GLSL-双面渲染技术(Two-sided rendering)

来源：互联网发布：eclipse java mars 编辑：程序博客网时间：2024/04/30 22:24

http://blog.csdn.net/xiajun07061225/article/details/7729871

当我们渲染一个完全封闭的物体的时候，多边形的背面都是隐藏的，我们无法看见。但是如果这个物体有一些空洞，那么一些背面就可以被看见了。然而因为这些多边形的法向量的方向在这个时候不是正确的，所以会渲染出错误的结果。为了合理的渲染这些背面，我们必须反转法向量，然后基于这些反转的法向量来计算光照。

下图显示了一个采用Phong光照模型绘制茶壶使用双面渲染与否的结果对比：

图一使用双面渲染与不使用的效果对比

一、固定管线OpenGL实现双面渲染

在固定管线中，我们需要用一个函数来启用双面光照：

glLightModeli(GL_LIGHT_MODEL_TWO_SIDE,GL_TRUE);

这样，OpenGL将会反转表面的法线的方向，表示背面多边形的法线。

实例：

[cpp] view plaincopyprint?

void GLWidget::initLight()
{
GLfloat light_ambient[]={0.7,0.8,0.9,1.0};
GLfloat light_diffuse[]={1.0,1.0,1.0,1.0};
GLfloat light_specular[]={1.0,1.0,1.0,1.0};
GLfloat light_position[]={2.0,2.0,2.0,0.0};
GLfloat mat_diffuse[]={0.8,0.8,0.8,1.0};
glLightfv(GL_LIGHT0,GL_AMBIENT,light_ambient);
glLightfv(GL_LIGHT0,GL_DIFFUSE,light_diffuse);
glLightfv(GL_LIGHT0,GL_SPECULAR,light_specular);
glLightfv(GL_LIGHT0,GL_POSITION,light_position);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glEnable(GL_AUTO_NORMAL);
glEnable(GL_NORMALIZE);
//glEnable(GL_CULL_FACE);
//启用双面光照
glLightModeli(GL_LIGHT_MODEL_TWO_SIDE,GL_TRUE);
glMaterialfv(GL_FRONT_AND_BACK,GL_DIFFUSE,mat_diffuse);
}

void GLWidget::initLight(){GLfloat light_ambient[]={0.7,0.8,0.9,1.0};   GLfloat light_diffuse[]={1.0,1.0,1.0,1.0};   GLfloat light_specular[]={1.0,1.0,1.0,1.0};   GLfloat light_position[]={2.0,2.0,2.0,0.0};GLfloat mat_diffuse[]={0.8,0.8,0.8,1.0}; glLightfv(GL_LIGHT0,GL_AMBIENT,light_ambient);   glLightfv(GL_LIGHT0,GL_DIFFUSE,light_diffuse);   glLightfv(GL_LIGHT0,GL_SPECULAR,light_specular);   glLightfv(GL_LIGHT0,GL_POSITION,light_position); glEnable(GL_LIGHTING);glEnable(GL_LIGHT0);glEnable(GL_AUTO_NORMAL);glEnable(GL_NORMALIZE);//glEnable(GL_CULL_FACE);//启用双面光照glLightModeli(GL_LIGHT_MODEL_TWO_SIDE,GL_TRUE);glMaterialfv(GL_FRONT_AND_BACK,GL_DIFFUSE,mat_diffuse);  }

效果：

二、可编程管线OpenGL实现双面渲染

假设我们的光照（采用Phong光照模型）计算是基于顶点的。在顶点着色器中，我们需要为每个顶点分别计算两次：front face和back face。两次计算采用的法向量不同。

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#version 400
layout (location = 0) in vec3 VertexPosition;
layout (location = 1) in vec3 VertexNormal;
out vec3 FrontColor;
out vec3 BackColor;
struct LightInfo{
vec4 Position;
vec3 La;
vec3 Ld;
vec3 Ls;
};
uniform LightInfo Light;
struct MaterialInfo{
vec3 Ka;
vec3 Kd;
vec3 Ks;
float Shininess;
};
uniform MaterialInfo Material;
uniform mat4 ModelViewMatrix;
uniform mat3 NormalMatrix;
uniform mat4 ProjectionMatrix;
uniform mat4 MVP;
//转化到视空间
void getEyeSpace(out vec3 tnorm,out vec4 eyeCoords)
{
//convert normal and position to eye coords
tnorm = normalize(NormalMatrix * VertexNormal);
eyeCoords = ModelViewMatrix * vec4(VertexPosition,1.0);
}
//计算Phong光照模型
vec3 phongModel(vec4 eyeCoords,vec3 tnorm)
{
vec3 s = normalize(vec3(Light.Position - eyeCoords));
vec3 v = normalize(-eyeCoords.xyz);
vec3 r = reflect(-s,tnorm);
vec3 ambient = Light.La * Material.Ka;
float sDotN = max(dot(s,tnorm),0.0);
vec3 diffuse = Light.Ld * Material.Kd * sDotN;
vec3 specular = vec3(0.0);
if(sDotN > 0)
specular = Light.Ls * Material.Ks *
pow(max(dot(r,v),0.0),Material.Shininess);
return (ambient + diffuse + specular);
}
void main()
{
vec3 tnorm;
vec4 eyeCoords;
getEyeSpace(tnorm,eyeCoords);
FrontColor = phongModel(eyeCoords,tnorm);
BackColor = phongModel(eyeCoords,-tnorm);
gl_Position = MVP * vec4(VertexPosition,1.0);
}

#version 400layout (location = 0) in vec3 VertexPosition;layout (location = 1) in vec3 VertexNormal;out vec3 FrontColor;out vec3 BackColor;struct LightInfo{vec4 Position;vec3 La;vec3 Ld;vec3 Ls;};uniform LightInfo Light;struct MaterialInfo{vec3 Ka;vec3 Kd;vec3 Ks;float Shininess;};uniform MaterialInfo Material;uniform mat4 ModelViewMatrix;uniform mat3 NormalMatrix;uniform mat4 ProjectionMatrix;uniform mat4 MVP;//转化到视空间void getEyeSpace(out vec3 tnorm,out vec4 eyeCoords){//convert normal and position to eye coordstnorm = normalize(NormalMatrix * VertexNormal);eyeCoords = ModelViewMatrix * vec4(VertexPosition,1.0);}//计算Phong光照模型vec3 phongModel(vec4 eyeCoords,vec3 tnorm){vec3 s = normalize(vec3(Light.Position - eyeCoords));vec3 v = normalize(-eyeCoords.xyz);vec3 r = reflect(-s,tnorm);vec3 ambient = Light.La * Material.Ka;float sDotN = max(dot(s,tnorm),0.0);vec3 diffuse = Light.Ld * Material.Kd * sDotN;vec3 specular = vec3(0.0);if(sDotN > 0)specular = Light.Ls * Material.Ks * pow(max(dot(r,v),0.0),Material.Shininess);return (ambient + diffuse + specular);}void main(){vec3 tnorm;vec4 eyeCoords;getEyeSpace(tnorm,eyeCoords);FrontColor = phongModel(eyeCoords,tnorm);BackColor = phongModel(eyeCoords,-tnorm);gl_Position = MVP * vec4(VertexPosition,1.0);}

在片断着色器中，我们根据关键字gl_FrontFacing的值来为片断赋予颜色值。

gl_FrontFacing是GLSL内置的关键字，它代表图元是面向光源还是背向光源。

片断着色器 twosided.frag：

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#version 400
in vec3 FrontColor;
in vec3 BackColor;
layout (location = 0) out vec4 FragColor;
void main()
{
if(gl_FrontFacing)
{
FragColor = vec4(FrontColor,1.0);
}
else
{
FragColor = vec4(BackColor,1.0);
}
}

#version 400in vec3 FrontColor;in vec3 BackColor;layout (location = 0) out vec4 FragColor;void main(){if(gl_FrontFacing){FragColor = vec4(FrontColor,1.0);}else{FragColor = vec4(BackColor,1.0);}}