3D游戏引擎系列十二

笔者介绍：姜雪伟，IT公司技术合伙人，IT高级讲师，CSDN社区专家，特邀编辑，畅销书作者，国家专利发明人;已出版书籍：《手把手教你架构3D游戏引擎》电子工业出版社和《Unity3D实战核心技术详解》电子工业出版社等。

CSDN课程视频网址：http://edu.csdn.net/lecturer/144

对于3D游戏产品都需要阴影技术的实现，阴影的运行效率也成为判定游戏研发技术水平的手段之一。游戏中实现阴影的方式有很多种，主要分三种：一种是对于静态物体比如建筑物可以使用LightMap渲染，将建筑的阴影直接渲染到地面上这种技术广泛应用在移动端，Unity引擎本身就提供了此功能。另一种是对于游戏中动态的物体，实现方式是在移动端或者在网页游戏中为了优化效率，直接用一张带有Alpha通道的贴图放到角色的下面，可以实时跟随角色移动。第三种实现方式是该书重点讲解的实时阴影渲染，实时阴影在PC端游特别是次时代网游中很常见，鉴于PC端硬件的强大处理能力，应用实时阴影技术对整个游戏场景进行渲染，为的是增加游戏场景的真实性。当然实时阴影技术的运用会对CPU和GPU有一定的消耗，所以对于实时阴影的渲染，可以通过摒弃掉不需要实时渲染的建筑物进行效率优化。实时渲染技术常用的是PSSM（Parallel-Split ShadowMap）算法，实现阴影的算法非常多的，我就不一一列举了。PSSM通过字面意思知道就是平行切分视锥，游戏中实时阴影的渲染效果如下图：

要实现如此的效果，得从PSSM实现的原理讲起，PSSM算法的核心就是把视椎体进行分割，然后分别渲染组合。语言讲解不如看图直观，先通过视锥体分割说起。效果如下图：

PSSM实时阴影的绘制首先需要灯光，在现实生活中，白天只有太阳出来了才可以看到影子。在虚拟世界中也是一样的，场景使用的是Directional（平行光）相当于现实世界的太阳光。上图左边部分显示的是视景体的投影，利用PSSM算法将其平行的分割成多个部分，然后对每个部分进行渲染，分割成的块数是可以自己设置的。右半部分是顶视角观看的分割效果，把物体分成三块进行实时阴影的渲染。渲染的计算是GPU中执行的，在GPU中执行的流程如下图:

上图的处理流程首先是场景中的灯光照射到需要投影的物体上，接下来程序对投影的物体顶点进行矩阵变换将其转换到投影空间中，再转换到裁剪空间进行视口的平行分割，最后将其分别渲染出来。渲染阴影流程讲完了接下来解决Shader渲染的问题，我们把平行分割的计算放到GPU中执行，需要编写Shader脚本文件，新建一个文本文件把其扩展名字改成.fx。Shader的完整内容如下：

float4x4 g_mViewProj;void VS_RenderShadowMap(  float4 vPos : POSITION,  out float4 vPosOut : POSITION,  out float3 vPixelOut : TEXCOORD0){  // pass vertex position through as usual  vPosOut = mul(vPos, g_mViewProj);  // output pixel pos  vPixelOut=vPosOut.xyz;}float4 PS_RenderShadowMap(float3 vPixelPos : TEXCOORD0): COLOR{  // write z coordinate (linearized depth) to texture  return vPixelPos.z;}// This technique is used when rendering meshes to the shadowmap// technique RenderShadowMap{  pass p0  {    // render back faces to hide artifacts    CullMode = CW;    VertexShader = compile vs_2_0 VS_RenderShadowMap();    PixelShader = compile ps_2_0 PS_RenderShadowMap();  }}float3 g_vLightDir;float3 g_vLightColor;float3 g_vAmbient;float g_fShadowMapSize;float4x4 g_mShadowMap;// no filtering in floating point texturesampler2D g_samShadowMap  =sampler_state{  MinFilter = Point;  MagFilter = Point;  MipFilter = None;  AddressU = Border;  AddressV = Border;  BorderColor = 0xFFFFFFFF;};void VS_Shadowed(  in float4 vPos : POSITION,  in float3 vNormal : NORMAL,  in float fAmbientIn : TEXCOORD0,  out float4 vPosOut : POSITION,  out float4 vShadowTex : TEXCOORD0,  out float fAmbientOut : TEXCOORD1,  out float3 vDiffuse : COLOR0){  // pass vertex position through as usual  vPosOut = mul(vPos, g_mViewProj);  // calculate per vertex lighting  vDiffuse = g_vLightColor * saturate(dot(-g_vLightDir, vNormal));  // coordinates for shadowmap  vShadowTex = mul(vPos, g_mShadowMap);  // ambient occlusion  fAmbientOut = saturate(0.5f+fAmbientIn);}float4 PS_Shadowed(  float4 vShadowTex : TEXCOORD0,  float fAmbientOcclusion : TEXCOORD1,  float4 vDiffuse : COLOR0) : COLOR{  float fTexelSize=1.0f/g_fShadowMapSize;  // project texture coordinates  vShadowTex.xy/=vShadowTex.w;  // 2x2 PCF Filtering  //   float fShadow[4];  fShadow[0] = (vShadowTex.z < tex2D(g_samShadowMap, vShadowTex).r);  fShadow[1] = (vShadowTex.z < tex2D(g_samShadowMap, vShadowTex + float2(fTexelSize,0)).r);  fShadow[2] = (vShadowTex.z < tex2D(g_samShadowMap, vShadowTex + float2(0,fTexelSize)).r);  fShadow[3] = (vShadowTex.z < tex2D(g_samShadowMap, vShadowTex + float2(fTexelSize,fTexelSize)).r);  float2 vLerpFactor = frac(g_fShadowMapSize * vShadowTex);  float fLightingFactor = lerp(lerp( fShadow[0], fShadow[1], vLerpFactor.x ),                               lerp( fShadow[2], fShadow[3], vLerpFactor.x ),                               vLerpFactor.y);  // multiply diffuse with shadowmap lookup value  vDiffuse*=fLightingFactor;  // final color  float4 vColor=1;  vColor.rgb = saturate(g_vAmbient*fAmbientOcclusion + vDiffuse).rgb;  return vColor;}// This technique is used to render the final shadowed meshes//technique Shadowed{  pass p0  {    // render front facesCullMode = CCW;  VertexShader = compile vs_2_0 VS_Shadowed();  PixelShader = compile ps_2_0 PS_Shadowed();  }}

理论讲了很多，Shader代码实现起来比较简单，为了消除阴影锯齿，使用了PCF Filtering过滤技术。其他的代码跟以前讲的很类似这里就不一一分析了。接下来通过C++函数接口将参数传递给Shader文件，C++代码核心函数实现如下所示：

void RenderScene(D3DXMATRIX &mView, D3DXMATRIX &mProj){  // Set constants  //  D3DXMATRIX mViewProj=mView * mProj;  _pEffect->SetMatrix("g_mViewProj",&mViewProj);  _pEffect->SetVector("g_vLightDir",&_vLightDir);  _pEffect->SetVector("g_vLightColor",&_vLightDiffuse);  _pEffect->SetVector("g_vAmbient",&_vLightAmbient);  _pEffect->SetFloat("g_fShadowMapSize",(FLOAT)_iShadowMapSize);  // enable effect  unsigned int iPasses=0;  _pEffect->Begin(&iPasses,0);  // for each pass in effect   for(unsigned int i=0;i<iPasses;i++)  {    // start pass    _pEffect->BeginPass(i);    {      // for each subset in mesh      for(DWORD j=0;j<_iMeshMaterials;j++)      {        // draw subset        _pMesh->DrawSubset(j);      }    }    // end pass    _pEffect->EndPass();  }  // disable effect  _pEffect->End();}

该函数主要是将Shader文件中需要使用的参数通过C++代码传递给GPU进行渲染，在介绍PSSM原理时对物体进行Split操作。在C++中的函数如下所示：

void CalculateSplitDistances(void){  // Reallocate array in case the split count has changed  //  delete[] _pSplitDistances;  _pSplitDistances=new float[_iNumSplits+1];  _fSplitSchemeLambda=Clamp(_fSplitSchemeLambda,0.0f,1.0f);  for(int i=0;i<_iNumSplits;i++)  {    float fIDM=i/(float)_iNumSplits;    float fLog=_fCameraNear*powf((_fCameraFar/_fCameraNear),fIDM);    float fUniform=_fCameraNear+(_fCameraFar-_fCameraNear)*fIDM;    _pSplitDistances[i]=fLog*_fSplitSchemeLambda+fUniform*(1-_fSplitSchemeLambda);  }  // make sure border values are right  _pSplitDistances[0]=_fCameraNear;  _pSplitDistances[_iNumSplits]=_fCameraFar;}

最后将上述实现的两个关键函数在Render函数中调用，完成最终的代码实现。渲染函数如下所示：

void Render(void){  // move camera, adjust settings, etc..  DoControls();  // calculate the light position  _vLightSource=D3DXVECTOR3(-200*sinf(_fLightRotation),120,200*cosf(_fLightRotation));  _vLightTarget=D3DXVECTOR3(0,0,0);  // and direction  _vLightDir=D3DXVECTOR4(_vLightTarget-_vLightSource,0);  D3DXVec4Normalize(&_vLightDir,&_vLightDir);  // calculate camera aspect  D3DPRESENT_PARAMETERS pp=GetApp()->GetPresentParams();  float fCameraAspect=pp.BackBufferWidth/(float)pp.BackBufferHeight;  AdjustCameraPlanes();  CalculateSplitDistances();  // Clear the screen  //  GetApp()->GetDevice()->Clear(0, NULL, D3DCLEAR_TARGET|D3DCLEAR_ZBUFFER, D3DXCOLOR(0.5f,0.5f,0.5f,0.5f), 1.0f, 0);for(int iSplit=0;iSplit<_iNumSplits;iSplit++)  {    // use numpad to skip rendering    if(GetKeyDown(VK_NUMPAD1+iSplit)) continue;    // near and far planes for current frustum split    float fNear=_pSplitDistances[iSplit];    float fFar=_pSplitDistances[iSplit+1];    // Calculate corner points of frustum split    float fScale=1.1f;    D3DXVECTOR3 pCorners[8];    CalculateFrustumCorners(pCorners,_vCameraSource,_vCameraTarget,_vCameraUpVector,                            fNear,fFar,_fCameraFOV,fCameraAspect,fScale);    // Calculate view and projection matrices    CalculateLightForFrustum(pCorners);    // Enable rendering to shadowmap    _ShadowMapTexture.EnableRendering();    // Clear the shadowmap    GetApp()->GetDevice()->Clear(0, NULL, D3DCLEAR_TARGET|D3DCLEAR_ZBUFFER, 0xFFFFFFFF, 1.0f, 0);    // Set up shaders    // To hide artifacts, only render back faces of the scene    _pEffect->SetTechnique("RenderShadowMap");    // Render the scene to the shadowmap    RenderScene(_mLightView,_mLightProj);    // Go back to normal rendering    _ShadowMapTexture.DisableRendering();    /////////////////////////////////////////////////////////////    // At this point we have the shadowmap texture rendered.   //    /////////////////////////////////////////////////////////////    // Calculate a matrix to transform points to shadowmap texture coordinates    // (this should be exactly like in your standard shadowmap implementation)    float fTexOffset=0.5f+(0.5f/(float)_iShadowMapSize);    D3DXMATRIX mTexScale(   0.5f,               0.0f,      0.0f,   0.0f,                            0.0f,              -0.5f,      0.0f,   0.0f,                            0.0f,               0.0f,      1.0f,   0.0f,                            fTexOffset,    fTexOffset,     0.0f,   1.0f );    D3DXMATRIX mShadowMap=_mLightView * _mLightProj * mTexScale;    // store it to the shader    _pEffect->SetMatrix("g_mShadowMap",&mShadowMap);    // Since the near and far planes are different for each    // rendered split, we need to change the depth value range    // to avoid rendering over previous splits    D3DVIEWPORT9 CameraViewport;    GetApp()->GetDevice()->GetViewport(&CameraViewport);    // as long as ranges are in order and don't overlap it should be all good...    CameraViewport.MinZ=iSplit/(float)_iNumSplits;    CameraViewport.MaxZ=(iSplit+1)/(float)_iNumSplits;    GetApp()->GetDevice()->SetViewport(&CameraViewport);    // use the current splits near and far plane    // when calculating matrices for the camera    CalculateViewProj(_mCameraView, _mCameraProj,                      _vCameraSource,_vCameraTarget,_vCameraUpVector,                      _fCameraFOV, fNear, fFar, fCameraAspect);    // setup shaders    _pEffect->SetTechnique("Shadowed");    // bind shadowmap as a texture    GetApp()->GetDevice()->SetTexture(0,_ShadowMapTexture.GetColorTexture());    // render the final scene    RenderScene(_mCameraView, _mCameraProj);    // unbind texture so we can render on it again    GetApp()->GetDevice()->SetTexture(0,NULL);    // draw the shadowmap texture to HUD    RenderSplitOnHUD(iSplit);  }  // render other HUD stuff  RenderHUD();}

整个PSSM的核心代码就实现完成了，最后本书实现了9级平行分割对物体阴影的实现，实现效果如下: