用OpenSceneGraph实现的NeHe OpenGL教程 - 第三十六课

• 简介

这节课NeHe课程向我们介绍了如何读取帧缓存中的像素值，并把 它作为场景中的纹理加载到几何体上。也就是我们常说的渲染到纹理(Render To Texture)功能，也称纹理烘培。这一功能主要有两个作用：第一是实现场景离屏渲染之后的后置处理；第二是实现多种不同场景的融合显示效果。

• 实现

首先创建我们场景中需要显示的Helix模型，代码如下：

osg::Geode*	createHelix()

接下来我们需要将这个模型加载的场景写到纹理中去，在OSG中可以通过创建渲染到纹理的相机完成这样的操作：

osg::Camera*	createRenderToTextureCamera(int w, int h)
{
	osg::Camera *rtt = new osg::Camera;
	rtt->setClearColor(osg::Vec4(0.0f, 0.0f, 0.5f, 0.5));	
	rtt->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	rtt->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
	rtt->setViewport(0, 0, w, h);
	rtt->setRenderOrder(osg::Camera::PRE_RENDER);
	rtt->setRenderTargetImplementation(osg::Camera::FRAME_BUFFER_OBJECT);
	rtt->setProjectionMatrixAsPerspective(45.0f, static_cast<double>(640)/static_cast<double>(480), 0.1f, 100.0f );
	rtt->setViewMatrixAsLookAt(osg::Vec3d(0, 5, 50), osg::Vec3d(0, 0, 0), osg::Vec3d(0, 1, 0));

	osg::MatrixTransform *zoomMT = new osg::MatrixTransform;
	zoomMT->setMatrix(osg::Matrix::translate(0,0,-50));
	osg::MatrixTransform *xRotMT = new osg::MatrixTransform;
	xRotMT->addUpdateCallback(new RotAxisCallback(osg::X_AXIS, 0.02));
	osg::MatrixTransform *yRotMT = new osg::MatrixTransform;
	yRotMT->addUpdateCallback(new RotAxisCallback(osg::Y_AXIS, 0.01));

	zoomMT->addChild(xRotMT);
	xRotMT->addChild(yRotMT);
	yRotMT->addChild(createHelix());

	rtt->addChild(zoomMT);
	
	return rtt;
}

通过设置相机渲染方式是PRE_RENDER可以保证在渲染其他场景之前先渲染这个相机中的场景，并通过下面的操作将其保存到纹理之中：

	g_Texture = createTexture(128,128);
	osg::Camera *rttCamera = createRenderToTextureCamera(128, 128);
	rttCamera->attach(osg::Camera::COLOR_BUFFER, g_Texture);

这样我们需要的纹理就创建好了。

接下来加载使用纹理的几何体：

osg::Node*	createBlur(int times, float inc)

同时在几何体的更新中不断修改纹理坐标的位置，实现微小的偏移，整个场景看起来像一种运动模糊的效果：

class BlurGeometryUpdateCallback : public osg::Drawable::UpdateCallback

最后把所有这些部分添加到场景根节点中，编译运行程序：

附：本课源码(源码中可能存在错误和不足，仅供参考)

#include "../osgNeHe.h"

#include <QtCore/QTimer>
#include <QtGui/QApplication>
#include <QtGui/QVBoxLayout>

#include <osgViewer/Viewer>
#include <osgDB/ReadFile>
#include <osgQt/GraphicsWindowQt>

#include <osg/MatrixTransform>
#include <osg/AnimationPath>

#include <osg/Material>
#include <osg/LightSource>

#include <osg/BlendFunc>
#include <osg/Texture2D>
#include <osg/State>


float		angle;
float     vertexes[4][3];
float     normal[3];
osg::Texture2D	*g_Texture;

//////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////

void ReduceToUnit(float vector[3])								// Reduces A Normal Vector (3 Coordinates)
{																// To A Unit Normal Vector With A Length Of One.
	float length;												// Holds Unit Length
	// Calculates The Length Of The Vector
	length = (float)sqrt((vector[0]*vector[0]) + (vector[1]*vector[1]) + (vector[2]*vector[2]));

	if(length == 0.0f)											// Prevents Divide By 0 Error By Providing
		length = 1.0f;											// An Acceptable Value For Vectors To Close To 0.

	vector[0] /= length;										// Dividing Each Element By
	vector[1] /= length;										// The Length Results In A
	vector[2] /= length;										// Unit Normal Vector.
}

void calcNormal(float v[3][3], float out[3])					// Calculates Normal For A Quad Using 3 Points
{
	float v1[3],v2[3];											// Vector 1 (x,y,z) & Vector 2 (x,y,z)
	static const int x = 0;										// Define X Coord
	static const int y = 1;										// Define Y Coord
	static const int z = 2;										// Define Z Coord

	// Finds The Vector Between 2 Points By Subtracting
	// The x,y,z Coordinates From One Point To Another.

	// Calculate The Vector From Point 1 To Point 0
	v1[x] = v[0][x] - v[1][x];									// Vector 1.x=Vertex[0].x-Vertex[1].x
	v1[y] = v[0][y] - v[1][y];									// Vector 1.y=Vertex[0].y-Vertex[1].y
	v1[z] = v[0][z] - v[1][z];									// Vector 1.z=Vertex[0].y-Vertex[1].z
	// Calculate The Vector From Point 2 To Point 1
	v2[x] = v[1][x] - v[2][x];									// Vector 2.x=Vertex[0].x-Vertex[1].x
	v2[y] = v[1][y] - v[2][y];									// Vector 2.y=Vertex[0].y-Vertex[1].y
	v2[z] = v[1][z] - v[2][z];									// Vector 2.z=Vertex[0].z-Vertex[1].z
	// Compute The Cross Product To Give Us A Surface Normal
	out[x] = v1[y]*v2[z] - v1[z]*v2[y];							// Cross Product For Y - Z
	out[y] = v1[z]*v2[x] - v1[x]*v2[z];							// Cross Product For X - Z
	out[z] = v1[x]*v2[y] - v1[y]*v2[x];							// Cross Product For X - Y

	ReduceToUnit(out);											// Normalize The Vectors
}




class RotAxisCallback : public osg::NodeCallback
{
public:
	RotAxisCallback(const osg::Vec3& axis, double rotSpeed = 0.0, double currentAngle = 0.0)
		: _rotAxis(axis), _rotSpeed(rotSpeed), _currentAngle(currentAngle){ }

	virtual void operator()(osg::Node* node, osg::NodeVisitor* nv)
	{
		osg::MatrixTransform *rotMT = dynamic_cast<osg::MatrixTransform*>(node);
		if (!rotMT)
			return;
		_currentAngle += _rotSpeed;
		rotMT->setMatrix(osg::Matrix::rotate(_currentAngle, _rotAxis));
		
		traverse(node, nv);
	}

	void setRotateSpeed(double speed)
	{
		_rotSpeed = speed;
	}

	double getRotateSpeed() const
	{
		return _rotSpeed;
	}

	double getCurrentAngle() const
	{
		return _currentAngle;
	}


private:
	osg::Vec3		_rotAxis;
	double			_currentAngle;
	double			_rotSpeed;
};

osg::Geode*	createHelix()
{
	GLfloat x;									// Helix x Coordinate
	GLfloat y;									// Helix y Coordinate
	GLfloat z;										// Helix z Coordinate
	GLfloat phi;									// Angle
	GLfloat theta;								// Angle
	GLfloat v,u;									// Angles
	GLfloat r = 1.5f;							// Radius Of Twist
	int twists = 5;								// 5 Twists
	
	osg::Geode *geode = new osg::Geode;
	osg::Geometry *geometry = new osg::Geometry;
	osg::Vec3Array *vertexArray = new osg::Vec3Array;
	osg::Vec3Array *normalArray = new osg::Vec3Array;

//////////////////////////////////////////////////////////////////////////
	for(int phi=0; phi <= 360; phi+=20.0)							// 360 Degrees In Steps Of 20
	{
		for(int theta=0; theta<=360*twists; theta+=20.0)			// 360 Degrees * Number Of Twists In Steps Of 20
		{
			v=(phi/180.0f*3.142f);								// Calculate Angle Of First Point	(  0 )
			u=(theta/180.0f*3.142f);							// Calculate Angle Of First Point	(  0 )

			x=float(cos(u)*(2.0f+cos(v) ))*r;					// Calculate x Position (1st Point)
			y=float(sin(u)*(2.0f+cos(v) ))*r;					// Calculate y Position (1st Point)
			z=float((( u-(2.0f*3.142f)) + sin(v) ) * r);		// Calculate z Position (1st Point)

			vertexes[0][0]=x;									// Set x Value Of First Vertex
			vertexes[0][1]=y;									// Set y Value Of First Vertex
			vertexes[0][2]=z;									// Set z Value Of First Vertex

			v=(phi/180.0f*3.142f);								// Calculate Angle Of Second Point	(  0 )
			u=((theta+20)/180.0f*3.142f);						// Calculate Angle Of Second Point	( 20 )

			x=float(cos(u)*(2.0f+cos(v) ))*r;					// Calculate x Position (2nd Point)
			y=float(sin(u)*(2.0f+cos(v) ))*r;					// Calculate y Position (2nd Point)
			z=float((( u-(2.0f*3.142f)) + sin(v) ) * r);		// Calculate z Position (2nd Point)

			vertexes[1][0]=x;									// Set x Value Of Second Vertex
			vertexes[1][1]=y;									// Set y Value Of Second Vertex
			vertexes[1][2]=z;									// Set z Value Of Second Vertex

			v=((phi+20)/180.0f*3.142f);							// Calculate Angle Of Third Point	( 20 )
			u=((theta+20)/180.0f*3.142f);						// Calculate Angle Of Third Point	( 20 )

			x=float(cos(u)*(2.0f+cos(v) ))*r;					// Calculate x Position (3rd Point)
			y=float(sin(u)*(2.0f+cos(v) ))*r;					// Calculate y Position (3rd Point)
			z=float((( u-(2.0f*3.142f)) + sin(v) ) * r);		// Calculate z Position (3rd Point)

			vertexes[2][0]=x;									// Set x Value Of Third Vertex
			vertexes[2][1]=y;									// Set y Value Of Third Vertex
			vertexes[2][2]=z;									// Set z Value Of Third Vertex

			v=((phi+20)/180.0f*3.142f);							// Calculate Angle Of Fourth Point	( 20 )
			u=((theta)/180.0f*3.142f);							// Calculate Angle Of Fourth Point	(  0 )

			x=float(cos(u)*(2.0f+cos(v) ))*r;					// Calculate x Position (4th Point)
			y=float(sin(u)*(2.0f+cos(v) ))*r;					// Calculate y Position (4th Point)
			z=float((( u-(2.0f*3.142f)) + sin(v) ) * r);		// Calculate z Position (4th Point)

			vertexes[3][0]=x;									// Set x Value Of Fourth Vertex
			vertexes[3][1]=y;									// Set y Value Of Fourth Vertex
			vertexes[3][2]=z;									// Set z Value Of Fourth Vertex

			calcNormal(vertexes,normal);						// Calculate The Quad Normal

			normalArray->push_back(osg::Vec3(normal[0],normal[1],normal[2]));
			normalArray->push_back(osg::Vec3(normal[0],normal[1],normal[2]));
			normalArray->push_back(osg::Vec3(normal[0],normal[1],normal[2]));
			normalArray->push_back(osg::Vec3(normal[0],normal[1],normal[2]));

			// Render The Quad
			vertexArray->push_back(osg::Vec3(vertexes[0][0],vertexes[0][1],vertexes[0][2]));
			vertexArray->push_back(osg::Vec3(vertexes[1][0],vertexes[1][1],vertexes[1][2]));
			vertexArray->push_back(osg::Vec3(vertexes[2][0],vertexes[2][1],vertexes[2][2]));
			vertexArray->push_back(osg::Vec3(vertexes[3][0],vertexes[3][1],vertexes[3][2]));
		}
	}
//////////////////////////////////////////////////////////////////////////

	geometry->setVertexArray(vertexArray);
	geometry->setNormalArray(normalArray, osg::Array::BIND_PER_VERTEX);
	geometry->addPrimitiveSet(new osg::DrawArrays(osg::PrimitiveSet::QUADS, 0, vertexArray->size()));

	osg::Material *helixMat = new osg::Material;
	helixMat->setAmbient(osg::Material::FRONT_AND_BACK, osg::Vec4(0.4f,0.2f,0.8f,1.0f));
	helixMat->setDiffuse(osg::Material::FRONT_AND_BACK, osg::Vec4(0.4f,0.2f,0.8f,1.0f));
	helixMat->setSpecular(osg::Material::FRONT_AND_BACK, osg::Vec4(1.0f,1.0f,1.0f,1.0f));
	helixMat->setShininess(osg::Material::FRONT, 128.0f);

	geometry->getOrCreateStateSet()->setAttributeAndModes(helixMat);
	geometry->getOrCreateStateSet()->setMode(GL_LIGHT0, osg::StateAttribute::ON);

	geode->addDrawable(geometry);

	return geode;
}


class ViewerWidget : public QWidget, public osgViewer::Viewer
{
public:
	ViewerWidget(osg::Node *scene = NULL)
	{
		QWidget* renderWidget = getRenderWidget( createGraphicsWindow(0,0,640,480), scene);

		QVBoxLayout* layout = new QVBoxLayout;
		layout->addWidget(renderWidget);
		layout->setContentsMargins(0, 0, 0, 1);
		setLayout( layout );

		connect( &_timer, SIGNAL(timeout()), this, SLOT(update()) );
		_timer.start( 10 );
	}

	QWidget* getRenderWidget( osgQt::GraphicsWindowQt* gw, osg::Node* scene )
	{
		osg::Camera* camera = this->getCamera();
		camera->setGraphicsContext( gw );

		const osg::GraphicsContext::Traits* traits = gw->getTraits();

		camera->setClearColor( osg::Vec4(0.0, 0.0, 0.0, 0.5) );
		camera->setViewport( new osg::Viewport(0, 0, traits->width, traits->height) );
		camera->setProjectionMatrixAsPerspective(45.0f, static_cast<double>(traits->width)/static_cast<double>(traits->height), 0.1f, 100.0f );
		camera->setViewMatrixAsLookAt(osg::Vec3d(0, 5, 50), osg::Vec3d(0, 0, 0), osg::Vec3d(0, 1, 0));

		this->setSceneData( scene );
		return gw->getGLWidget();
	}

	osgQt::GraphicsWindowQt* createGraphicsWindow( int x, int y, int w, int h, const std::string& name="", bool windowDecoration=false )
	{
		osg::DisplaySettings* ds = osg::DisplaySettings::instance().get();
		osg::ref_ptr<osg::GraphicsContext::Traits> traits = new osg::GraphicsContext::Traits;
		traits->windowName = name;
		traits->windowDecoration = windowDecoration;
		traits->x = x;
		traits->y = y;
		traits->width = w;
		traits->height = h;
		traits->doubleBuffer = true;
		traits->alpha = ds->getMinimumNumAlphaBits();
		traits->stencil = ds->getMinimumNumStencilBits();
		traits->sampleBuffers = ds->getMultiSamples();
		traits->samples = ds->getNumMultiSamples();

		return new osgQt::GraphicsWindowQt(traits.get());
	}

	virtual void paintEvent( QPaintEvent* event )
	{ 
		frame(); 
	}

protected:

	QTimer _timer;
};



class BlurGeometryUpdateCallback : public osg::Drawable::UpdateCallback
{
public:
	BlurGeometryUpdateCallback(int renderTimes, float inc) : _times(renderTimes), _inc(inc){ }

	void update(osg::NodeVisitor*, osg::Drawable* d)
	{
		osg::Geometry *geometry = dynamic_cast<osg::Geometry*>(d);
		if (!geometry)
			return;
		
		osg::Vec2Array *texArray = dynamic_cast<osg::Vec2Array*>(geometry->getTexCoordArray(0));
		if(!texArray)
			return;
		
		osg::Vec4Array *colorArray = dynamic_cast<osg::Vec4Array*>(geometry->getColorArray());
		if(!colorArray)
			return;

		float spost = 0.0f;	
		float alphainc = 0.9f / _times;
		float alpha = 0.2f;
		alphainc = alpha / _times;

		texArray->clear();
		colorArray->clear();
		
		for (int num = 0;num < _times;num++)
		{
			colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
			texArray->push_back(osg::Vec2(0+spost,1-spost));

			colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
			texArray->push_back(osg::Vec2(0+spost,0+spost));

			colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
			texArray->push_back(osg::Vec2(1-spost,0+spost));

			colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
			texArray->push_back(osg::Vec2(1-spost,1-spost));

			spost += _inc;					
			alpha = alpha - alphainc;	
		}

		texArray->dirty();
		colorArray->dirty();
	}

	int _times;
	float _inc;
};


osg::Node*	createBlur(int times, float inc)
{
	float spost = 0.0f;
	float alphainc = 0.9f / times;
	float alpha = 0.2f;
	alphainc = alpha / times;

	osg::BlendFunc *blendFunc = new osg::BlendFunc(osg::BlendFunc::SRC_ALPHA, osg::BlendFunc::ONE);
	osg::Geode *geode = new osg::Geode;
	osg::Geometry *geometry = new osg::Geometry;
	geometry->setUpdateCallback(new BlurGeometryUpdateCallback(times, inc));
	osg::Vec2Array *vertexArray = new osg::Vec2Array;
	osg::Vec2Array *texArray = new osg::Vec2Array;
	osg::Vec4Array *colorArray = new osg::Vec4Array;

	for (int num = 0;num < times;num++)
	{
		colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
		vertexArray->push_back(osg::Vec2(0,0));
		texArray->push_back(osg::Vec2(0+spost,1-spost));

		colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
		vertexArray->push_back(osg::Vec2(0,480));
		texArray->push_back(osg::Vec2(0+spost,0+spost));

		colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
		vertexArray->push_back(osg::Vec2(640,480));
		texArray->push_back(osg::Vec2(1-spost,0+spost));

		colorArray->push_back(osg::Vec4(1.0f, 1.0f, 1.0f, alpha));
		vertexArray->push_back(osg::Vec2(640,0));
		texArray->push_back(osg::Vec2(1-spost,1-spost));
	}

	geometry->setVertexArray(vertexArray);
	geometry->setTexCoordArray(0, texArray, osg::Array::BIND_PER_VERTEX);
	geometry->setColorArray(colorArray, osg::Array::BIND_PER_VERTEX);
	geometry->addPrimitiveSet(new osg::DrawArrays(osg::PrimitiveSet::QUADS, 0, vertexArray->size()));
	geometry->getOrCreateStateSet()->setAttributeAndModes(blendFunc);
	geometry->getOrCreateStateSet()->setTextureAttributeAndModes(0, g_Texture);
	geometry->getOrCreateStateSet()->setMode(GL_DEPTH_TEST, osg::StateAttribute::OFF);
	geometry->getOrCreateStateSet()->setMode(GL_LIGHT0, osg::StateAttribute::ON);
	geode->addDrawable(geometry);

	return geode;
}


osg::Camera* createBlurHUD()
{
	osg::Camera* camera = new osg::Camera;
	camera->setProjectionMatrix(osg::Matrix::ortho2D(0,640,0,480));
	camera->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
	camera->setViewMatrix(osg::Matrix::identity());
	camera->setRenderOrder(osg::Camera::POST_RENDER);
	camera->setAllowEventFocus(false);

	camera->addChild(createBlur(25, 0.02f));

	return camera;
}


osg::Texture2D*  createTexture(int w, int h)
{
	osg::Texture2D *texture = new osg::Texture2D;
	texture->setInternalFormat(GL_RGBA);
	texture->setFilter(osg::Texture::MIN_FILTER, osg::Texture::LINEAR);
	texture->setFilter(osg::Texture::MAG_FILTER, osg::Texture::LINEAR);
	texture->setTextureSize(w,h);
	return texture;
}


osg::Camera*	createRenderToTextureCamera(int w, int h)
{
	osg::Camera *rtt = new osg::Camera;
	rtt->setClearColor(osg::Vec4(0.0f, 0.0f, 0.5f, 0.5));	
	rtt->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	rtt->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
	rtt->setViewport(0, 0, w, h);
	rtt->setRenderOrder(osg::Camera::PRE_RENDER);
	rtt->setRenderTargetImplementation(osg::Camera::FRAME_BUFFER_OBJECT);
	rtt->setProjectionMatrixAsPerspective(45.0f, static_cast<double>(640)/static_cast<double>(480), 0.1f, 100.0f );
	rtt->setViewMatrixAsLookAt(osg::Vec3d(0, 5, 50), osg::Vec3d(0, 0, 0), osg::Vec3d(0, 1, 0));

	osg::MatrixTransform *zoomMT = new osg::MatrixTransform;
	zoomMT->setMatrix(osg::Matrix::translate(0,0,-50));
	osg::MatrixTransform *xRotMT = new osg::MatrixTransform;
	xRotMT->addUpdateCallback(new RotAxisCallback(osg::X_AXIS, 0.02));
	osg::MatrixTransform *yRotMT = new osg::MatrixTransform;
	yRotMT->addUpdateCallback(new RotAxisCallback(osg::Y_AXIS, 0.01));

	zoomMT->addChild(xRotMT);
	xRotMT->addChild(yRotMT);
	yRotMT->addChild(createHelix());

	rtt->addChild(zoomMT);
	
	return rtt;
}



osg::Node*	buildScene()
{
	GLfloat light0pos[4]=     {0.0f, 5.0f, 10.0f, 1.0f};
	GLfloat light0ambient[4]= {0.2f, 0.2f,  0.2f, 1.0f};
	GLfloat light0diffuse[4]= {0.3f, 0.3f,  0.3f, 1.0f};
	GLfloat light0specular[4]={0.8f, 0.8f,  0.8f, 1.0f};	

	osg::Group *root = new osg::Group;
	
	osg::Light *light = new osg::Light;
	light->setLightNum(0);
	light->setAmbient(osg::Vec4(light0ambient[0],light0ambient[1],light0ambient[2],light0ambient[3]));
	light->setDiffuse(osg::Vec4(light0diffuse[0],light0diffuse[1],light0diffuse[2],light0diffuse[3]));
	light->setPosition(osg::Vec4(light0pos[0], light0pos[1],light0pos[2],light0pos[3]));
	light->setSpecular(osg::Vec4(light0specular[0],light0specular[1],light0specular[2],light0specular[3]));
	osg::LightSource *lightSource = new osg::LightSource;
	lightSource->setLight(light);
	root->addChild(lightSource);

	osg::MatrixTransform *zoomMT = new osg::MatrixTransform;
	zoomMT->setMatrix(osg::Matrix::translate(0,0,-50));
	osg::MatrixTransform *xRotMT = new osg::MatrixTransform;
	xRotMT->addUpdateCallback(new RotAxisCallback(osg::X_AXIS, 0.02));
	osg::MatrixTransform *yRotMT = new osg::MatrixTransform;
	yRotMT->addUpdateCallback(new RotAxisCallback(osg::Y_AXIS, 0.01));
	
	zoomMT->addChild(xRotMT);
	xRotMT->addChild(yRotMT);
	yRotMT->addChild(createHelix());


	g_Texture = createTexture(128,128);
	osg::Camera *rttCamera = createRenderToTextureCamera(128, 128);
	rttCamera->attach(osg::Camera::COLOR_BUFFER, g_Texture);
	root->addChild(rttCamera);
	root->addChild(zoomMT);
	root->addChild(createBlurHUD());
	
	return root;
}



int main( int argc, char** argv )
{
	QApplication app(argc, argv);
	ViewerWidget* viewWidget = new ViewerWidget(buildScene());
	viewWidget->setGeometry( 100, 100, 640, 480 );
	viewWidget->show();
	return app.exec();
}