OpenGL: 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::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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