Theme
At first I have several inspiration pictures, and later I decided to majorly focus on using materials to simulate different kinds of clues that can show something's age.
The major content I want to render is a rusted typewriter, with different parts showing different kinds of rust and wear.
In contrast, in the scene there're also things from modern age, shinny objects, and objects made of futuristic materials around it.
Thus, I need to add support for texture, extend the class of materials by adding new BSDFs and allowing different BSDFs to be combined so that I can simulate complex surface appearances, and I also need volumetric path tracing to simulate the appearance of futuristic materials.
Implemented Features
Simple Features
Environmental Light(2 pts)
Simple environmental light with latitude-longitude format input, with uniform sphere sampling.
Related source file : env.cpp, bmp2d.h, bmp2dc.h
Moderate Features
New BSDFs(6 pts)
I added anisotropic microfacet, dielectric coating, and oren-nayar BRDFs.
Anisotropic Phong
For anisotropic, I implemented the BRDF and importance sampling method described in the paper "An Anisotropic Phong BRDF Model".Results in Warptest(sigma_v = sigma_u * 2):
Some extra change to basecode is needed to produce correct result for anisotropic, because by default local frame doesn't ensure a continuous tangent.
When intersecting, I forced local frame to align with 1 derivative of position over U or V, so that it won't have the discontinuity.
I think a better way should be to use a tangent map.
Oren-Nayar
For Oren-nayar I used a combination of cosine hemisphere and uniform hemisphere based on the roughness.
Oren-nayar converges to diffuse when roughness=0:
Coating BRDF
The coating BRDF is based on what's described in paper "Arbitrarily Layered Micro-Facet Surfaces", and taking the implementation in Mitsuba renderer as reference, I also added some simple methods to change the appearance and compensate the energy loss (not physically correct as it could add more light than incoming light).
Related source files : phong_aniso.cpp , coating2.cpp, orennayar.cpp, warp.cpp, warp.h, bvh.cpp, mesh.h
Texture support and node-based material system(6 pts)
Inspired by the coating BRDF, I also implemented a BRDF that can combine different BRDFs based on given probability to interact with either of them, with weighted pdf and sampling methods.
As the weight is manually specified, it might not be physically correct, but it obeys energy conservation.
Then I added texture mapping to use textures to interact with BSDFs, for example, texture can now be used as mask for certain BSDFs, or be passed in as parameter to blend or combine different BRDFs.
BSDF and texture nodes can be combined to create a multi-layered material with tree structure, which allows me to create complex materials.
Related source file : bmp2d.h, bmp2dc.h, bridge.cpp
Advanced Features
Volumetric Path Tracing(12 pts)
I have implemented volumetric path tracing in homogeneous media with support for isotropic, Henyey-Greenstein, Rayleigh phase functions.
In medium it's doing phase function sampling, and after exiting the medium it will start MIS. The depth is controlled by Russian roulette.
I tested the phase functions and parameter g for Henyey-Greenstein, and the effect of the parameter is as expected.
I also tested the high-scatterring medium in cornell box, and it converges to diffuse surface.
When both sigmaA and sigmaS are 0, it's just a pass-through medium, and there's more noise due to the dielectric pass-through BRDF of the bounding object.
For Heterogeneous medium I implemented Woodcock tracking, and it can either load voxel data or use perlin noise function as sigma values.
When I set sigma to be the same in any position, for some sigma the result is darker than the result produced by homogeneous medium, thus there still exists some error in the implementation of heterogeneous media.
Related source files : path_mis.cpp, homogeneous.cpp, heterogeneous.cpp, medium.cpp, isotropic.cpp, HG.cpp, RL.cpp
Final Rendering Competition Submission
For the rendering competition submission, to replicate the appearance of rust, I used pre-generated noise to blend different kinds of materials multiple times to simulate different kinds of rust and wear, to get a mixture of rough and smooth materials. I used anisotropic microfacet model for the shinny metal objects, and volumetric render for the futuristic materials. The models are from Blendswap.com and the texture for environmental light is from hdrlabs.com.
