【Ray Tracing The Next Week 超详解】 光线追踪2-8 Volume

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 Preface

今天有两个东东,一个是体积烟雾,一个是封面图

下一篇我们总结项目代码

 

Chapter 8:Volumes

我们需要为我们的光线追踪器添加新的物体——烟、雾,也称为participating media。 我们还需要补充一个材质——次表面散射材质,它有点像物体内的浓雾。

体渲染通常的做法是,在体的内部有很多随机表面,来实现散射的效果。比如一束烟可以表示为,在这束烟的内部任意位置,都可以存在一个面,以此来实现烟、雾

 

对于一个恒定密度体,一条光线通过其中的时候,在烟雾体中传播的时候也会发生散射,光线在烟雾体中能传播多远,也是由烟雾体的密度决定的,密度越高,光线穿透性越差,光线传播的距离也越短。从而实现烟雾的透光性。

引用书中一张图(光线可穿透可散射)

【Ray Tracing The Next Week 超详解】 光线追踪2-8 Volume

当光线通过体积时,它可能在任何点散射。 光线在任何小距离dL中散射的概率为:

概率= C * dL,其中C与体积的光密度成比例。

对于恒定体积,我们只需要密度C和边界。 

 

/// isotropic.hpp

// -----------------------------------------------------
// [author]        lv
// [begin ]        2019.1
// [brief ]        the isotropic-class for the ray-tracing project
//                from the 《ray tracing the next week》
// -----------------------------------------------------

#pragma once


namespace rt
{

class isotropic :public material
    {
public:
    isotropic(texture* tex) :_albedo(tex) {  }

    virtual bool scatter(const ray& InRay, const hitInfo& info, rtvec& attenuation, ray& scattered)const override
        {
        scattered = ray(info._p, lvgm::random_unit_sphere());
        attenuation = _albedo->value(info._u, info._v, info._p);
        return true;
        }

private:
    texture * _albedo;
    };

} // rt namespace

 

这个材质的散射原理和漫反射磨砂材质的大同小异,均属于碰撞点转换为新视点,沿任意方向发射新的视线,只不过漫反射的视线方向向量指向外相切球体表面,而isotropic的视线方向指向以碰撞点为球心的单位球体表面

区别就在于

漫反射的散射光线不可能指到物体内部,它一定是散射到表面外部(视线方向指向外切球体表面)

isotropic材质的散射光线可以沿原来的方向一往前,以此视线透光性

因为烟雾内部只是颗粒而不存在真正不可穿透的几何实体,所以漫反射实体不可穿透,只能散射到表面外部,而烟雾可穿透

 

接下来我们看一下烟雾体

/// constant_medium.hpp

// -----------------------------------------------------
// [author]        lv
// [begin ]        2019.1
// [brief ]        the constant_dedium-class for the ray-tracing project
//                from the 《ray tracing the next week》
// -----------------------------------------------------


#pragma once

namespace rt
{

class constant_medium :public intersect
    {
public:
    constant_medium(intersect* p, rtvar d, texture* tex);

    virtual bool hit(const ray& sight, rtvar t_min, rtvar t_max, hitInfo& info)const override;

    virtual aabb getbox()const override;

private:
    intersect* _item;

    rtvar _density;    //烟雾密度

    material* _materialp;
    };



inline constant_medium::constant_medium(intersect* p, rtvar d, texture* tex)
    :_item(p)
    ,_density(d)
    ,_materialp(new isotropic(tex))
    {
    }

aabb constant_medium::getbox()const
    {
    return _item->getbox();
    }

bool constant_medium::hit(const ray& sight, rtvar t_min, rtvar t_max, hitInfo& info)const 
    {
    hitInfo info1, info2;
    if (_item->hit(sight, -rtInf(), rtInf(), info1)) {
        if (_item->hit(sight, info1._t + 0.0001, rtInf(), info2)) {
            if (info1._t < t_min)
                info1._t = t_min;
            if (info2._t > t_max)
                info2._t = t_max;
            if (info1._t >= info2._t)
                return false;
            if (info1._t < 0)
                info1._t = 0;
            float distance_inside_boundary = (info2._t - info1._t)*sight.direction().normal();
            float hit_distance = -(1 / _density)*log(lvgm::rand01());
            if (hit_distance < distance_inside_boundary) {
                info._t = info1._t + hit_distance / sight.direction().normal();
                info._p = sight.go(info._t);
                info._n = rtvec(1, 0, 0);  // arbitrary
                info._materialp = _materialp;
                return true;
                }
            }
        }
    return false;
    }

} // rt namespace

 

 

hit函数里面是一些边界合法性检测

 

场景测试代码

intersect* cornell_smoke()
{
    intersect ** list = new intersect*[9];

    int cnt = 0;
    material* red = new lambertian(new constant_texture(rtvec(0.65, 0.05, 0.05)));
    material * blue = new lambertian(new constant_texture(rtvec(0.05, 0.05, 0.73)));
    material* white = new lambertian(new constant_texture(rtvec(0.73, 0.73, 0.73)));
    material* green = new lambertian(new constant_texture(rtvec(0.12, 0.45, 0.15)));
    material* light = new areaLight(new constant_texture(rtvec(7, 7, 7)));

    list[cnt++] = new xz_rect(113, 443, 127, 432, 550, light);
    list[cnt++] = new flip_normal(new xz_rect(113, 443, 127, 432, 550, light));
    list[cnt++] = new flip_normal(new yz_rect(0, 555, 0, 555, 555, green));
    list[cnt++] = new yz_rect(0, 555, 0, 555, 0, red);
    list[cnt++] = new flip_normal(new xz_rect(0, 555, 0, 555, 555, white));
    list[cnt++] = new xz_rect(0, 555, 0, 555, 0, white);
    list[cnt++] = new flip_normal(new xy_rect(0, 555, 0, 555, 555, blue));

    intersect* box1 = new translate(new rotate_y(new box(rtvec(), rtvec(165, 165, 165), white), -18), rtvec(130, 0, 65));
    intersect* box2 = new translate(new rotate_y(new box(rtvec(), rtvec(165, 320, 165), white), 15), rtvec(265, 0, 295));

    list[cnt++] = new constant_medium(box2, 0.006, new constant_texture(rtvec(0.8, 0.58, 0.)));
    list[cnt++] = new constant_medium(box1, 0.008, new constant_texture(rtvec(0.9, 0.2, 0.72)));

    return new intersections(list, cnt);
}

 

下面是效果:sample -> 1000

【Ray Tracing The Next Week 超详解】 光线追踪2-8 Volume

 

注释 // arbitrary处为随机方向,之前为(1,0,0)

我觉得改为(rand01(),rand01(),rand01())较为合适,下面是改之后的效果

【Ray Tracing The Next Week 超详解】 光线追踪2-8 Volume

 

Chapter 9:A Scene Testing All New Features

最后的封面图是这样一个场景:

体积雾:有一个蓝色的次表面散射球体,但是这个东西并没有单独实现,而是把它包裹在了一个玻璃球内。

体积雾:整个场景是由层薄薄的雾气笼盖着的

长方体:地面是一堆随机长方体

玻璃球:一个作为蓝色烟雾的容器,一个是单纯的玻璃球

映射纹理:地球纹理球体

过程纹理:柏林噪声纹理球体

运动模糊球体:有一个棕色的运动球体

镜面球体:银白色的镜面球

区域光照:顶部是一个长方形的区域光源

其他未说明材质的都是漫反射

渲染器中剩下的最大缺陷是没有阴影射线,但这就是为什么我们容易获得焦散和次表面散射效果的原因。

 

下面是渲染代码,关于相机参数设置还需等待渲染结果出来才能公布

VS四开,渲染了一天还没完,我也实属无奈

 

intersect* finalScene()
{
    int nb = 20;
    intersect ** list = new intersect*[30];
    intersect ** boxlist = new intersect*[2000];
    intersect ** boxlist2 = new intersect*[2000];
    
    material * white = new lambertian(new constant_texture(rtvec(0.73, 0.73, 0.73)));
    material * ground = new lambertian(new constant_texture(rtvec(0.48, 0.83, 0.53)));

    int b = 0;
    for (int i = 0; i < nb; ++i)
        for (int j = 0; j < nb; ++j)
        {
            rtvar w = 100;
            rtvar x0 = -1000 + i*w;
            rtvar z0 = -1000 + j*w;
            rtvar y0 = 0;
            rtvar x1 = x0 + w;
            rtvar y1 = 100 * (lvgm::rand01() + 0.01);
            rtvar z1 = z0 + w;
            boxlist[b++] = new box(rtvec(x0, y0, z0), rtvec(x1, y1, z1), ground);
        }

    int l = 0;
    list[l++] = new bvh_node(boxlist, b, 0, 1);
    material * light = new areaLight(new constant_texture(rtvec(10, 10, 10)));
    list[l++] = new xz_rect(123, 423, 147, 412, 550, light);
    rtvec center(400, 400, 200);
    list[l++] = new moving_sphere(center, center + rtvec(30, 0, 0), 0, 1, 50, new lambertian(new constant_texture(rtvec(0.7, 0.3, 0.1))));
    list[l++] = new sphere(rtvec(260, 150, 45), 50, new dielectric(1.5));
    list[l++] = new sphere(rtvec(0, 150, 145), 50, new metal(new constant_texture(rtvec(0.8, 0.8, 0.9)), 10.0));
        
    intersect * boundary = new sphere(rtvec(360, 150, 145), 70, new dielectric(1.5));
    list[l++] = boundary;
    list[l++] = new constant_medium(boundary, 0.2, new constant_texture(rtvec(0.2, 0.4, 0.9)));
    boundary = new sphere(rtvec(), 5000, new dielectric(1.5));
    list[l++] = new constant_medium(boundary, 0.0011, new constant_texture(rtvec(1., 1., 1.)));
    
    int x, y, n;
    unsigned char * tex = stbi_load("earthmap.jpg", &x, &y, &n, 0);
    material * emat = new lambertian(new image_texture(tex, x, y));
    list[l++] = new sphere(rtvec(400, 200, 400), 100, emat);
    texture * pertext = new noise_texture(0.1);
    list[l++] = new sphere(rtvec(220, 280, 300), 80, new lambertian(pertext));
    int ns = 1000;
    for (int j = 0; j < ns; ++j)
        boxlist2[j] = new sphere(rtvec(165 * lvgm::rand01(), 165 * lvgm::rand01(), lvgm::rand01()), 10, white);

    list[l++] = new translate(new rotate_y(new bvh_node(boxlist2, ns, 0, 1), 15), rtvec(-100, 270, 395));

    return new intersections(list, l);
}

 

 

未完待续。。。