Hardware Occlusion Query         Intro to HOQ with Ogre



Hardware Occlusion Queries (HOQ) have many uses. A HOQ is the process of determining whether one or more rendered objects are visible on a per-pixel level (one or more occluder convex or concave can occlude it). In this tutorial we test whether a Billboard is being hidden in some way from the camera. This could happen if the Billboard is outside the frustum, or there could be an object between the Billboard and the camera. In some cases, the Billboard might be slated to be drawn when it shouldn't be. Or you might have game play that changes depending on whether the Billboard is visible.

 

NOTE:

The techniques that are described in the article below (Extending the scene manager) are not necessary anymore as of Ogre Version 1.8. Ogre 1.8 provides a RenderObjectListener that allows you to simply hook in when the object that you want to occlusion test is being rendered. For more information how this works, take a look at the "Sample_Lighting" sample which demonstrates occlusion queries for lens flares.

Lens Flares

Lens Flares - occur when the camera looks directly at a light source. The tricky part is the flare places an overlay on top of objects between the camera and the light source. The intensity of the lens flare depends on how much the light source is obstructed. The technical difficulty is knowing how much the light source is occluded. One method to determine whether the light source is occluded is to place a small transparent billboard at the light source. This billboard should have culling turned off in the material script otherwise HOQ will return a non-sense occlusion value of 10000 when the billboard is outside the frustum. 10000 could be a valid value when the billboard is inside the frustum. A HOQ is performed on this billboard to control the intensity of the lens flare. HOQs return a number of pixels. Depending on the camera and field of value this number can fluctuate. There is more discussion about this on the Ogre forums in tgraupmann's post on HardwareOcclusionQuery.

Transparent Billboard Material Script

A material is needed to render a transparent billboard for the HOQ. The material should reference a transparent texture that should be a power of 2. 2x2 is recommended. Culling needs to be disabled so that the HOQ can return correct counts when the billboard leaves the frustum.

material hoq_blank
{
    technique
    {
        pass
        {
            lighting off
            scene_blend src_alpha dest_alpha
            depth_write off

            cull_hardware none
            cull_software none

            texture_unit
            {
                tex_address_mode clamp
                texture hoq_blank.png
            }
        }
    }
}

Example Animation

This is an example of HOQ being used for a lens flare. The transparent billboard should have been made somewhat smaller so that the lens flare fades more quickly when the asteroid flies in front of the camera. In this picture the transparent billboard was 1000x1000 with the star being 100,000 units away from the camera. Faster fade effects occur with smaller transparent billboards. If the transparent billboard gets too small the lens flare will pop (appear and disappear within a 1 pixel change). The larger the transparent billboard the more gradually the lens flare will fade in and out.

HardwareOcclusionQuery.gif

Calculating Max Pixel Count

Keep in mind the pixel counts returned from the HOQ will change if the camera field of view changes. The pixel count will also change if other resolutions are used (640x480, 800x600, 1024x768, etc). So you'll need to be able to calculate what the maximum pixel count is for your camera and resolution.

I haven't quite figured this process out yet. I assume that I can calculate the pixel area of the transparent billboard using screen coordinates.

#include <OgreCamera.h>
#include <OgreVector2.h>
#include <OgreVector3.h>

inline Ogre::Vector2 ToScreenCoordinates(Ogre::Camera *_camera, const Ogre::Vector3 &_position)
{ 
    Ogre::Vector3 hcsPosition = _camera->getProjectionMatrix() * _camera->getViewMatrix() * _position;
    return Ogre::Vector2(0.5 - hcsPosition.x * 0.5f, hcsPosition.y * 0.5f + 0.5);
}

Square Peg Round Hole

In this example we are using a transparent billboard which happens to be a square. This transparent billboard is used to approximate the occlusion of a round light source. The transparent billboard can be rotated at a fast rate which will cause some fluctuation in the pixel counts returned in the HOQ. If the pixel counts are averaged over a brief amount of time, the mean should approximate a circle instead of a square.

HOQ Methods

As scenes increase in complexity and applications make full use of Ogre features it becomes necessary to extend an existing Ogre::SceneManager to do a hardware query on only the Ogre::SceneNode you are interested in. The general way to do a hardware occlusion query is to use 3 steps. Step 1 begin the query. Step 2 draw the Ogre::SceneNode. Step 3 end the query. Unfortunately, the existing Scene Managers only provide an interface to render the whole scene or a render group. If you have a simple scene you can put each HOQ into a render group but you may be using Ogre in a way that was not intended. Or you may see overlays outside the scene affecting your HOQ pixel count. An alternative, is to extend an Ogre::SceneManager and perform HOQ queries during the Render Ogre::SceneNode method.

Extend SceneManger - Custom Scene Manager

The nice part of Ogre being open source is that you have access to the Scene Manager source code. If you extend an Ogre::SceneManager, you can have the extended class do exactly the same thing as the class you derived from. In this case, only the constructor, destructor, getTypeName, and renderSingleObject need to be implemented to extend the class.

CustomSceneManager.h

The header for the CustomSceneManager.h definition looks as follows. Notice there wasn't a need to modify the interface. These are all the same parameters compared with Ogre::DefaultSceneManager.

#ifndef __CUSTOM_SCENE_MANAGER__H__
#define    __CUSTOM_SCENE_MANAGER__H__

#include <OgreNoMemoryMacros.h>
#include <OgreSceneManagerEnumerator.h>
#include <OgreMemoryMacros.h>

class CustomSceneManagerFactory : public Ogre::SceneManagerFactory
{
protected:
    void initMetaData(void) const;
public:
    CustomSceneManagerFactory() {}
    ~CustomSceneManagerFactory() {}
    /// Factory type name
    static const Ogre::String FACTORY_TYPE_NAME;
    Ogre::SceneManager* createInstance(const Ogre::String& instanceName);
    void destroyInstance(Ogre::SceneManager* instance);
};

class CustomSceneManager : public Ogre::SceneManager
{
public:
    CustomSceneManager(const Ogre::String& name);
    ~CustomSceneManager();
    const Ogre::String& getTypeName(void) const;

    /** Internal utility method for rendering a single object. 
    @remarks
        Assumes that the pass has already been set up.
    @param rend The renderable to issue to the pipeline
    @param pass The pass which is being used
    @param doLightIteration If true, this method will issue the renderable to
        the pipeline possibly multiple times, if the pass indicates it should be
        done once per light
    @param manualLightList Only applicable if doLightIteration is false, this
        method allows you to pass in a previously determined set of lights
        which will be used for a single render of this object.
    */
    virtual void renderSingleObject(const Ogre::Renderable* rend, const Ogre::Pass* pass, 
        bool doLightIteration, const Ogre::LightList* manualLightList = 0);
};

class RenderableOcclusionPair
{
public:
    RenderableOcclusionPair(Ogre::Renderable* _renderable);
    ~RenderableOcclusionPair();

    ///
    /// Returns the pixel count, and changes the state
    /// to continue the next hardware query
    ///
    unsigned int GetPixelCount();

    ///
    /// Returns the renderable to query
    ///
    Ogre::Renderable* GetRenderable();

    ///
    /// Start the query, if the state
    /// is ready to render.
    ///
    void BeginOcclusionQuery();

    ///
    /// Finish the query, if the state
    /// is ready to render. Changes
    /// the state to wait for access
    ///
    void EndOcclusionQuery();

protected:
    
    ///
    /// HardwareOcclusionQuery
    ///
    Ogre::HardwareOcclusionQuery*    m_HardwareOcclusionQuery;

    ///
    /// Renderable
    ///
    Ogre::Renderable*                m_Renderable;

    ///
    /// Pixel Count
    ///
    unsigned int                    m_PixelCount;

    ///
    /// Pixel State
    ///
    enum PixelState
    {
        READY_FOR_RENDER,
        QUERY_STARTED,
        READY_FOR_ACCESS,
        __ENUM_MAX__ // must be last
    };

    ///
    /// PixelState
    ///
    PixelState                        m_PixelState;
};

#endif    // __CUSTOM_SCENE_MANAGER__H__

CustomSceneManager.cpp

The implementation for the CustomSceneManager.cpp is mostly the same as Ogre::DefaultSceneManager.

#include "CustomSceneManager.h"

#include <OgreNoMemoryMacros.h>
#include "OgreCamera.h"
#include "OgreString.h"
#include "OgreSceneNode.h"
#include "OgrePlane.h"
#include "OgreQuaternion.h"
#include "OgreColourValue.h"
#include "OgreCommon.h"
#include "OgreSceneQuery.h"
#include "OgreAutoParamDataSource.h"
#include "OgreAnimationState.h"
#include "OgreRenderQueue.h"
#include "OgreRenderQueueSortingGrouping.h"
#include "OgreRenderSystem.h"
#include "OgreRectangle2D.h"
#include "OgrePixelFormat.h"
#include "OgreRoot.h"
#include <OgreHardwareOcclusionQuery.h>
#include "OgreResourceGroupManager.h"
#include "OgreTexture.h"
#include <OgreMemoryMacros.h>

///
/// STATIC OCCLUSION QUERY LIST
///
std::list<RenderableOcclusionPair*> g_RenderableOcclusionPairList;

void CustomSceneManagerFactory::initMetaData(void) const
{
    mMetaData.typeName = FACTORY_TYPE_NAME;
    mMetaData.description = "The custom scene manager";
    mMetaData.sceneTypeMask = Ogre::ST_GENERIC;
    mMetaData.worldGeometrySupported = false;
}

const Ogre::String CustomSceneManagerFactory::FACTORY_TYPE_NAME = "CustomSceneManager";

Ogre::SceneManager* CustomSceneManagerFactory::createInstance(const Ogre::String& instanceName)
{
    return new CustomSceneManager(instanceName);
}

void CustomSceneManagerFactory::destroyInstance(Ogre::SceneManager* instance)
{
    delete instance;
}

LogDefineModule(CustomSceneManager);

CustomSceneManager::CustomSceneManager(const Ogre::String& name)
    : Ogre::SceneManager(name)
{
}

CustomSceneManager::~CustomSceneManager()
{
}

const Ogre::String& CustomSceneManager::getTypeName(void) const
{
    return CustomSceneManagerFactory::FACTORY_TYPE_NAME;
}

void CustomSceneManager::renderSingleObject(const Ogre::Renderable* rend, const Ogre::Pass* pass, 
                                            bool doLightIteration, const Ogre::LightList* manualLightList)
{
    /// 
}

RenderableOcclusionPair::RenderableOcclusionPair(Ogre::Renderable* _renderable) :
    m_Renderable                    (_renderable),
    m_PixelCount                    (0),
    m_PixelState                    (RenderableOcclusionPair::READY_FOR_RENDER)
{
    ///
    /// CREATE HardwareOcclusionQuery
    ///
    m_HardwareOcclusionQuery = Ogre::Root::getSingleton().getRenderSystem()->createHardwareOcclusionQuery();
    assert (m_HardwareOcclusionQuery);
}

RenderableOcclusionPair::~RenderableOcclusionPair()
{
    ///
    /// DELETE HardwareOcclusionQuery
    ///
    Ogre::Root::getSingleton().getRenderSystem()->destroyHardwareOcclusionQuery(m_HardwareOcclusionQuery);
    m_HardwareOcclusionQuery = 0;
}

///
/// Returns the pixel count, and changes the state
/// to continue the next hardware query
///
unsigned int RenderableOcclusionPair::GetPixelCount()
{
    switch (m_PixelState)
    {
    case RenderableOcclusionPair::READY_FOR_ACCESS :
        {
            ///
            /// GET THE PIXEL COUNT
            ///
            m_HardwareOcclusionQuery->pullOcclusionQuery(&m_PixelCount);

            ///
            /// CHANGE THE STATE
            ///
            m_PixelState = RenderableOcclusionPair::READY_FOR_RENDER;
        }
    }

    return m_PixelCount;
}

///
/// Returns the renderable to query
///
Ogre::Renderable* RenderableOcclusionPair::GetRenderable()
{
    return m_Renderable;
}

///
/// Start the query, if the state
/// is ready to render.
///
void RenderableOcclusionPair::BeginOcclusionQuery()
{
    switch (m_PixelState)
    {
    case RenderableOcclusionPair::READY_FOR_RENDER :
        {
            ///
            /// START THE QUERY
            ///
            m_HardwareOcclusionQuery->beginOcclusionQuery();

            ///
            /// CHANGE THE STATE
            ///
            m_PixelState = RenderableOcclusionPair::QUERY_STARTED;
        }
    }
}

///
/// Finish the query, if the state
/// is ready to render. Changes
/// the state to wait for access
///
void RenderableOcclusionPair::EndOcclusionQuery()
{
    switch (m_PixelState)
    {
    case RenderableOcclusionPair::QUERY_STARTED :
        {
            ///
            /// START THE QUERY
            ///
            m_HardwareOcclusionQuery->endOcclusionQuery();

            ///
            /// CHANGE THE STATE
            ///
            m_PixelState = RenderableOcclusionPair::READY_FOR_ACCESS;
        }
    }
}

CustomSceneManager Interface

Any class that wants to add a HOQ query for their Ogre::SceneNode just simply adds a transparent billboard to the extern g_RenderableOcclusionPairList inside a RenderableOcclusionPair.

#include "CustomSceneManager.h"

///
/// STATIC OCCLUSION QUERY LIST
///
extern std::list<RenderableOcclusionPair*> g_RenderableOcclusionPairList;

RenderableOcclusionPair Setup

The following snippet shows adding an Ogre::BillboardSet to the list to be queried.

///
    /// CREATE THE OCCLUSION TEST SCENE NODE
    ///
    m_OcclusionTestSceneNode =_mgr->getRootSceneNode()->createChildSceneNode(m_Position);
    assert (m_OcclusionTestSceneNode);

    ///
    /// INITIALIZE THE OCCLUSION TEST BILLBOARD SYSTEM
    ///
    m_OcclusionTestBillboardSet = new BillboardSet("A super unique name", 1);
    assert (m_OcclusionTestBillboardSet);

    ///
    /// SET THE OCCLUSION TEST MATERIAL
    ///
    m_OcclusionTestBillboardSet->setMaterialName("hoq_blank");

    ///
    /// SET THE OCCLUSION TEST BILLBOARD ORIGIN
    ///
    m_OcclusionTestBillboardSet->setBillboardOrigin(BBO_CENTER);

    ///
    /// CREATE EACH BILLBOARD IN THE OCCLUSION TEST SET
    ///
    m_OcclusionTestBillboard = m_OcclusionTestBillboardSet->createBillboard(Ogre::Vector3::ZERO);
    assert (m_OcclusionTestBillboard);

    ///
    /// SET OCCLUSION TEST DIMENSIONS
    ///
    m_OcclusionTestBillboard->setDimensions(m_OcclusionTestSize.x, m_OcclusionTestSize.y);

    ///
    /// UPDATE OCCLUSION TEST BOUNDS
    ///
    m_OcclusionTestBillboardSet->_updateBounds();

    ///
    /// ATTACH THE OCCLUSION TEST BILLBOARD SET
    ///
    m_OcclusionTestSceneNode->attachObject(m_OcclusionTestBillboardSet);

    //////
    /// OCCLUSION STUFF
    //////
    m_RenderableOcclusionPair =
        new RenderableOcclusionPair(m_OcclusionTestBillboardSet);
    assert (m_RenderableOcclusionPair);

    ///
    /// ADD TO EXTERN LIST
    ///
    g_RenderableOcclusionPairList.push_back(m_RenderableOcclusionPair);

Access to Pixel Count

The following snippet shows how to access the pixel count from the HOQ query. The following is called periodically.

if (m_MaxPixelCount)
        {
            float pixelRatio = float(m_RenderableOcclusionPair->GetPixelCount()) / float(m_MaxPixelCount);
        }



Credits

This document was created by tgraupmann. Feel free to email any questions you might have to tgraupmann@yahoo.com.

Implementation

Lee04 has created a commercial hardware occlusion query query enabled render plug-in is availble for Ogre. Feel free to email any questions you have to colorod (at) gmail.com mailto:colorod@gmail.com.
More about the plug-in: Lee's HOQ plug-in is very easy to use and doesn't require materials to have depth_write off etc, it's handled in the back ground. Since it does all the HOQ on a first Z pass, Ogre and you are free to render as usual on top of the depth buffer. It also enables early z out optimization on the GPU. Ogre sub entities that has the same hardware occlusion ID is automagically group together, which is good for different kind of scene manager implementations and optimizations and this can change dynamically. Also all objects in the scene is occluders by default. HOQ tests are also filtered when objects are visible from previous frame to save performance.


Alias: Hardware_Occlusion_Query