阴影算法

图中有两条光线，一条沿视角方向，另一条偏右

采用阴影体算法，实现了多光源，多物体的阴影。

这个算法没有采用顶点shader，所以速度比较慢，学习算法用，在实际工程中用效率就太低了。。。

算法还有个缺陷，只对凸多面体有效，如果物体是凹多面体阴影就会错误。因为我不知道如何确定凹多面体的轮廓，从而构造一个封闭的阴影体。非常希望有牛人能指点一二！

主要代码如下

计算轮廓

/* 计算轮廓（silhouette edge），采用的算法，伪代码如下 for 三角形tri in 所有三角形 如果tri是前向面 for 边e1 in 三角形tri的三条边 如果e1在轮廓列表中，从轮廓列表中移除e1 否则把e1加入轮廓列表中 默认物体是凸多面体。。。 对凹多面体，如何用算出来的轮廓构造阴影体，至今还不知道，希望有前辈能提点提点。*/void ShadowVolume::Build(const Model &model, const Vector3 &lightPos) {m_lightPos = Vector3(lightPos);m_silhouetteEdges.clear();m_silhouettePoints.clear();for(int j = 0; j < model.m_indexNum; j += 3) {Vector3 v1 = model.m_vbuf[model.m_ibuf[j]];Vector3 v2 = model.m_vbuf[model.m_ibuf[j + 1]];Vector3 v3 = model.m_vbuf[model.m_ibuf[j + 2]];Vector3 n = (v2 - v1).crossProduct(v3 - v1);n.normalise();if(n.dotProduct(lightPos - v1) > EPSILON) {// 注意此处要减去v1，才是正确的可见面判别。否则在离物体很近的时候会出现误差AddEdge(SilEdge(v1, v2));AddEdge(SilEdge(v2, v3));AddEdge(SilEdge(v3, v1));}}list<SilEdge>::iterator it = m_silhouetteEdges.begin();m_silhouettePoints.push_back((*it).edges[0]);m_silhouettePoints.push_back((*it).edges[1]);m_silhouetteEdges.erase(it);bool erased = true;// 把轮廓边缘的点提取出来，构造出多边形的点集合。下面的方法很糟糕，可是我想不到更好的办法了。。while(m_silhouetteEdges.size() != 0 && erased) {erased = false;for(it = m_silhouetteEdges.begin();it != m_silhouetteEdges.end(); it++) {SilEdge &edge = *it;if(edge.edges[0] == m_silhouettePoints[m_silhouettePoints.size() - 1]) {if(edge.edges[1] != m_silhouettePoints[0]) {m_silhouettePoints.push_back(edge.edges[1]);}m_silhouetteEdges.erase(it);erased = true;break;}if(edge.edges[1] == m_silhouettePoints[m_silhouettePoints.size() - 1]) {if(edge.edges[0] != m_silhouettePoints[0]) {m_silhouettePoints.push_back(edge.edges[0]);}m_silhouetteEdges.erase(it);erased = true;break;}if(edge.edges[1] == m_silhouettePoints[0] || edge.edges[0] == m_silhouettePoints[0]) {m_silhouetteEdges.erase(it);erased = true;break;}}}}

绘制阴影体

void DrawVolume() {m_infinitePoints.clear();for(size_t i = 0; i < m_silhouettePoints.size(); i++) {Vector3 a = m_silhouettePoints[i] - m_lightPos;Vector3 infinite = a;infinite.normalise();infinite *= 100;infinite += a;m_infinitePoints.push_back(infinite);}glBegin(GL_POLYGON);for(int i = m_infinitePoints.size() - 1; i >= 0; i--) {glVertex3f(m_infinitePoints[i].x, m_infinitePoints[i].y, m_infinitePoints[i].z);}glEnd();glBegin(GL_POLYGON);for(size_t i = 0; i < m_silhouettePoints.size(); i++) {glVertex3f(m_silhouettePoints[i].x, m_silhouettePoints[i].y, m_silhouettePoints[i].z);}glEnd();glBegin(GL_QUADS);int n = m_silhouettePoints.size();for(int i = 0; i < n; i++) {glVertex3f(m_silhouettePoints[(i+1) % n].x, m_silhouettePoints[(i+1) % n].y, m_silhouettePoints[(i+1) % n].z);glVertex3f(m_silhouettePoints[i].x, m_silhouettePoints[i].y, m_silhouettePoints[i].z);glVertex3f(m_infinitePoints[i].x, m_infinitePoints[i].y, m_infinitePoints[i].z);glVertex3f(m_infinitePoints[(i+1) % n].x, m_infinitePoints[(i+1) % n].y, m_infinitePoints[(i+1) % n].z);}glEnd();}

用蒙版缓存绘制阴影

// 用蒙版缓存(stencil buffer)的方法绘制阴影。void Render() {// 绘制没有光线时的场景for(size_t k = 0; k < mLight.size(); k++) {glDisable(GL_LIGHT0 + k);}glBlendFunc(GL_ONE, GL_ONE);drawWall();for(size_t i = 0; i < mObjects.size(); i++) {mObjects[i]->Render();}//处理每条光线for(size_t k = 0; k < mLight.size(); k++) {glClear(GL_STENCIL_BUFFER_BIT);// 计算每一个物体阴影体for(size_t i = 0; i < mObjects.size(); i++) {mObjects[i]->BuildShadow(mLight[k]->position);}glDepthFunc(GL_LESS);// 绘制阴影体，采用卡马克的depth fail算法glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);glDepthMask(GL_FALSE);glEnable(GL_CULL_FACE);glEnable(GL_STENCIL_TEST);glEnable(GL_POLYGON_OFFSET_FILL);// 对重叠表面的处理glPolygonOffset(0.0f, 100.0f);glCullFace(GL_FRONT);glStencilFunc(GL_ALWAYS, 0x0, 0xff);glStencilOp(GL_KEEP, GL_INCR, GL_KEEP);for(size_t i = 0; i < mObjects.size(); i++) {mObjects[i]->RenderShadow();}glCullFace(GL_BACK);glStencilFunc(GL_ALWAYS, 0x0, 0xff);glStencilOp(GL_KEEP, GL_DECR, GL_KEEP);for(size_t i = 0; i < mObjects.size(); i++) {mObjects[i]->RenderShadow();}glDepthFunc(GL_LEQUAL);glDisable(GL_CULL_FACE);glDisable(GL_POLYGON_OFFSET_FILL);// 绘制整个场景，如果蒙版缓存的值为1，说明有阴影，不绘制glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);glDepthMask(GL_TRUE);glStencilFunc(GL_EQUAL, 0x0, 0xff);glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);// 开启当前光线的光照效果glEnable(GL_BLEND);glBlendFunc(GL_ONE, GL_ONE);glEnable(GL_LIGHT0 + k);for(size_t i = 0; i < mObjects.size(); i++) {mObjects[i]->Render();}drawWall();glDisable(GL_LIGHT0 + k);glDisable(GL_BLEND);}}

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