extent practicable, remove from the Collective Work any credit as required by clause 4(c), as requested. If You create a Derivative Work, upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work any credit as required by clause 4(c), as requested.
  • You may distribute, publicly display, publicly perform, or publicly digitally perform a Derivative Work only under the terms of this License, a later version of this License with the same License Elements as this License, or a Creative Commons iCommons license that contains the same License Elements as this License (e.g. Attribution-ShareAlike 2.5 Japan). You must include a copy of, or the Uniform Resource Identifier for, this License or other license specified in the previous sentence with every copy or phonorecord of each Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Derivative Works that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder, and You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Derivative Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Derivative Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Derivative Work itself to be made subject to the terms of this License.
  • If you distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or (ii) if the Original Author and/or Licensor designate another party or parties (e.g. a sponsor institute, publishing entity, journal) for attribution in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; the title of the Work if supplied; to the extent reasonably practicable, the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, a credit identifying the use of the Work in the Derivative Work (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.

5. Representations, Warranties and Disclaimer

UNLESS OTHERWISE AGREED TO BY THE PARTIES IN WRITING, LICENSOR OFFERS THE WORK AS-IS AND MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND CONCERNING THE MATERIALS, EXPRESS, IMPLIED, STATUTORY OR OTHERWISE, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF TITLE, MERCHANTIBILITY, FITNESS FOR A PARTICULAR PURPOSE, NONINFRINGEMENT, OR THE ABSENCE OF LATENT OR OTHER DEFECTS, ACCURACY, OR THE PRESENCE OF ABSENCE OF ERRORS, WHETHER OR NOT DISCOVERABLE. SOME JURISDICTIONS DO NOT ALLOW THE EXCLUSION OF IMPLIED WARRANTIES, SO SUCH EXCLUSION MAY NOT APPLY TO YOU.

6. Limitation on Liability.

EXCEPT TO THE EXTENT REQUIRED BY APPLICABLE LAW, IN NO EVENT WILL LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

7. Termination

  • This License and the rights granted hereunder will terminate automatically upon any breach by You of the terms of this License. Individuals or entities who have received Derivative Works or Collective Works from You under this License, however, will not have their licenses terminated provided such individuals or entities remain in full compliance with those licenses. Sections 1, 2, 5, 6, 7, and 8 will survive any termination of this License.
  • Subject to the above terms and conditions, the license granted here is perpetual (for the duration of the applicable copyright in the Work). Notwithstanding the above, Licensor reserves the right to release the Work under different license terms or to stop distributing the Work at any time; provided, however that any such election will not serve to withdraw this License (or any other license that has been, or is required to be, granted under the terms of this License), and this License will continue in full force and effect unless terminated as stated above.

8. Miscellaneous

  • Each time You distribute or publicly digitally perform the Work or a Collective Work, the Licensor offers to the recipient a license to the Work on the same terms and conditions as the license granted to You under this License.
  • Each time You distribute or publicly digitally perform a Derivative Work, Licensor offers to the recipient a license to the original Work on the same terms and conditions as the license granted to You under this License.
  • If any provision of this License is invalid or unenforceable under applicable law, it shall not affect the validity or enforceability of the remainder of the terms of this License, and without further action by the parties to this agreement, such provision shall be reformed to the minimum extent necessary to make such provision valid and enforceable.
  • No term or provision of this License shall be deemed waived and no breach consented to unless such waiver or consent shall be in writing and signed by the party to be charged with such waiver or consent.
  • This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You.