Level of Detail (LOD)

Description

This chapter briefly describes Level Of Detail techniques supported by Candera.

Chapters

See also:

Example Solution:

 

Introduction 

Level of Detail

Level of detail (LOD) is a discipline of interactive computer graphics attempts 
to bridge complexity and performance by regulating the amount of detail used 
to represent the virtual world.

(Level of Detail for 3D Graphics, David Luebke, Morgan Kaufmann Publisher, 2003)

Picture source: http://milkostoev.co.uk/2009/01/cobra-gt500/cobragt500wire

 

Motivation

Reduce render complexity in order to increase runtime performance with minimal or without visual penalty

Detail Simplifications

 

 

Level of Detail - Geometry Reduction Types 

Discrete LOD

Continuous or progressive LOD

View-dependent LOD

Picture source: http://research.microsoft.com/en-us/um/people/hoppe/proj/vdrpm/default.htm

Level of Detail - Chain 

Typical LOD Chain

The following figure depicts a typical LOD chain:

Level 0 .. 2: Meshes. Level 3: Billboard. Level 4: null-Node

 

Candera - Discrete LOD Concept 

Candera::LodNode, LoD Level

The class Candera::LodNode implements discrete LOD technique. Any Candera class derived from class Candera::Node is accepted as LOD representation.

Each LOD level in Candera::LodNode defines:

LOD level index, Node, Lower bound of its visibility range, Upper bound of its visibility range.

Picture source: http://www.opensg.org/wiki/Tutorial/OpenSG1/NodeCores

 

LOD Render Strategy

The LOD render strategy defines the transition between adjacent LOD levels.

LOD Criterion

The LOD criterion delivers a source value to map onto the LOD nodes visibility boundaries.

 

 

Candera - LOD Render Strategy 

Discrete LOD Render Strategy

Stepwise transition between LOD levels Non-adjacent boundaries between contiguous LOD levels define a Hysteresis.

 

Blend Transition LOD Render Strategy

Alpha blended transitions between LOD levels Non-adjacent boundaries between contiguous LOD levels define transition phase. During blend transition one alternating LOD node is always opaque to avoid 'see-through' effects.

 

 

 

Candera - LOD Architecture Overview 

 

 

Example - LOD in SceneComposer 

Example - Level of Detail in SceneComposer

This chapter shows a typical LOD use case and explains how the transition from two different LODs can be realized with CGI Studio SceneComposer.

To experiment with LOD transition, the following can be used:

Use Case

The example solution is based on following use case:

You can experiment with following node properties:

See also:

 

 

Example - Blended LOD 

Blended LOD

Change CameraDistance or start CameraPosition animation to see the smooth transition.

In application code the LodNode is set up as follows:

Initializing the LodNode

    // init LodNode bounds
    static_cast<void>(lodNode->SetLodLevel(0, lodNode->GetLodLevel(0).node, m_lowBoundHigh, m_upBoundHigh));
    static_cast<void>(lodNode->SetLodLevel(1, lodNode->GetLodLevel(1).node, m_lowBoundLow, m_upBoundLow));
    // end init

Define Criterion and RenderStrategy

        // Define DistanceToCameraLodCriterion and BlendLodRenderStrategy

        static DistanceToCameraLodCriterion cameraLodCriterion; // LOD is automatically selected by distance to camera.
        static BlendLodRenderStrategy blendLodStrategy; // LOD is automatically blended between non-adjacent boundaries

        // End Define DistanceToCameraLodCriterion and BlendLodRenderStrategy

Link pieces together

        // Init BlendLodRenderStrategy and set to LodNode

        blendLodStrategy.SetLodCriterion(&cameraLodCriterion);
        lodNode->SetLodRenderStrategy(&blendLodStrategy);

       // End Init BlendLodRenderStrategy and set to LodNode

 

 

Example - Step-wise LOD 

Step-wise LOD

Change CriterionValue property appropriate to see the transition.

In application code the LodNode is set up as follows:

Initializing the LodNode

    // init LodNode bounds
    static_cast<void>(lodNode->SetLodLevel(0, lodNode->GetLodLevel(0).node, m_lowBoundHigh, m_upBoundHigh));
    static_cast<void>(lodNode->SetLodLevel(1, lodNode->GetLodLevel(1).node, m_lowBoundLow, m_upBoundLow));
    // end init

Define Criterion and RenderStrategy

        // Define GenericLodCriterion and DiscreteLodRenderStrategy

        static GenericValueLodCriterion genericLodCriterion; // Application defined criterion value can be set.
        static DiscreteLodRenderStrategy discreteLodStrategy; // Stepwise LOD switch.

        // End Define GenericLodCriterion and DiscreteLodRenderStrategy

Link pieces together

        // Init DiscreteLodRenderStrategy and set to LodNode

        genericLodCriterion.SetCriterionValue(m_criterionValue); // Set application defined criterion value.
        discreteLodStrategy.SetLodCriterion(&genericLodCriterion);
        lodNode->SetLodRenderStrategy(&discreteLodStrategy);

        // End Init DiscreteLodRenderStrategy and set to LodNode

 

 

Example - Set LOD directly 

Set LOD directly

// Create LodNode object with 2 empty LOD levels.
m_lodNode = LodNode::Create(2);
// Set LOD 0 active and deactivate LOD 1. No RenderCriterion is attached.
m_lodNode->GetLodLevel(0).node->SetRenderingEnabled(true);
m_lodNode->GetLodLevel(1).node->SetRenderingEnabled(false);