osg 使用整理 （7）：体渲染效果

2025-10-30 17:23:44

osg 使用整理 （7）：体渲染效果

​ ​ 体渲染技术可用于医学成像、计算流体力学、有限元、地球物理学、遥感等领域，数据通常来源于CT扫描、核磁共振MRI、卫星成像和声纳等设备，大概分为三种：直接体渲染技术（光线投射法、抛雪球法、错切变形法）、间接体绘制技术和最大密度投影技术。

1 光线投射法（ray casting)

​ re:《GPU编程与CG语言之阳春白雪下里巴人》

​ 光线投射法是基于图像序列的直接体绘制算法。从图像的每一个像素，沿着视线方向发射一条光线，光线穿越整个图像序列，并在这个过程中，对图像序列进行采样获取颜色信息，同时依据光线吸收模型将颜色值进行累加，直至光线穿越整个图像序列，最后得到的颜色值就是渲染图像的颜色。

1.1 投射方向

​ 首先创建立方体几何，作为体纹理载体，体纹理通过三维纹理坐标和模型一一对应，然后由视点向模型上的点连射线，该射线穿越模型空间等价于射线穿越了体纹理，需要注意OpenGL和Direct3D使用的体纹理坐标并不相同。

​ 射线起始点为相机位置，终止于立方体背面点，射线方向由此求出。开启OpengGL正面剔除设置：

glEnable(GL_CULL_FACE);

glCullFace(GL_BACK);//GL_BACK 背面 GL_FRONT 正面 GL_FRONT_AND_BACK 正面和反面

​ 穿越体数据过程中做等距采样，采样密度由外界设置，根据视点和出射点计算投射距离，除以采样密度得到采样次数，这样在每一次采样循环过程中按照颜色合成公式进行反复累加。

1.2 透明度合成公式

​ 透明度代表光穿透物体的能力，如果穿透多个物体，则这种变化是累加的，称为alpha混合技术。如果有多个透明物体，通常需要对物体进行排序。从背面到前面进行采样合成的公式为：

\[C_i^Δ=(1-A_i)C_{i+1}^Δ+C_i\\

A_i^Δ=(1-A_i)C_{i+1}^Δ+A_i

\]

​ 直到透明度累加超过1或者采样距离大于投射距离或者采样次数超过设置最大采样次数时，结束颜色混合迭代。

1.3 osg中光线投射法shader实现

​ 顶点着色器代码：

#version 330

layout(location = 0) in vec3 Postion;

layout(location = 1) in vec4 Color;

layout(location = 8) in vec3 TexCoord;

uniform mat4 osg_ModelViewProjectionMatrix;

uniform mat4 osg_ViewMatrixInverse;

uniform mat4 toTextureMatrix;

out vec4 cameraPos;

out vec4 vertexPos;

out vec4 baseColor;

out vec3 lightDir;

void main(void)

{

gl_Position=osg_ModelViewProjectionMatrix*vec4(Postion,1.0);

cameraPos = toTextureMatrix*osg_ViewMatrixInverse * vec4(0,0,0,1);

vertexPos=vec4(TexCoord,1.0);

baseColor=Color;

vec4 lightPosition = osg_ViewMatrixInverse * gl_LightSource[0].position;

if (lightPosition[3]==0.0)

{

lightDirection = -normalize(lightPosition.xyz);

}

else

{

lightDirection = normalize((lightPosition-vertexPos).xyz);

}

}

​ 片段着色器代码 （带颜色转移函数）

#version 330

uniform sampler3D baseTexture;

uniform sampler1D tfTexture;

uniform float tfScale;

uniform float tfOffset;

uniform float SampleDensityValue;

uniform float TransparencyValue;

uniform float AlphaFuncValue;

in vec4 cameraPos;

in vec4 vertexPos;

in vec4 baseColor;

out vec4 FragColor;

void main(void)

{

vec4 t0 = vertexPos;

vec4 te = cameraPos;

if (te.x>=0.0 && te.x<=1.0 &&

te.y>=0.0 && te.y<=1.0 &&

te.z>=0.0 && te.z<=1.0)

{

// do nothing... te inside volume

}

else

{

if (te.x<0.0)

{

float r = -te.x / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.x>1.0)

{

float r = (1.0-te.x) / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.y<0.0)

{

float r = -te.y / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.y>1.0)

{\n"

float r = (1.0-te.y) / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.z<0.0)

{

float r = -te.z / (t0.z-te.z);

te = te + (t0-te)*r;

}

if (te.z>1.0)

{

float r = (1.0-te.z) / (t0.z-te.z);

te = te + (t0-te)*r;

}

}

const float min_iteratrions = 2.0;

const float max_iteratrions = 2048.0;

float num_iterations = ceil(length((te-t0).xyz)/SampleDensityValue);

if (num_iterations

else if (num_iterations>max_iteratrions) num_iterations = max_iteratrions;

vec3 deltaTexCoord=(t0-te).xyz/float(num_iterations-1.0);

vec3 texcoord = te.xyz;

vec4 fragColor = vec4(0.0, 0.0, 0.0, 0.0);

while(num_iterations>0.0)

{

float v = texture( baseTexture, texcoord).a * tfScale + tfOffset;

vec4 color = texture1D( tfTexture, v);

float r = color[3]*TransparencyValue;

if (r>AlphaFuncValue)

{

fragColor.xyz = fragColor.xyz*(1.0-r)+color.xyz*r;

fragColor.w += r;

}

if(fragColor.w>1.0)break;

texcoord += deltaTexCoord;

--num_iterations;

}

fragColor.w *= TransparencyValue;

if (fragColor.w>1.0) fragColor.w = 1.0;

fragColor *= baseColor;

if (fragColor.w

FragColor = fragColor;

};

​ iso等值面片段着色器

#version 330

uniform sampler3D baseTexture;

uniform sampler1D tfTexture;

uniform float tfScale;

uniform float tfOffset;

uniform float SampleDensityValue;

uniform float TransparencyValue;

uniform float IsoSurfaceValue;

in vec4 cameraPos;

in vec4 vertexPos;

in vec4 baseColor;

in vec3 lightDirection;

out vec4 FragColor;

void main(void)

{

vec4 t0 = vertexPos;

vec4 te = cameraPos;

if (te.x>=0.0 && te.x<=1.0 &&

te.y>=0.0 && te.y<=1.0 &&

te.z>=0.0 && te.z<=1.0)

{

// do nothing... te inside volume

}

else

{

if (te.x<0.0)

{

float r = -te.x / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.x>1.0)

{

float r = (1.0-te.x) / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.y<0.0)

{

float r = -te.y / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.y>1.0)

{\n"

float r = (1.0-te.y) / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.z<0.0)

{

float r = -te.z / (t0.z-te.z);

te = te + (t0-te)*r;

}

if (te.z>1.0)

{

float r = (1.0-te.z) / (t0.z-te.z);

te = te + (t0-te)*r;

}

}

const float min_iteratrions = 2.0;

const float max_iteratrions = 2048.0;

float num_iterations = ceil(length((te-t0).xyz)/SampleDensityValue);

if (num_iterations

else if (num_iterations>max_iteratrions) num_iterations = max_iteratrions;

vec3 deltaTexCoord=(t0-te).xyz/float(num_iterations-1.0);

vec3 texcoord = te.xyz;

float previousV = texture( baseTexture, texcoord).a;

float normalSampleDistance = 1.0/512.0;

vec3 deltaX = vec3(normalSampleDistance, 0.0, 0.0);

vec3 deltaY = vec3(0.0, normalSampleDistance, 0.0);

vec3 deltaZ = vec3(0.0, 0.0, normalSampleDistance);

vec4 fragColor = vec4(0.0, 0.0, 0.0, 0.0);

while(num_iterations>0.0)

{

float v = texture3D( baseTexture, texcoord).a;

float m = (previousV-IsoSurfaceValue) * (v-IsoSurfaceValue);

if (m <= 0.0)

{

float r = (IsoSurfaceValue-v)/(previousV-v);

texcoord = texcoord - r*deltaTexCoord;

v = texture( baseTexture, texcoord).a * tfScale + tfOffset;

vec4 color = texture( tfTexture, v);

float px = texture( baseTexture, texcoord + deltaX).a;

float py = texture( baseTexture, texcoord + deltaY).a;

float pz = texture( baseTexture, texcoord + deltaZ).a;

float nx = texture( baseTexture, texcoord - deltaX).a;

float ny = texture( baseTexture, texcoord - deltaY).a;

float nz = texture( baseTexture, texcoord - deltaZ).a;

vec3 grad = vec3(px-nx, py-ny, pz-nz);

if (grad.x!=0.0 || grad.y!=0.0 || grad.z!=0.0)

{

vec3 normal = normalize(grad);

float lightScale = 0.1 + max(0.0, dot(normal.xyz, lightDirection))*0.9;

color.x *= lightScale;

color.y *= lightScale;

color.z *= lightScale;

}

color *= baseColor;

FragColor = color;

return ;

}

previousV=v;

texcoord += deltaTexCoord;

--num_iterations;

}

discard;

};

​ light光照片段着色器

#version 330

uniform sampler3D baseTexture;

uniform sampler1D tfTexture;

uniform float tfScale;

uniform float tfOffset;

uniform float SampleDensityValue;

uniform float TransparencyValue;

uniform float AlphaFuncValue;

in vec4 cameraPos;

in vec4 vertexPos;

in vec4 baseColor;

in vec3 lightDirection;

out vec4 FragColor;

void main(void)

{

vec4 t0 = vertexPos;

vec4 te = cameraPos;

if (te.x>=0.0 && te.x<=1.0 &&

te.y>=0.0 && te.y<=1.0 &&

te.z>=0.0 && te.z<=1.0)

{

// do nothing... te inside volume

}

else

{

if (te.x<0.0)

{

float r = -te.x / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.x>1.0)

{

float r = (1.0-te.x) / (t0.x-te.x);

te = te + (t0-te)*r;

}

if (te.y<0.0)

{

float r = -te.y / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.y>1.0)

{\n"

float r = (1.0-te.y) / (t0.y-te.y);

te = te + (t0-te)*r;

}

if (te.z<0.0)

{

float r = -te.z / (t0.z-te.z);

te = te + (t0-te)*r;

}

if (te.z>1.0)

{

float r = (1.0-te.z) / (t0.z-te.z);

te = te + (t0-te)*r;

}

}

const float min_iteratrions = 2.0;

const float max_iteratrions = 2048.0;

float num_iterations = ceil(length((te-t0).xyz)/SampleDensityValue);

if (num_iterations

else if (num_iterations>max_iteratrions) num_iterations = max_iteratrions;

vec3 deltaTexCoord=(t0-te).xyz/float(num_iterations-1.0);

vec3 texcoord = te.xyz;

float normalSampleDistance = 1.0/512.0;

vec3 deltaX = vec3(normalSampleDistance, 0.0, 0.0);

vec3 deltaY = vec3(0.0, normalSampleDistance, 0.0);

vec3 deltaZ = vec3(0.0, 0.0, normalSampleDistance);

vec4 fragColor = vec4(0.0, 0.0, 0.0, 0.0);

while(num_iterations>0.0)

{

float v = texture( baseTexture, texcoord).a * tfScale + tfOffset;

vec4 color = texture1D( tfTexture, v);

float a=v;

float px = texture( baseTexture, texcoord + deltaX).a;

float py = texture( baseTexture, texcoord + deltaY).a;

float pz = texture( baseTexture, texcoord + deltaZ).a;

float nx = texture( baseTexture, texcoord - deltaX).a;

float ny = texture( baseTexture, texcoord - deltaY).a;

float nz = texture( baseTexture, texcoord - deltaZ).a;

vec3 grad = vec3(px-nx, py-ny, pz-nz);

if (grad.x!=0.0 || grad.y!=0.0 || grad.z!=0.0)

{

vec3 normal = normalize(grad);

float lightScale = 0.1 + max(0.0, dot(normal.xyz, lightDirection))*0.9;

color.x *= lightScale;

color.y *= lightScale;

color.z *= lightScale;

}

float r = color[3]*TransparencyValue;

if (r>AlphaFuncValue)

{

fragColor.xyz = fragColor.xyz*(1.0-r)+color.xyz*r;

fragColor.w += r;

}

if (fragColor.w

{

fragColor = color;

}

texcoord += deltaTexCoord;

--num_iterations;

}

fragColor.w *= TransparencyValue;

if (fragColor.w>1.0) fragColor.w = 1.0;

fragColor *= baseColor;

if (fragColor.w

FragColor = fragColor;

};