[(source)](https://www.masterclass.com/articles/all-about-soft-boiled-eggs-how-long-to-boil-eggs-and-easy-recipe)
Different people have different preferences for how they enjoy their boiled eggs - hard-boiled, soft-boiled, or somewhere in between!
You can never be sure if an egg is up to your standards just from the outside; it’s what’s on the inside that counts!
Our goal for this project was to render a few eggs boiled for different amounts of time.
(##) Features
* Homogeneous volumetric path tracing/subsurface scattering:
* with multiple scattering
* with index-matched or dielectric boundary
* with isotropic or Henyey-Greenstein phase function
* with scattering/absorption coefficients as colors
* with next-event estimation (NEE)
* Custom yolk material: a homogeneous medium encased in a rough dielectric with custom refraction
* Environment lighting: using an environment map image mapped onto a sky sphere
* Normal mapping: using a normal map image to modify surface normals
(##) Volumetric path tracer
* We implemented a volumetric path tracer in integrators/path_tracer_vol.cpp and with next-event estimation in integrators/path_tracer_vol_nee.cpp.
* We added new classes for media in medium.h and media/homogeneous.cpp, and we augmented the Material class to contain pointers for the meda "inside" and "outside" of the material.
* We added new material classes materials/volumetric.cpp (for a volumetric object with an index-matched boundary), materials/volumetric_with_mat.cpp (for volumetric object encased in another material like a dielectric), and materials/yolk.cpp (a custom yolk material to be discussed in the next section).
* We added new classes for isotropic and Henyey-Greenstein phase functions in phase_function.h, phase_functions/isotropic.cpp, and phase_functions/hg.cpp.
Validation of subsurface scattering (in the sphere on the right) against Jensen box reference.
Using a dielectric boundary and a medium with absorption/scattering coefficients set to 0.
Validation of strongly absorbing vs strongly scattering media.
Validation of forward vs backward scattering media (using Henyey-Greenstein phase function).
Validation of grey vs colored media.
Validation of next-event estimation (NEE).
(##) Custom yolk material
We created a custom wet yolk material in materials/yolk.cpp.
The yolk consists of a homogeneous medium encased in a rough dielectric boundary.
We further customized the yolk by making light refract on the way in, but not on the way out.
This allows for shadow connections from within the yolk, while still getting sharp specular highlights on the wet yolk.
(##) Environment mapping
We used a sky sphere with an environment map image for environment lighting.
The implementation is in materials/environment_light.cpp. The material gets the albedo value from an image texture.
The material only emits from the non-normal-facing side because it assumes it is applied to a surface that surrounds the scene (with normals facing outwards, but we want it to emit light inwards).
(##) Normal mapping
We implemented normal mapping in textures/normal_mapped_texture.cpp and modified code in materials/lambertian.cpp and the various surfaces classes.
We originally planned to use normal mapping for the egg shells in our image;
however, we ultimately decided not to use this feature in our final image because we didn't find a texture that looked good on our egg shells.
We still implemented the feature, and the validation images are shown below.
Validation of normal mapping.
Validation of normal mapping.
Validation of normal mapping for a sphere.
(##) Final image
Our final image consists of 3 eggs cooked at different consistencies.
We rendered it on 30 cores with 100 samples per pixel for each, and then averaged them to obtain the final image.
* All 3 egg whites consist of a forward-scattering medium with a high albedo.
* The yolk of the hardest-boiled egg is simulated simply using a constant-colored Lambertian material.
* The yolks of the soft-boiled egg and the runny egg are simulated using the custom yolk material.
* The egg shells are simulated with a Lambertian material with an image texture for the albedo.
* The table material is a Fresnel blend that uses a marble image texture for the base.
(##) Challenges
Our biggest challenge was setting the egg yolk color because it required tuning the scattering and absorption coefficients, which aren't completely intuitive.
In addition, we tuned the image by rendering it with much fewer samples per pixel than our final image had,
and so when we rendered the image with many samples per pixel, the resulting colors were much brighter than we expected.
This made it particularly difficult to set the appropriate coefficients, and it also prevented us from rendering our image with even more samples per pixel.
Other general challenges included:
* Refactoring the code to add new features. Sometimes values which needed to be modified were declared as consts, so we had to change these or add new variables/parameters.
* Creating the detailed obj models, setting up the scene in Blender, and adjusting the camera angle.
(##) Division of labor
We worked together on the implementation of each feature. Jacob handled the modeling in Blender, while Tanli spent more time on the volumetric path tracing implementation.
