Q107：Linux系统下GDB对PBRT-V3进行debug

——————-Notes for the first debug of pbrt——————-

1.在pbrt-v3下新建一个名为build.debug的文件夹，定位打该文件夹;

    cd ~/pbrt-v3/build.debug/

2.用cmake生成debug版本的makefile;

    cmake ../ -DCMAKE_BUILD_TYPE=Debug

3.编译生成debug般的可执行文件pbrt;

    make -j4

4.链接可执行文件;

    sudo ln -s ~/pbrt-v3/build.debug/pbrt /usr/bin/pbrt.debug

5.用gdb开始对pbrt.debug进行debug;开始进入gdb环境;

    gdb pbrt.debug

6.在gdb环境中给pbrt.debug设置参数;

set args --nthreads=1 ~/pbrt/killeroos/killeroo-simple.pbrt

7.打开一个新的终端，查找所有实现“Li()”的类;（因为当前不知道会用到什么tracer，所以只有在每个tracer的Li()代码中加断点）

grep -rn "::Li(" ~/pbrt-v3/src/ 

得到的搜索结果：

/home/lbzeng/pbrt-v3/src/integrators/path.cpp:64:Spectrum PathIntegrator::Li(const RayDifferential &r, const Scene &scene,
/home/lbzeng/pbrt-v3/src/integrators/ao.cpp:57:Spectrum AOIntegrator::Li(const RayDifferential &r, const Scene &scene,
/home/lbzeng/pbrt-v3/src/integrators/volpath.cpp:55:Spectrum VolPathIntegrator::Li(const RayDifferential &r, const Scene &scene,
/home/lbzeng/pbrt-v3/src/integrators/directlighting.cpp:62:Spectrum DirectLightingIntegrator::Li(const RayDifferential &ray,
/home/lbzeng/pbrt-v3/src/integrators/whitted.cpp:44:Spectrum WhittedIntegrator::Li(const RayDifferential &ray, const Scene &scene,
/home/lbzeng/pbrt-v3/src/core/stats.h:199:    "SamplerIntegrator::Li()",

8.回到之前的终端，一次设置这些断点（.h文件中不用设置）

b /home/lbzeng/pbrt-v3/src/integrators/path.cpp:65
b /home/lbzeng/pbrt-v3/src/integrators/ao.cpp:58
b /home/lbzeng/pbrt-v3/src/integrators/volpath.cpp:56
b /home/lbzeng/pbrt-v3/src/integrators/directlighting.cpp:63
b /home/lbzeng/pbrt-v3/src/integrators/whitted.cpp:45

可以通过info b查看当前的断点信息

info b

Num     Type           Disp Enb Address    What
1       breakpoint     keep y   0x08381a45 in pbrt::PathIntegrator::Li(pbrt::RayDifferential const&, pbrt::Scene const&, pbrt::Sampler&, pbrt::MemoryArena&, int) const at /home/lbzeng/pbrt-v3/src/integrators/path.cpp:65
2       breakpoint     keep y   0x08390783 in pbrt::AOIntegrator::Li(pbrt::RayDifferential const&, pbrt::Scene const&, pbrt::Sampler&, pbrt::MemoryArena&, int) const at /home/lbzeng/pbrt-v3/src/integrators/ao.cpp:58
3       breakpoint     keep y   0x08391931 in pbrt::VolPathIntegrator::Li(pbrt::RayDifferential const&, pbrt::Scene const&, pbrt::Sampler&, pbrt::MemoryArena&, int) const at /home/lbzeng/pbrt-v3/src/integrators/volpath.cpp:56
4       breakpoint     keep y   0x0839700a in pbrt::DirectLightingIntegrator::Li(pbrt::RayDifferential const&, pbrt::Scene const&, pbrt::Sampler&, pbrt::MemoryArena&, int) const at /home/lbzeng/pbrt-v3/src/integrators/directlighting.cpp:63
5       breakpoint     keep y   0x08397ebf in pbrt::WhittedIntegrator::Li(pbrt::RayDifferential const&, pbrt::Scene const&, pbrt::Sampler&, pbrt::MemoryArena&, int) const at /home/lbzeng/pbrt-v3/src/integrators/whitted.cpp:45

9.运行（run）

r

然后会在第四个断点处停住：

Thread 1 "pbrt.debug" hit Breakpoint 4, pbrt::DirectLightingIntegrator::Li (
    this=0x8a54310, ray=..., scene=..., sampler=..., arena=..., depth=0)
    at /home/lbzeng/pbrt-v3/src/integrators/directlighting.cpp:64
64                                        MemoryArena &arena, int depth) const {

所以，可以确定当前场景用到的积分器（或者说tracer）是directlighting

10.查看断点处的代码;

l 64

然后按几下“回车”（“回车”即“重复上一条指令”）
终端上输出如下信息：

(gdb) l 64
59      }
60  }
61  
62  Spectrum DirectLightingIntegrator::Li(const RayDifferential &ray,
63                                        const Scene &scene, Sampler &sampler,
64                                        MemoryArena &arena, int depth) const {
65      ProfilePhase p(Prof::SamplerIntegratorLi);
66      Spectrum L(0.f);
67      // Find closest ray intersection or return background radiance
68      SurfaceInteraction isect;
(gdb) 
69      if (!scene.Intersect(ray, &isect)) {
70          for (const auto &light : scene.lights) L += light->Le(ray);
71          return L;
72      }
73  
74      // Compute scattering functions for surface interaction
75      isect.ComputeScatteringFunctions(ray, arena);
76      if (!isect.bsdf)
77          return Li(isect.SpawnRay(ray.d), scene, sampler, arena, depth);
78      Vector3f wo = isect.wo;
(gdb) 
79      // Compute emitted light if ray hit an area light source
80      L += isect.Le(wo);
81      if (scene.lights.size() > 0) {
82          // Compute direct lighting for _DirectLightingIntegrator_ integrator
83          if (strategy == LightStrategy::UniformSampleAll)
84              L += UniformSampleAllLights(isect, scene, arena, sampler,
85                                          nLightSamples);
86          else
87              L += UniformSampleOneLight(isect, scene, arena, sampler);
88      }
(gdb) 
89      if (depth + 1 < maxDepth) {
90          Vector3f wi;
91          // Trace rays for specular reflection and refraction
92          L += SpecularReflect(ray, isect, scene, sampler, arena, depth);
93          L += SpecularTransmit(ray, isect, scene, sampler, arena, depth);
94      }
95      return L;
96  }
97  
98  DirectLightingIntegrator *CreateDirectLightingIntegrator(
(gdb) 

11.简单看一下代码（这个需要一定的基础，本人之前有学过《Ray Tracing from the Ground Up》）
可以看出最终的颜色值L由四部分组成：漫反射光、光源、高光反射光、折射光。

然后，可以在这些位置分别设置断点：

b 75
b 80
b 84
b 87
b 92
b 93

当程序在某断点处停止时，如果想看看断点处函数的具体实现，可以：

s（进入函数）
n（进入函数后，n指令查看该函数中的下一行代码）
l N（刚进入函数时，对应代码的行数N，指令“l N”显示N行附近的代码，然后，按几次回车，基本可以显示整个函数的代码）
bt（打印出，程序跑至此处的callstack）

12.相关callstack