osg 使用整理（6）：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<min_iteratrions) num_iterations = min_iteratrions;
    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<AlphaFuncValue) discard;
    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<min_iteratrions) num_iterations = min_iteratrions;
    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<min_iteratrions) num_iterations = min_iteratrions;
    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<color.w)
        {
            fragColor = color;
        }
        texcoord += deltaTexCoord;

        --num_iterations;
    }

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

    fragColor *= baseColor;
    if (fragColor.w<AlphaFuncValue) discard;
    FragColor = fragColor;
};

原文地址：https://www.cnblogs.com/wangxydela/p/17658509.html

osg 使用整理 （6）：体渲染效果

osg 使用整理 （6）：体渲染效果

1 光线投射法（ray casting)

1.1 投射方向

1.2 透明度合成公式

1.3 osg中光线投射法shader实现

osg 使用整理（6）：体渲染效果

osg 使用整理（6）：体渲染效果