osg使用整理（11）：延迟渲染
一、基础概念
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前向渲染流程：顶点着色器->图元装配成点线三角形->几何着色器->裁剪剔除->光栅化（片元着色器）->透明度测试、深度测试。
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延迟渲染流程：顶点着色器->图元装配成点线三角形->几何着色器->裁剪剔除->光栅化输出G-Buffer，存储每个像素的属性信息（位置、法线、颜色）->深度测试->光照计算->片元着色器->透明度测试。
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渲染管线的差异：
a. 延迟渲染需要两个pass，先生成G-Buffer后进行光照计算。
b. 延迟渲染不能使用MSAA算法抗锯齿。
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优劣势：
a. 延迟渲染先进行深度测试，确定了可见像素后再进行光照计算，而不是对所有图元进行光栅化再光照计算，避免了大量无效计算。
b. 延迟渲染在一个Pass中处理多光源计算，提高了渲染效率。
c. 延迟渲染的G-Buffer占用带宽较大，需要合并一些纹理通道、减少buffer位数、将两个pass合并为OnePassDeferred。
d. 延迟渲染通常只能使用相同光照的效果，灵活性低。
e. 延迟渲染中透明物体需要单独的Pass来处理。
二、G-Buffer
如下图所示，延迟渲染首先生成称为G-Buffer的一系列纹理，常包含世界坐标系下的位置向量、颜色向量、顶点法线向量等等。
osg中要实现延迟渲染，首先准备离屏相机：
osg::ref_ptr<RttCamera> createRTTCamera(osg::Camera::BufferComponent buffer,osg::Texture* tex,int width,int height)
{
osg::ref_ptr<RttCamera> camera=new RttCamera(width,height);
camera->setRenderTargetImplementation(osg::Camera::RenderTargetImplementation::FRAME_BUFFER_OBJECT);
camera->setClearMask(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
camera->setPostDrawCallBack(new FBOPostDrawCallback);
camera->setRenderOrder(osg::Camera::PRE_RENDER,20);
camera->setViewPort(0,0,width,height);
if(tex)
{
tex->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
tex->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
camera->setViewPort(0,0,tex->getTextureWidth(),tex->getTextureHeight());
camera->attach(buffer,tex);
}
return camera;
}
注意到，离屏相机渲染目标设置为FBO，同时渲染次序设置为PRE_RENDER。然后准备附着在离屏相机上的颜色和深度纹理，简单认为有三维位置坐标纹理、法线方向纹理、基础颜色纹理三个。为了提高纹理精度，实现HDR渲染，我们可以使用浮点数缓冲，其内部格式通常设为GL_RGB16F, GL_RGBA16F, GL_RGB32F 或者GL_RGBA32F。浮点数缓冲可以存储超过0.0到1.0范围的浮点值。
//创建颜色附着纹理
osg::ref_ptr<osg::Texture2D> createColorTexture(int width,int height)
{
osg::ref_ptr<osg::Texture2D> texture=new osg::Texture2D;
texture->setTextureSize(width,height);
texture->setInternalFormat(GL_RGBA32F);
texture->setSourceFormat(GL_RGBA);
texture->setSourceType(GL_FLOAT);//浮点数精度更高
tex->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINEAR);
tex->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINEAR);
return texture;
}
//创建深度附着纹理
osg::ref_ptr<osg::Texture2D> createDepthTexture(int width,int height)
{
osg::ref_ptr<osg::Texture2D> texture=new osg::Texture2D;
texture->setTextureSize(width,height);
texture->setInternalFormat(GL_DEPTH_COMPONENT32F);
texture->setSourceFormat(GL_DEPTH_COMPONENT32F);
texture->setSourceType(GL_FLOAT);
tex->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINEAR);
tex->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINEAR);
return texture;
}
创建延迟渲染相机，首先渲染到G-buffer上，然后创建HUD相机渲染到和屏幕同样大小的矩形上。
/*延迟渲染相机*/
osg::ref_ptr<RttCamera> createDeferCamera(osg::Camera::BufferComponent buffer1,osg::Texture* tex1,
osg::Camera::BufferComponent buffer2,osg::Texture* tex2,
osg::Camera::BufferComponent buffer3,osg::Texture* tex3,int width,int height)
{
osg::ref_ptr<RttCamera> camera=new RttCamera(width,height);
camera->setRenderTargetImplementation(osg::Camera::RenderTargetImplementation::FRAME_BUFFER_OBJECT);
camera->setClearMask(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
camera->setPostDrawCallBack(new FBOPostDrawCallback);
camera->setRenderOrder(osg::Camera::PRE_RENDER,20);
camera->setViewPort(0,0,width,height);
if(tex1)
{
tex1->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
tex1->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
camera->attach(buffer1,tex1);
}
if(tex2)
{
tex2->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
tex2->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
camera->attach(buffer2,tex2);
}
if(tex3)
{
tex3->setFilter(osg::Texture::MIN_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
tex3->setFilter(osg::Texture::MAG_FILTER,osg::Texture::LINER_MIPMAP_NEAREST);
camera->attach(buffer3,tex3);
}
///顶点着色器
const char* vertCode=R"(
#version 330
layout(location = 0) in vec3 Position;
layout(location = 2) in vec3 normal;
layout(location = 3) in vec3 TexCoord;
uniform mat4 osg_ModelViewProjectionMatrix;
uniform mat4 osg_ModelViewMatrix;
uniform mat4 osg_NormalMatrix;
out vec3 vNormal;
out vec2 texCoord;
out vec4 fragPos;
void main()
{
texCoord=TexCoord;
fragPos=osg_ModelViewMatrix*vec4(Position,1.0);
vec4 viewNorm=transpose(inverse(osg_ModelViewMatrix))*vec4(-normal,1.0);
vNormal=normalize(viewNorm.xyz);
gl_Position=osg_ModelViewProjectionMatrix*vec4(Position,1.0);
}
)";
const char* fragCode=R"(
#version 330 core
uniform vec3 frontCol=vec3(1.0,0.0,0.2);
layout (location = 0) out vec4 gColor;
layout (location = 1) out vec4 gNormal;
layout (location = 2) out vec4 gPosition;
in vec2 texCoord;
in vec4 fragPos;
in vec3 vNormal;
void main()
{
// Store the fragment position vector in the first gbuffer texture
gPosition.xyz = fragPos.xyz;
// Also store the per-fragment normals into the gbuffer
gNormal = vec4(vNormal,1.0);
gColor=vec4(frontCol,1.0);
}
)";
osg::ref_ptr<osg::Shader> vertShader=new osg::Shader(osg::Shader::VERTEX,vertCode);
osg::ref_ptr<osg::Shader> fragShader=new osg::Shader(osg::Shader::FRAGMENT,fragCode);
osg::ref_ptr<osg::Program> program=new osg::Program;
program->addShader(vertShader);
program->addShader(fragShader);
camera->getOrCreateStateSet()->setAttributeAndModes(program,OVERRIDE_ON);
return camera;
}
HUD相机用于将纹理混合结果输出到屏幕上，需要注意绑定一个矩形几何为子节点，并正确设置渲染目标，设置RenderOrder为POST_RENDER。
/*延迟HUD相机*/
osg::ref_ptr<RttCamera> createHUDCamera(osg::Texture* tex1,osg::Texture* tex2,int width,int height)
{
osg::ref_ptr<RttCamera> camera=new RttCamera(width,height);
camera->setClearMask(GL_DEPTH_BUFFER_BIT);
camera->setPostDrawCallBack(new FBOPostDrawCallback);
camera->setRenderOrder(osg::Camera::POST_RENDER,100);
camera->setReferenceFrame(osg::Transform::ABSOLUTE_RF)
camera->setProjectMatrix(osg::Matrix::ortho2D(width,-width,-height,height))
camera->setViewPort(0,0,width,height);
camera->addChild(createQuadGeode(tex1,tex2,width,height));
return camera;
}
/*平铺的四边形几何*/
osg::ref_ptr<osg::Geode> createQuadGeode(osg::Texture* baseTexture,osg::Texture* modelTexture,int width,int height)
{
///创建四边形顶点
osg::ref_ptr<osg:Vec3Array> vertices= new osg::Vec3Array;
vertices->push_back(osg::Vec3(-width,-height,0.f));
vertices->push_back(osg::Vec3(width,-height,0.f));
vertices->push_back(osg::Vec3(width,height,0.f));
vertices->push_back(osg::Vec3(width,-height,0.f));
///创建四边形法线
osg::ref_ptr<osg:Vec3Array> normals= new osg::Vec3Array;
normals->push_back(osg::Vec3(0.0,0.0,2.f));
///创建四边形纹理坐标
osg::ref_ptr<osg:Vec2Array> texCoords= new osg::Vec2Array;
texCoords->push_back(osg::Vec2(1.0,0.f));
texCoords->push_back(osg::Vec2(0.0,0.f));
texCoords->push_back(osg::Vec2(0.0,1.f));
texCoords->push_back(osg::Vec2(1.0,1.f));
///创建四边形几何
osg::ref_ptr<osg:Geometry> quad= new osg::Geometry;
quad->setVertexArray(vertices);
quad->setNormalArray(normals);
quad->setTexCoordArray(0,texCoords);
quad->addPrimitiveSet(new osg::DrawArrays(GL_QUADS,0,4));
///创建四边形节点
osg::ref_ptr<osg::Geode> quadGeode=new osg::Geode;
quadGeode->addDrawable(quad);
quadGeode->getOrCreateStateSet()->addUniform(new osg::Uniform("baseTexture",0)));
quadGeode->getOrCreateStateSet()->setTextureAttributeAndModes(0,baseTexture);
quadGeode->getOrCreateStateSet()->addUniform(new osg::Uniform("modelTexture",1)));
quadGeode->getOrCreateStateSet()->setTextureAttributeAndModes(1,modelTexture);
const char* vertCode=R"(
#version 330
layout(location = 0) in vec3 Position;
layout(location = 2) in vec3 normal;
layout(location = 3) in vec3 TexCoord;
uniform mat4 osg_ModelViewProjectionMatrix;
uniform mat4 osg_ModelViewMatrix;
uniform mat4 osg_NormalMatrix;
out vec2 texCoord;
void main()
{
texCoord=TexCoord;
gl_Position=osg_ModelViewProjectionMatrix*vec4(Position,1.0);
}
)";
const char* fragCode=R"(
uniform sampler2D baseTexture;
uniform sampler2D modelTexture;
in vec2 texCoord;
out vec4 fragColor;
void main()
{
vec4 modelCol=texture(modelTexture,texCoord);
vec4 baseCol=texture(baseTexture,texCoord);
fragColor=vec4(mix(modelCol.rgb,baseCol.rgb,baseCol.a),baseCol.a+modelCol.a);
}
)";
osg::ref_ptr<osg::Shader> vertShader=new osg::Shader(osg::Shader::VERTEX,vertCode);
osg::ref_ptr<osg::Shader> fragShader=new osg::Shader(osg::Shader::FRAGMENT,fragCode);
osg::ref_ptr<osg::Program> program=new osg::Program;
program->addShader(vertShader);
program->addShader(fragShader);
quadGeode->getOrCreateStateSet()->setAttributeAndModes(program,OVERRIDE_ON);
return quadGeode;
}